Dean's Analytical Chemistry Handbook

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SECTION 1

PRELIMINARY OPERATIONS OF ANALYSIS 1.1 SAMPLING 1.1.1 Handling the Sample in the Laboratory 1.1.2 Sampling Methodology 1.2 MIXING AND REDUCTION OF SAMPLE VOLUME 1.2.1 Introduction 1.2.2 Coning and Quartering Figure 1.1 Coning Samples Figure 1.2 Quartering Samples 1.2.3 Riffles 1.3 CRUSHING AND GRINDING 1.3.1 Introduction 1.3.2 Pulverizing and Blending Table 1.1 Sample Reduction Equipment Table 1.2 Properties of Grinding Surfaces 1.3.3 Precautions in Grinding Operations 1.4 SCREENING AND BLENDING Table 1.3 U.S. Standard Sieve Series 1.5 MOISTURE AND DRYING 1.5.1 Forms of Water in Solids 1.5.2 Drying Samples Table 1.4 Drying Agents Table 1.5 Solutions for Maintaining Constant Humidity 1.5.3 Drying Collected Crystals Table 1.6 Concentrations of Solutions of H2SO4, NaOH, and CaCl2 Giving Specified Vapor Pressures and Percent Humidities at 25°C 1.5.4 Drying Organic Solvents Table 1.7 Relative Humidity from Wet- and Dry-Bulb Thermometer Readings Table 1.8 Relative Humidity from Dew-Point Readings 1.5.5 Freeze-Drying 1.5.6 Hygroscopic lon-Exchange Membrane 1.5.7 Microwave Drying Table 1.9 Chemical Resistance of a Hygroscopic lon-Exchange Membrane 1.5.8 Critical-Point Drying Table 1.10 Transitional and Intermediate Fluids for Critical-Point Drying 1.5.9 Karl Fischer Method for Moisture Measurement 1.6 THE ANALYTICAL BALANCE AND WEIGHTS 1.6.1 Introduction Table 1.11 Classification of Balances by Weighing Range 1.6.2 General-Purpose Laboratory Balances Table 1.12 Specifications of Balances 1.6.3 Mechanical Analytical Balances 1.6.4 Electronic Balances 1.6.5 The Weighing Station 1.6.6 Air Buoyancy 1.6.7 Analytical Weights Table 1.13 Tolerances for Analytical Weights

1.2 1.2 1.3 1.6 1.6 1.6 1.7 1.7 1.7 1.8 1.8 1.8 1.9 1.10 1.11 1.11 1.12 1.12 1.13 1.14 1.14 1.15 1.15 1.16 1.16 1.17 1.18 1.19 1.19 1.19 1.20 1.20 1.21 1.21 1.22 1.22 1.23 1.23 1.23 1.24 1.24 1.26 1.27 1.27 1.27

1.1

1.2

SECTION ONE

1.7 METHODS FOR DISSOLVING THE SAMPLE 1.7.1 Introduction 1.7.2 Decomposition of Inorganic Samples Table 1.14 Acid Digestion Bomb-Loading Limits Table 1.15 The Common Fluxes Table 1.16 Fusion Decompositions with Borates in Pt or Graphite Crucibles 1.7.3 Decomposition of Organic Compounds Table 1.17 Maximum Amounts of Combustible Material Recommended for Various Bombs Table 1.18 Combustion Aids for Accelerators 1.7.4 Microwave Technology Table 1.19 Typical Operating Parameters for Microwave Ovens 1.7.5 Other Dissolution Methods Table 1.20 Dissolution with Complexing Agents Table 1.21 Dissolution with Cation Exchangers (H Form) Table 1.22 Solvents for Polymers 1.8 FILTRATION 1.8.1 Introduction 1.8.2 Filter Media Table 1.23 General Properties of Filter Papers and Glass Microfibers Table 1.24 Membrane Filters Table 1.25 Membrane Selection Guide Table 1.26 Hollow-Fiber Ultrafiltration Cartridge Selection Guide Table 1.27 Porosities of Fritted Glassware Table 1.28 Cleaning Solutions for Fritted Glassware 1.8.3 Filtering Accessories 1.8.4 Manipulations Associated with the Filtration Process 1.8.5 Vacuum Filtration 1.9 SPECIFICATIONS FOR VOLUMETRIC WARE 1.9.1 Volumetric Flasks Table 1.29 Tolerances of Volumetric Flasks 1.9.2 Volumetric Pipettes Table 1.30 Pipette Capacity Tolerances 1.9.3 Micropipettes Table 1.31 Tolerances of Micropipettes (Eppendorf) 1.9.4 Burettes Table 1.32 Burette Accuracy Tolerances

1.1

1.28 1.28 1.29 1.31 1.33 1.34 1.34 1.36 1.36 1.38 1.39 1.41 1.41 1.42 1.42 1.42 1.42 1.43 1.44 1.47 1.47 1.48 1.49 1.49 1.49 1.50 1.51 1.52 1.52 1.52 1.52 1.53 1.53 1.53 1.54 1.54

SAMPLING

1.1.1 Handling the Sample in the Laboratory Each sample should be completely identified, tagged, or labeled so that no question as to its origin or source can arise. Some of the information that may be on the sample is as follows: 1. The number of the sample. 2. The notebook experiment-identification number. 3. The date and time of day the sample was received.

PRELIMINARY OPERATIONS OF ANALYSIS

4. 5. 6. 7.

1.3

The origin of the sample and cross-reference number. The (approximate) weight or volume of the sample. The identifying code of the container. What is to be done with the sample, what determinations are to be made, or what analysis is desired?

A computerized laboratory data management system is the solution for these problems. Information as to samples expected, tests to be performed, people and instruments to be used, calculations to be performed, and results required are entered and stored directly in such a system. The raw experimental data from all tests can be collected by the computer automatically or can be entered manually. Status reports as to the tests completed, work in progress, priority work lists, statistical trends, and so on are always available automatically on schedule and on demand.

1.1.2 Sampling Methodology The sampling of the material that is to be analyzed is almost always a matter of importance, and not infrequently it is a more important operation than the analysis itself. The object is to get a representative sample for the determination that is to be made. This is not the place to enter into a discussion on the selection of the bulk sample from its original site, be it quarry, rock face, stockpile, production line, and so on. This problem has been outlined elsewhere.1–5 In practice, one of the prime factors that tends to govern the bulk sampling method used is that of cost. It cannot be too strongly stressed that a determination is only as good as the sample preparation that precedes it. The gross sample of the lot being analyzed is supposed to be a miniature replica in composition and in particle-size distribution. If it does not truly represent the entire lot, all further work to reduce it to a suitable laboratory size and all laboratory procedures are a waste of time. The methods of sampling must necessarily vary considerably and are of all degrees of complexity. No perfectly general treatment of the theory of sampling is possible. The technique of sampling varies according to the substance being analyzed and its physical characteristics. The methods of sampling commercially important materials are generally very well prescribed by various societies interested in the particular material involved, in particular, the factual material in the multivolume publications of the American Society for Testing Materials, now known simply as ASTM, its former acronym. These procedures are the result of extensive experience and exhaustive tests and are generally so definite as to leave little to individual judgment. Lacking a known method, the analyst can do pretty well by keeping in mind the general principles and the chief sources of trouble, as discussed subsequently. If moisture in the original material is to be determined, a separate sample must usually be taken. 1.1.2.1 Basic Sampling Rules. No perfectly general treatment of the theory of sampling is possible. The technique of sampling varies according to the substance being analyzed and its physical characteristics. The methods of sampling commercially important materials are generally very well prescribed by various societies interested in the particular material involved: water and sewage by the American Public Health Association, metallurgical products, petroleum, and materials of construction by the ASTM, road building materials by the American Association of State Highway Officials, agricultural materials by the Association of Official Analytical Chemists (AOAC), and so on. A large sample is usually obtained, which must then be reduced to a laboratory sample. The size of the sample must be adequate, depending upon what is being measured, the type of measurement being made, and the level of contaminants. Even starting with a well-gathered sample, errors can 1

G. M. Brown, in Methods in Geochemistry, A. A. Smales and L. R. Wager, eds., Interscience, New York, 1960, p. 4. D. J. Ottley, Min. Miner. Eng. 2:390 (1966). C. L. Wilson and D. W. Wilson, Comprehensive Analytical Chemistry, Elsevier, London, 1960; Vol. 1A, p. 36. 4 C. A. Bicking, “Principles and Methods of Sampling,” Chap. 6, in Treatise on Analytical Chemistry, I. M. Kolthoff and P. J. Elving, eds., Part 1, Vol. 1, 2d ed., Wiley-Interscience, New York, 1978; pp. 299–359. 5 G. M. Brown, in Methods in Geochemistry, A. A. Smales and L. R. Wager, eds., Interscience, New York, 1960, p. 4. 2 3

1.4

SECTION ONE

occur in two distinct ways. First, errors in splitting the sample can result in bias with concentration of one or more of the components in either the laboratory sample or the discard material. Second, the process of attrition used in reducing particle sizes will almost certainly create contamination of the sample. By disregarding experimental errors, analytical results obtained from a sample of n items will be distributed about m with a standard devitation s

s⫽

(1.1)

n

In general, s and m are not known, but s can be used as an estimate of s, and the average of analytical results as an estimate of m. The number of samples is made as small as compatible with the desired accuracy. If a standard deviation of 0.5% is assigned as a goal for the sampling process, and data obtained from previous manufacturing lots indicate a value for s that is 2.0%, then the latter serves as an estimate of s. By substituting in Eq. (1.1), 0.5 =

2.0 n

(1.2)

and n =16, number of samples that should be selected in a random manner from the total sample submitted. To include the effect of analytical error on the sampling problem requires the use of variances. The variance of the analysis is added to the variance of the sampling step. Assuming that the analytical method has a standard deviation of 1.0%, then s2 ⫽

(s

2 ⫹s 2 s a

)

n

(1.3)

where the numerator represents the variance of the sampling step plus the variance of the analysis. Thus (0.5) 2 ⫽

[(2.0) 2⫹ (1.0) 2 ] n

(1.4)

and n = 20, the number of samples required. The above discussion is a rather simple treatment of the problem of sampling. 1.1.2.2 Sampling Gases.6 Instruments today are uniquely qualified or disqualified by the Environmental Protection Agency. For a large number of chemical species there are as yet no approved methods. The size of the gross sample required for gases can be relatively small because any inhomogeneity occurs at the molecular level. Relatively small samples contain tremendous quantities of molecules. The major problem is that the sample must be representative of the entire lot. This requires the taking of samples with a “sample thief ” at various locations of the lot, and then combining the various samples into one gross sample. Gas samples are collected in tubes [250 to 1000 milliliter (mL) capacity] that have stopcocks at both ends. The tubes are either evacuated or filled with water, or a syringe bulb attachment may be used to displace the air in the bottle by the sample. For sampling by the static method, the sampling bottle is evacuated and then filled with the gas from the source being sampled, perhaps a cylinder. These steps are repeated a number of times to obtain the desired sampling accuracy. For sampling by the dynamic method, the gas is allowed to flow through the sampling container at a slow, steady rate. The container is flushed out and the gas reaches equilibrium with the walls of the sampling lines and container with respect to moisture. When equilibrium has been reached, the stopcocks on the sampling container are 6 J. P. Lodge, Jr., ed., Methods of Air Sampling and Analysis, 3d ed., Lewis, Chelsea, Michigan, 1989. Manual of methods adopted by an intersociety committee.

PRELIMINARY OPERATIONS OF ANALYSIS

1.5

closed—the exit end first followed by the entrance end. The sampling of flowing gases must be made by a device that will give the correct proportion of the gases in each annular increment. Glass containers are excellent for inert gases such as oxygen, nitrogen, methane, carbon monoxide, and carbon dioxide. Stainless-steel containers and plastic bags are also suitable for the collection of inert gases. Entry into the bags is by a fitting seated in and connected to the bag to form an integral part of the bag. Reactive gases, such as hydrogen sulfide, oxides of nitrogen, and sulfur dioxide, are not recommended for direct collection and storage. However, TedlarTM bags are especially resistant to wall losses for many reactive gases. In most cases of atmospheric sampling, large volumes of air are passed through the sampling apparatus. Solids are removed by filters; liquids and gases are either adsorbed or reacted with liquids or solids in the sampling apparatus. A flowmeter or other device determines the total volume of air that is represented by the collected sample. A manual pump that delivers a definite volume of air with each stroke is used in some sampling devices. 1.1.2.3 Sampling Liquids. For bottle sampling a suitable glass bottle of about 1-L capacity, with a 1.9-centimeter (cm) opening fitted with a stopper, is suspended by clean cotton twine and weighted with a 560-gram (g) lead or steel weight. The stopper is fitted with another length of twine. At the appropriate level or position, the stopper is removed with a sharp jerk and the bottle permitted to fill completely before raising. A cap is applied to the sample bottle after the sample is withdrawn. In thief sampling a thief of proprietary design is used to obtain samples from within about 1.25 cm of the tank bottom. When a projecting stem strikes the bottom, the thief opens and the sample enters at the bottom of the unit and air is expelled from the top. The valves close automatically as the thief is withdrawn. A core thief is lowered to the bottom with valves open to allow flushing of the interior. The valves shut as the thief hits the tank bottom. When liquids are pumped through pipes, a number of samples can be collected at various times and combined to provide the gross sample. Care should be taken that the samples represent a constant fraction of the total amount pumped and that all portions of the pumped liquid are sampled. Liquid solutions can be sampled relatively easily provided that the material can be mixed thoroughly by means of agitators or mixing paddles. Homogeneity should never be assumed. After adequate mixing, samples can be taken from the top and bottom and combined into one sample that is thoroughly mixed again; from this the final sample is taken for analysis. For sampling liquids in drums, carboys, or bottles, an open-ended tube of sufficient length to reach within 3 mm of the bottom of the container and of sufficient diameter to contain from 0.5 to 1.0 L may be used. For separate samples at selected levels, insert the tube with a thumb over the top end until the desired level is reached. The top hole is covered with a thumb upon withdrawing the tube. Alternatively the sample may be pumped into a sample container. Specially designed sampling syringes are used to sample microquantities of air-sensitive materials. For suspended solids, zone sampling is very important. A proprietary zone sampler is advantageous. When liquids are pumped through pipes, a number of samples can be collected at various times and combined to provide the gross sample. Take care that the samples represent a constant fraction of the total amount pumped and that all portions of the pumped liquid are sampled. 1.1.2.4 Sampling Compact Solids. In sampling solids particle size introduces a variable. The size/weight ratio b can be used as a criterion of sample size. This ratio is expressed as b⫽

weight of largest particle ⫻ 100 weight of sample

(1.5)

A value of 0.2 is suggested for b; however, for economy and accuracy in sampling, the value of b should be determined by experiment. The task of obtaining a representative sample from nonhomogeneous solids requires that one proceeds as follows. A gross sample is taken. The gross sample must be at least 1000 pounds (lb) if the pieces are greater than 1 inch (in) (2.54 cm), and must be subdivided to 0.75 in (1.90 cm) before reduction to 500 lb (227 kg), to 0.5 in (1.27 cm) before reduction to 250 lb (113 kg), and so on, down

1.6

SECTION ONE

to the 15-lb (6.8-kg) sample, which is sent to the laboratory. Mechanical sampling machines are used extensively because they are more accurate and faster than hand-sampling methods described below. One type removes part of a moving steam of material all of the time. A second type diverts all of stream of material at regular intervals. For natural deposits or semisoft solids in barrels, cases, bags, or cake form, an auger sampler of post-hole digger is turned into the material and then pulled straight out. Core drilling is done with special equipment; the driving head should be of hardened steel and the barrel should be at least 46 cm long. Diamond drilling is the most effective way to take trivial samples of large rock masses. For bales, boxes, and similar containers, a split-tube thief is used. The thief is a tube with a slot running the entire length of the tube and sharpened to a cutting edge. The tube is inserted into the center of the container with sufficient rotation to cut a core of the material. For sampling from conveyors or chutes, a hand scoop is used to take a cross-sectional sample of material while in motion. A gravity-flow auger consists of a rotating slotted tube in a flowing mass. The material is carried out of the tube by a worm screw. 1.1.2.5 Sampling Metals. Metals can be sampled by drilling the piece to be sampled at regular intervals from all sides, being certain that each drill hole extends beyond the halfway point. Additional samples can be obtained by sawing through the metal and collecting the “sawdust.” Surface chips alone will not be representative of the entire mass of a metallic material because of differences in the melting points of the constituents. This operation should be carried out dry whenever possible. If lubrication is necessary, wash the sample carefully with benzene and ether to remove oil and grease. For molten metals the sample is withdrawn into a glass holder by a sample gun. When the sample cools, the glass is broken to obtain the sample. In another design the sampler is constructed of two concentric slotted brass tubes that are inserted into a molten or powdered mass. The outer tube is rotated to secure a representative solid core.

1.2

MIXING AND REDUCTION OF SAMPLE VOLUME

1.2.1 Introduction The sample is first crushed to a reasonable size and a portion is taken by quartering or similar procedures. The selected portion is then crushed to a somewhat smaller size and again divided. The operations are repeated until a sample is obtained that is large enough for the analyses to be made but not so large as to cause needless work in its final preparation. This final portion must be crushed to a size that will minimize errors in sampling at the balance yet is fine enough for the dissolution method that is contemplated. Every individual sample presents different problems in regard to splitting the sample and grinding or crushing the sample. If the sample is homogeneous and hard, the splitting procedure will present no problems but grinding will be difficult. If the sample is heterogeneous and soft, grinding will be easy but care will be required in splitting. When the sample is heterogeneous both in composition and hardness, the interactions between the problems of splitting and grinding can be formidable. Splitting is normally performed before grinding in order to minimize the amount of material that has to be ground to the final size that is suitable for subsequent analysis.

1.2.2 Coning and Quartering A good general method for mixing involves pouring the sample through a splitter repeatedly, combining the two halves each time by pouring them into a cone. When sampling very large lots, a representative sample can be obtained by coning (Fig. 1.1) and quartering (Fig. 1.2). The first sample is formed into a cone, and the next sample is poured onto the apex of the cone. The result is then mixed and flattened, and a new cone is formed. As each successive

PRELIMINARY OPERATIONS OF ANALYSIS

1.7

FIGURE 1.1 Coning samples. (From Shugar and Dean, The Chemist’s Ready Reference Handbook, McGraw-Hill, 1990.)

sample is added to the re-formed cone, the total is mixed thoroughly and a new cone is formed prior to the addition of another sample. After all the samples have been mixed by coning, the mass is flattened and a circular layer of material is formed. This circular layer is then quartered and the alternate quarters are discarded. This process can be repeated as often as desired until a sample size suitable for analysis is obtained. The method is easy to apply when the sample is received as a mixture of small, equal-sized particles. Samples with a wide range of particle sizes present more difficulties, especially if the large, intermediate, and small particles have appreciably different compositions. It may be necessary to crush the whole sample before splitting to ensure accurate splitting. When a coarsesized material is mixed with a fine powder of greatly different chemical composition, the situation demands fine grinding of a much greater quantity than is normal, even the whole bulk sample in many cases. Errors introduced by poor splitting are statistical in nature and can be very difficult to identify except by using duplicate samples. 1.2.3

Riffles Riffles are also used to mix and divide portions of the sample. A riffle is a series of chutes directed alternately to opposite sides. The starting material is divided into two approximately equal portions. One part may be passed repeatedly through until the sample size is obtained.

FIGURE 1.2 Quartering samples. The cone is flattened, opposite quarters are selected, and the other two quarters are discarded. (From Shugar and Dean, 1990.)

1.8

SECTION ONE

1.3

CRUSHING AND GRINDING

1.3.1 Introduction In dealing with solid samples, a certain amount of crushing or grinding is sometimes required to reduce the particle size. Unfortunately, these operations tend to alter the composition of the sample and to introduce contaminants. For this reason the particle size should be reduced no more than is required for homogeneity and ready attack by reagents. If the sample can be pulverized by impact at room temperature, the choices are the following: 1. Shatterbox for grinding 10 to 100 mL of sample 2. Mixers or mills for moderate amounts to microsamples 3. Wig-L-Bug for quantities of 1 mL or less For brittle materials that require shearing as well as impact, use a hammer–cutter mill for grinding wool, paper, dried plants, wood, and soft rocks. For flexible or heat-sensitive samples, such as polymers or tissues, chill in liquid nitrogen and grind in a freezer mill or use the shatterbox that is placed in a cryogenic container. For hand grinding, use boron carbide mortars. Many helpful hints involving sample preparation and handling are in the SPEX Handbook.7

1.3.2 Pulverizing and Blending Reducing the raw sample to a fine powder is the first series of steps in sample handling. Sample reduction equipment is shown in Table 1.1, and some items are discussed further in the following sections along with containment materials, the properties of which are given in Table 1.2. 1.3.2.1 Containment Materials. The containers for pulverizing and blending must be harder than the material being processed and should not introduce a contaminant into the sample that would interfere with subsequent analyses. The following materials are available. Agate is harder than steel and chemically inert to almost anything except hydrofluoric acid. Although moderately hard, it is rather brittle. Use is not advisable with hard materials, particularly aluminum-containing samples, or where the silica content is low and critical; otherwise agate mortars are best for silicates. Agate mortars are useful when organic and metallic contaminations are equally undesirable. Silicon is the major contaminant, accompanied by traces of aluminum, calcium, iron, magnesium, potassium, and sodium. Alumina ceramic is ideal for extremely hard samples, especially when impurities from steel and tungsten carbide are objectionable. Aluminum is the major contaminant, accompanied by traces of calcium, magnesium, and silicon. However, because alumina ceramic is brittle, care must be taken not to feed “uncrushable” materials such as scrap metal, hardwoods, and so on into crushers or mills. Boron carbide is very low wearing but brittle. It is probably most satisfactory for general use in mortars, although costly. Major contaminants are boron and carbide along with minor amounts of aluminum, iron, silicon, and possibly calcium. The normal processes of decomposition used in subsequent stages of the analysis usually convert the boron carbide contaminant into borate plus carbon dioxide, after which it no longer interferes with the analysis. Plastic containers (and grinding balls) are usually methacrylate or polystyrene. Only traces of organic impurities are added to the sample. Steel (hardened plain-carbon) is used for general-purpose grinding. Iron is the major contaminant, accompanied by traces of carbon, chromium, manganese, and silicon. Stainless steel is less subject to chemical attack, but contributes nickel and possibly sulfur as minor contaminants. 7 R.

H. Obenauf et al., SPEX Handbook of Sample Preparation and Handling, 3d ed., SPEX Industries, Edison, N. J., 1991.

1.9

䊏

䊏

䊏

䊏 䊐

䊐

1M HClO4 or HNO3

Al, Fe(III); bronze and rocks

pH 9.7 + butyrate + EDTA

CHCl3

Dithizone in CCl4

Al, Cu, Fe(III), Zn

pH 7–8 plus EDTA

Acetylacetone

Alkaline earths, phosphate

pH 4–5

Acetylacetone in benzene

(Continued)

L. Ducret and P. Sequin, Anal. Chim. Acta 17:213 (1957) L. C. Pasztor, J. D. Bode, and Q. Fernando, Anal. Chem. 32:277 (1960) Z. Marczenko, M. Mojski, and K. Kasiura, Zh. Anal. Khim. 22:1805 (1967) J. B. Mullin and J. P. Riley, Nature 174:42 (1954) See also S. Bajo and A. Wyttenbach, Anal. Chem. 49:158 (1977) H. Bode and K. Wulff, Z. Anal. Chem. 219:32 (1966) A. W. Wylie, J. Chem. Soc. 1951, 1474 L. E. Glendenin et al., Anal. Chem. 27:59 (1955) F. Culkin and J. P. Riley, Anal. Chim. Acta 32:197 (1965)

H. Bode and G. Henrich, Z. Anal. Chem. 135:98 (1952)

Chem. Lett. 1979, 601 T. Y. Toribara and P. S. Chen, Jr., Anal. Chem. 24:539 (1952); T. Y. Toribara and R. E. Sherman, ibid. 25:1594 (1953) I. P. Alimarin and I. M. Gibalo, Zh. Anal. Khim. 11:389 (1956); C. W. Sill and C. P. Willis, Anal. Chem. 31:598 (1959) S. Banerjee, A. K. Sundaram, and H. D. Sharma, Anal. Chim. Acta 10:256 (1954); A. L. Markman and L. L. Galkina, Anal. Abst. 10:5071 (1963); 12:1628 (1965) T. Bidleman, Anal. Chim. Acta 56:221 (1971)

PRELIMINARY SEPARATION METHODS

2.43

Copper

0.1M–1M HCl + ascorbic acid

5% triphenylphosphite in CCl4

pH 6.5

Diethyldithiocarbamate and CCl4

0.1M–0.2M HCl

5M NaSCN

MIBK

0.05% dithizone

Cr, Fe(III), Ti, V; seawater, biomaterials, rock, and soils

0.02M citrate and pH 8

As, Cd, Ga, Ge, Fe, Mg, Mn, Mo, Ni, P, Pb, Sb, Se, V, W, Zn; steel and biomaterials Highly selective; Hg, platinum metals, Zn

Fe, Ca, P

Ni

Fe(III) and Ni when back extracted with strong HCl

Cobalt

Thiocyanate

Alkaline earths, As(V), Cd, Ce(IV), Co, Cr(III), Cu, Fe(III), Ga, lanthanoids, Mn, Mo, Ni, Sc, U(VI), V(V), Zn Co, Ni

Hg, Mo(VI), V(V)

V; Fe if F− present

Lanthanoids, Th, U

Separated from

pH 4.5–8.0

Aliquat 336 in CCl4

Chromium(III)

1M HCl

0.02M H2O2; pH 1.74, 0°C

1M–2M HCl, 5°C, 30 s shaking

0.5M H2SO4

Aqueous phase

1% 2-Nitroso-1-naphthol in CHCl3, or CCl4, or toluene 0.05% dithizone in CCl4

Tribenzylamine in CHCl3

MIBK

Thenoyltrifluoroacetone and xylene MIBK

Organic phase

Chromium

Chromium(VI)

Cerium(IV)

Element

TABLE 2.21 Extraction Procedures for the Elements (Continued )

2.44

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T. H. Handley and J. A. Dean, Anal. Chem. 33:1087 (1961)

G. J. de Jong and U. A. T. Brinkman, J. Radioanal. Chem. 35:223 (1977) E. Boyland, Analyst 71:230 (1946); O. K. Borggaard et al., ibid. 107:1479 (1982) H. R. Marston and D. W. Dewey, Austr. J. Exptl. Biol. Med. Sci. 18:343 (1940); V. D. Anand, G. S. Deshmukh, and C. M. Pandey, Anal. Chem. 33:1933 (1961) R. A. Sharp and G. Wilkinson, J. Am. Chem. Soc. 77:6519 (1955) G. H. Ellis and J. F. Thompson, Ind. Eng. Chem. Anal. Ed. 17:254 (1945) K. J. Hahn, D. J. Tuma, and J. L. Sullivan, Anal. Chem. 40:974 (1968)

G. W. Smith and F. L. Moore, Anal. Chem. 29:448 (1957) H. A. Bryan and J. A. Dean, Anal. Chem. 29:1289 (1957); J. A. Dean and M. L. Beverly, ibid. 30:977 (1958); P. D. Blundy, Analyst 83:555 (1958); E. S. Pilkington and P. R. Smith, Anal. Chim. Acta 39:321 (1967) R. K. Brookshier and H. Freund, Anal. Chem. 23:1110 (1951); N. Ichinose et al., Anal. Chim. Acta 96:391 (1978) G. B. Fasolo, R. Malvano, and A. Massaglia, Anal. Chim. Acta 29:569 (1963); E. M. Donaldson, Talanta 27:779 (1980)

Reference

PRELIMINARY SEPARATION METHODS

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

`

Diethyl ether

Gold

3M–6M HCl

9M HCl

Cupferron, 1M H2SO4

CHCl3

CCl4

6M–8M HCl

MIBK

Germanium

5.5M–6M HCl

Diisopropyl ether

Gallium

Acetic acid solution

Diisopropyl ether (four extractions)

10M HCl

Pentyl acetate, isopentyl acetate, tributyl phosphate, trioctylphosphine in cyclohexane CHCl3 or CCl4

3.3M HF plus 6M HCl

Methyl isobutyl ketone

pH 9.2 (tartrate–NaOH)

Dithizone in CHCl3

Niobium

Dimethylglyoxime, pH 4.8–12 (tartrate medium) or pH 7.2–12 (citrate medium)

Aqueous phase

CHCl3

7M HCl

Nickel

Organic phase MIBK

Molybdenum

Element

TABLE 2.21 Extraction Procedures for the Elements (Continued ) Reference W. Doll and H. Specker, Z. Anal. Chem. 161:354 (1958); G. R. Waterbury and C. F. Metz, Talanta 6:237 (1960) E. B. Sandell and R. W. Perlich, Ind. Eng. Chem. Anal. Ed. 11:309 (1939); A Claassen and L. Bastings, Rec. Trav. Chim. 73:783 (1954) G. L. Hubbard and T. E. Green, Anal. Chem. 38:428 (1966); B. Morsches and G. Tölg, Z. Anal. Chem. 250:81 (1970) J. R. Werning et al., Ind. Eng. Chem. 46:646 (1954) T. V. Ramakrishna et al., Talanta 16:847 (1969); A. K. De and A. K. Sen, ibid. 13:853 (1966); K. Ilsemann and R. Bock, Z. Anal. Chem. 274:185 (1975) R. D. Sauerbrunn and E. B. Sandell, J. Am. Chem. Soc. 75:4170 (1953); Anal. Chim. Acta 9:86 (1953); G. Goldstein, D. L. Manning, O. Menis, and J. A. Dean, Talanta 7:296 (1961) W. Geilmann and R. Neeb, Z. Anal. Chem. 156:420 (1957) R. S. Young, Analyst 76:49 (1951); see F. E. Beamish, Talanta 14:991 (1967); S. J. Al-Bazi and A. Chow, ibid. 31:815 (1984) F. I. Danilova et al., Zh. Anal. Khim. 29:2150 (1974) E. M. Donaldson and M. Wang, Talanta 33:35 (1986) A. D. Langade and V. M. Shinde, Analyst 107:708 (1982) C. Wadelin and M. G. Mellon, Anal. Chem. 38:1668 (1953)

PRELIMINARY SEPARATION METHODS

CHCl3 or CCl4

Diethyl ether (three extractions)

Ruthenium(VIII)

Scandium

Tributyl phosphate

1-Butyl-3-methyl pyrazole in toluene

Rhodium(III)

MIBK

7M NH4SCN and 0.5M HCl; add 5 mL 2M HCl per 100 mL for each extraction 10M HCl

As RuO4: Ag(II) oxide, Ce(IV), or KIO4 in HNO3 or H2SO4

0.01M HCl

Tetraphenylarsonium chloride, pH 7–13. Tetraphenylphosphonium chloride similar HCl

0.01M Na tetraphenylborate, 0.1M NaClO4

Nitrobenzene

CHCl3

Dipicrylamine, pH 7.0–9.0

Nitrobenzene

Rhenium(VII)

104 1.6 106 >105 33 1720 2230 52 1600 120 * 1.1 104 4700 >105 >104 5460 14 >105 1850 >105

0.5M

1.0M

4.0M

1900 1.6 0.1 2930 117 * 790 84 >105 460 262 99 420 370 3400 3040 0.9 64 105 19 530 610 28 450

318 2.2 0.4 590 42 105 297 860 230 7 >104 510 >105

33 * 0.8 6 217 105 39 102 44 5 1460 64 105

61 3.8 0.8 127 13 1 42 1 265 21 27 19 18 20 35 43 0.3 14 265 4 21 20 6 22 1.4 15 * 0.6 2 60 2050 12 19 7 1 145 16 7250

0.2 12 2 1 5 11 3 3 2 2 2 0.4 0.2 11 3 2 3

3 (2M) 0.7 7 67 2 3 0.3 9 2 15

* Precipitate forms. References: F. W. E. Strelow, Anal. Chem. 32:1185 (1960); see also J. Korkisch and S. S. Ahluwalia, Talanta 14:155 (1967).

metals Co, Cu, Mn, Ni, and Zn, which is unsatisfactory in HNO3, becomes feasible in HCl. Separation of Fe(III) from Ti(IV) or Be is considerably better in HNO3 than in HCl. K and Na, which accompany Cd and In when these are eluted with 0.5M HCl to separate them from Al, Cu, Fe(III), Ni, and Zn, can be separated easily from Cd and In in HNO3 medium. A separation of Hg(I) and Hg(II) is feasible in HNO3. Distribution coefficients in H2SO4 are normally distinctly higher than those in either HCl or HNO3. On the other hand, a number of cations show marked complex formation or ion association

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PRELIMINARY SEPARATION METHODS

PRELIMINARY SEPARATION METHODS

2.73

TABLE 2.31 Distribution Coefficients (Dg) of Metal Ions on AG 50W-X8 Resin in Perchloric Acid Solutions Metal ion Ag(I) Al(III) Ba Be Bi(III) Ca Cd Ce(III) Co(II) Cr(III) Cu(II) Dy(III) Fe(II) Fe(III) Ga(III) Hg(I) Hg(II) In(III) La Mg Mn(II) Mo(VI) Mo(VI)* Ni(II) Pb(II) Sn(IV) Sr Th(IV) Ti(IV) Tl(I) Tl(III) U(VI) V(IV) V(V) V(V)* W(VI)* Y Yb(III) Zn Zr

0.2M 90 5250 2280 206 >104 636 423 >104 378 8410 378 >104 389 7470 5870 4160 937 6620 >104 312 387 22 0.7 387 1850 Precipitate 870 >104 549 131 1550 276 201 9.8 9.3 0.4 >104 >104 361 >104

1.0M 20 106 127 14 243 50 36 459 31 120 30 258 32 119 112 147 85 128 475 24 32 5.5 0.4 32 117 Precipitate 67 5870 19 23 176 29 18 2.2 3 0.4 246 205 30 >104

4.0M 5.8 11 19 1.9 42 7.7 6.3 53 4.8 11 4.5 39 5.2 12 11 9 23 14 58 3 4.7 4.5 1.3 5 17 7.5 10 686 5.7 2.7 41 18 4.4 0.8 1 0.4 24 20 5 333

* Hydrogen peroxide present. Reference: F. W. E. Strelow and H. Sondorp, Talanta 19:1113 (1972).

in sulfuric acid. Cations such as Zr, Th(IV), U(VI), Ti(IV), Sc, and to a lesser extent Fe(III), In, and Cr(III) show the sulfate complexing effect. Uranium(VI) can be separated from Be, Mg, Co, Cu, Fe(II), Fe(III), Mn(II), Al, rare earths, Th, and other elements, and Ti(IV) can be separated from the same elements, as well as Nb(V), V(V), and Mo(VI). Scandium can readily be separated from Y, La, and the other rare-earth elements. V(IV) is most easily separated from V(V) or V(IV) from Mo(VI) and Nb(V) with H2SO4 as eluent.

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PRELIMINARY SEPARATION METHODS

2.74

SECTION TWO

TABLE 2.32 Distribution Coefficients (Dg) of Metal Ions on AG 50W-X8 Resin in Nitric Acid Solutions Total resin capacity ratio q = 0.4, except for Te(IV) where q = 0.2 Metal ion

0.1M

0.2M

0.5M

1.0M

4.0M

Ag(I) Al As(III) Ba Be Bi(III) Ca Cd Ce(III) Co(II) Cr(III) Cs Cu(II) Er Fe(III) Ga Gd Hf Hg(I) Hg(II) In(III) K La Li Mg Mn(II) Mo(VI) Na Nb(V) Ni(II) Pb(II) Pd(II) Rb Rh(III) Sc Se(IV) Sm Sr Te(IV) Th(IV) Ti(IV) Tl(I) U(VI) V(IV) V(V) Y Yb Zn Zr

156 >104 104 1260 5100 148 1080 >104 >104 >104 >104 >104 >104 4700 >104 99 >104 33 794 1240 ppt 54 12 1140 >104 97 118 78 >104 104 3100 40 >104 1410 173 659 495 20 >104 >104 1020 >104

86 3900 104 392 1620 81 356 >104 4100 4200 >104 >104 7600 1090 >104 59 >104 19 295 389 5 29 6 384 1420 62 68 45 3300 104 775 20 >104 461 91 262 157 11 >104 >104 352 >104

36 392 104 640 121 680 26 1870 8 71 89 3 13 1 91 183 23 29 19 500 104 71 41 69 36 5 1020 1150 83 104

18 79 104 1240 1790 3190 86 >104 28 484 610 5 48 7 590 71 91 49 1050 104 31 3900 225 236 1490 118 490 15 >104 >104 550 474

540 104 135 98

126 104 1600 >104 >104 >104 2690 7900 >104 138 >104 48 1300 1590 ppt 81 14 1390 109 148 80 5600 104 ppt* >104 395 452 6500 596 1230 27 >104 >104 1570 546

* ppt denotes precipitate. Reference: F. W. E. Strelow, R. Rethemeyer, and C. J. C. Bothma, Anal. Chem. 37:106 (1965).

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4.0N 5 104 510 105

HClO4

HNO3

90 5250

86 3900 104 636 423 >104 378 8410 378 >104 389 7470 5870 4160 937 6620 >104 312 387 22 387 1850

1560 183 305 480 392 >104 392 1620 356

precipitate 870 >104 549 131 1550 276 201 10 0.4 >104 >104 361 >104

H2SO4 8300 104 540 >104 433 176 505

4100 4200 7600 1090 >104 >104 295 389 5 384 1420 62 45 3300

2050 3500

>104 461 91 262

3900 225 236 1490 118

11

15

1790 3190 >104 484 610 5 590 71 49 1050

>104

>104

361 >104

550 474

References For HCl: F. W. E. Strelow, Anal. Chem. 32:1185 (1960); J. Korkisch and S. S. Ahluwalia, Talanta 14:155 (1967). For HClO4: F. W. E. Strelow and H. Sondorp, Talanta 19:1113 (1972). For HNO3: F. W. E. Strelow, R. Rethemeyer, and C. J. C. Bothma, Anal. Chem. 37:106 (1965); J. Korkisch, F. Feik, and S. S. Ahluwalia, Talanta 14:1069 (1967). For H2SO4: F. W. E. Strelow, R. Rethemeyer, and C. J. C. Bothma, Anal. Chem. 37:106 (1965).

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PRELIMINARY SEPARATION METHODS

PRELIMINARY SEPARATION METHODS

2.77

2.3.4.5 Anion Exchange of Metal Complexes. Many metal ions may be converted to a negatively charged complex through suitable masking systems. This fact, coupled with the greater selectivity of anion exchangers, makes anion exchange a logical tool for handling certain metals. The negatively charged metal complexes are initially adsorbed by the exchanger from a high concentration of complexing agent, the eluted stepwise by lowering the concentration of the complexing agent in the eluent sufficiently to cause dissociation of the least stable of the metal complexes, and so on. If interconversion of the anion complex is fast, control of ligand concentration affords a powerful tool for control of absorbability, since ligand concentration controls the fraction of the metal present as adsorbable complex. For each metal and complexing ligand, there is a characteristic curve of log Dv versus molarity of complexing agent. Examples are given in Tables 2.35 and 2.36 for metals that form chloride and sulfate complexes. Similar studies are available for fluoride solution22 and nitrate solutions.23 It is possible to devise a number of separation schemes in which a group of metals is adsorbed on a resin from a concentration of the complexing agent, and then each metal in turn is eluted by progressively lowering the complexing agent concentration (Table 2.37). Thus a cation that forms no, or only a very weak, anionic complex is readily displaced by several column volumes of the complexing agent, while its companions are retained. For separating two ions, it is advisable to choose a concentration of the complexing agent for which the separation factor is maximal, yet, at the same time, it is important that the volume distribution ratio of the eluting ion not be higher than unity. When Dv = 1, the peak maximum emerges within approximately two column volumes. The separation of nickel, manganese, cobalt, copper, iron(III), and zinc ions is done as follows.24 The mixture of cations in 12M HCl is poured onto the column bed, which has been previously washed with 12M HCl. Nickel, which forms no chloro complex, elutes within several column volumes of a 12M HCl solution. The receiver is changed and manganese(II) is eluted with several column volumes of 6M HCl. This procedure is repeated using successively 4M HCl to elute cobalt(II), 2.5M HCl for copper(II), 0.5M HCl for iron(III), and lastly, 0.005M HCl for zinc. Table 2.38 contains selected applications of ion exchange for the separation of a particular element from other metals or anions. A discussion of ion-exchange chromatographic methods for analysis is reserved for Sec. 4.5. 2.3.4.6 Ligand-Exchange Chromatography.25 In this method a cation-exchange resin, saturated with a complex-forming metal, such as Cu(II), Fe(III), Co(II), Ni(II), or Zn(II), acts as a solid adsorbent. Thus, even though they are bound to an exchanger, these metals retain their ability to be the central atom of a coordination compound. Ligands, which may be anions or neutral molecules such as ammonia, amines, amino acids, or olefins, are removed from the liquid phase by formation of complexes with the metal attached to the resin and subsequent displacement of water or other solvents coordinated to the metal ion. Although the ordinary strongly acidic and weakly acidic cation exchangers undergo very satisfactory ligand-exchange reactions, the chelating resins that have iminodiacetate functional groups attached to a styrene matrix are ideally suited for ligand-exchange work. Strong complex formers, such as nickel, copper, or zinc, are tightly bound to the iminodiacetate exchanger. Consequently, leakage of metal ions from chelating resins by ordinary ion-exchange reactions with cationic materials in eluting solutions is held to a minimum. Ligands with a stronger complexing tendency are more strongly retained. It is an efficient way to separate ligands from nonligands. Elution development uses a ligand in the eluent that complexes with the metal less strongly than the ligands of the mixture. Separations by ligand-exchange chromatography are outlined in Table 2.39.

22 J.

P. Faris, Anal. Chem. 32:520 (1960). P. Faris and R. F. Buchman, Anal. Chem. 36:1157 (1964). 24 K. A. Kraus and G. E. Moore, J. Am. Chem. Soc. 75:1460 (1953). 25 F. Helfferich, Nature 189:1001 (1961). 23 J.

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PRELIMINARY SEPARATION METHODS

2.78

SECTION TWO

TABLE 2.35 Distribution Coefficients (Dv) of Metal Ions on AG 1-10X in HCl Solutions Slight adsorption observed for Cr(III), Sc, Ti(III), Tl(I), and V(IV) in 12M HCl (0.3 ≤ Dv ≤ 1). No adsorption observed for Al, Ba, Be, Ca, Cs, K, La, Li, Mg, Na, Ni(II), Po, Rb, Th, and Y in 0.1–12M HCl. Metal ions Ag As(III) As(V) Au(III) Bi(III) Cd Co(II) Cr(VI) Cu(II) Fe(III) Ga Hf Hg(II) In Mn(II) Mo(VI) Nb(V) Os(IV) Pb(II) Pd(II) Pt(IV) Re(VII) Rh(IV) Ru(IV) Sb(III) Sb(V) Se(IV) Sn(II) Sn(IV) Ta(V) Te(IV) Ti(IV) Tl(III) U(IV) U(VI) V(V) W(VI) Zn Zr

2M

4M

100 6 ⫻ 105 5 000 1 300

10 2 13 000 12 10 10 16 4 000 16 500 1 600 320 630 2 500 1 630 1 600 8

105 3

1 000

6M

10 3 0.3 6 2 2 2 × 105 79 000 630 100 630 100 63 (10M) 100 ppm), Fe(III), Ge, Ni (> 40 ppm), − Sn(II); AsO3− 4 , F (> 25 ppm), SiO2− 3 , W(VI), V(V). (b) As, Ge, Si do not interfere. As(V), Ba, Bi, Fe(III) (< 200 ppm), Pb, Sb, Sn, W(VI); nitrate.

Al, Cr(VI), Ni; borate, phosphate (> 0.005M); formaldehyde.

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(Continued)

S. T. Volkov, Zavod. Lab. 5:1429 (1939); V. S. Zemel, ibid. 5:1433 (1939).

R. E. Kitson and M. G. Mellon, Ind. Eng. Chem., Anal. Ed. 16:379 (1944); O. B. Michelson, Anal. Chem. 29:60 (1957). R. J. Jakubiec and D. F. Boltz, Mikrochim. Acta 1969:181.

P. Pakalns, Anal. Chim. Acta 40:1 (1968); H. M. Theakston and W. R. Bandi, Anal. Chem. 38:1764 (1966); C. H. Locke and D. F. Boltz, ibid. 30:183 (1958).

D. F. Boltz and M. G. Mellon, Ind. Eng. Chem., Anal. Ed. 19:873 (1947); Talanta 27:263 (1980).

(a) D. F. Boltz and M. G. Mellon, Anal. Chem. 29:749 (1948). (b) C. Wadelin and M. G. Mellon, ibid. 25:1668 (1953).

H. A. Mottola, B. E. Simpson, and G. Gorin, Anal. Chem. 42:410 (1970).

W. C. Wolfe, Anal. Chem. 34:1328 (1962); C. B. Allsopp, Analyst 66:371 (1941).

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.39

(2) N2H4: To boiling solution (pH 8–10) add 2 mL 85% N2H4 · H2O, heat 2–5 min, cool, dilute to 50 mL. (3) Iodine method: In 25.0 mL, 2N H+. (a) Add 2 mL 5% CdI2. (b) Add 2 mL 1% CdI2–0.25% starch. Dilute both to volume, stand 5 min. (4) 3,3⬘-Diaminobenzidine (R): 50 mL, pH 2–3 with formate buffer, 2 mL 0.5% R, stand 30–50 min, adjust to pH 6–7 with NH3, extract with 10.0 mL toluene. (5) 2,2⬘-Dianthrimide (R): 5 mL 0.05% R, 12 mL conc. H2SO4, heat 5 h at 90°C.

(1) Molybdosilicic acid: To 50 mL sample, 2 mL 10% (NH4)6Mo7O24, stand 10 min, 1.5 mL 10% tartrate. (2) Heteropoly blue: 90 mL sample, 1 mL 7.5% (NH4)6Mo7O24, stand 5 min, 4 mL 10% tartaric acid, 1 mL 0.0016% 1-amino-2-naphthol4-sulfonic acid– 10% Na2SO3–10% NaHSO3, pH 1.6, stand 20 min.

(1) Methylene blue: 2% Zn(OAc)2, 0.05% p-aminoN,N-dimethylaniline in 2.7M H2SO4, FeCl3 catalyst followed by (NH4)HPO4. (2) Add N,N-Dimethyl-p-phenylene-diamine plus Cr2O2− 7 in acid solution, which gives methylene blue.

Silicon

Sulfur (H2S)

Procedure

Selenium

Element determined

0.005–0.5

0.005–0.5

745

0.1–1.5

815

745

1–6

56–2500

605

350

1–10

0.2–1

2–18

Range,

mg ⋅ mL−1

420

(b) 615

(a) 352

260

l, nm

Interferences

Cu(II).

Ba, Bi, Pb, Sb cause turbidity.

Ge, phosphate.

Specific method for Se(IV).

Cu(II) and Fe(III) unless masked with oxalate and fluoride, respectively. Cr(VI), V(V). Substances that reduce or complex Se(IV).

TABLE 6.16 Spectrophotometric Methods for the Determination of Nonmetals (Continued )

6.40

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A. E. Sands et al., U.S. Bur. Mines Rept. 4547, Washington, D.C., 1949; R. Pomeroy, Sew. Works J. 13:498 (1941).

D. F. Boltz and M. G. Mellon, Ind. Eng. Chem., Anal. Ed. 19:873 (1947); L. A. Trudell and D. F. Boltz, Anal. Chem. 35:2122 (1963).

L. G. Hargis, Anal. Chem. 42:1494, 1497 (1970).

F. J. Langmyhr and I. Dahl, Anal. Chim. Acta 29:377 (1963).

K. L. Cheng, Anal. Chem. 28:1738 (1956); J. Hoste and J. Gillis, Anal. Chim. Acta 12:58 (1955).

J. L. Lambert, P. Arthur, and T. E. Moore, Anal. Chem. 23:1101 (1951).

R. W. Haisty, M. S. Thesis, University of Illinois, Urbana, 1955.

References

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

(1) Barium chloranilate (R): 10 mL 1% KH phthalate (pH 4), 45% EtOH, 100 mL total volume; 0.3 g R, centrifuge or filter. (2) Benzidine sulfate precipitated; benzidine portion, 0.2M HCl, 0.1% NaNO2, 0.5% H2NSO2ONH4, 0.1% N(1-naphthyl)ethylenediamine HCl.

(1) Sn(II); see selenium, method (1) (2) Iodotellurite: 2M KI, 0.15M– 0.4M HCl, stand 20 min. (3) HPH2O2: To boiling solution, 1–8 meq HCl, 5 mL 3M HPH2O2, digest 100°C for 15 min, 3 mL 4% gum arabic, dilute to 50 mL.

Sulfur (SO4)

Tellurium

2–14

0.4–2

335 240–290

2–20

0.3–3 as S

550

400

0.5–10 10–300

(75 mL)

510

320 or 520

0.2–1

560

* SPADNS = 4,5-Dihydroxy-3-(2-hydroxy-5-sulfophenylazo)-2,7-naphthalenedisulfonic acid.

(1) Pararosaniline–SO2–HCHO: SO2 absorbed in 0.1M K2HgCl4, 2 mL 0.04% pararosaniline, 2 mL 0.2% HCHO, 25 mL total volume, stand 25 min. (2) Fe(II)-1,10-phenanthroline: 10 mL 0.001M Fe(III), 10 mL 0.03M 1,10-phenanthroline, pH 5.5, 1 mL 1-octanol, 50˚C, transfer to 100 mL, 2 mL NH4HF2.

Sulfur (SO2)

Chloride and phosphate.

Remove cations by cation exchange resin in H form.

NO2 (removed by H2NSO2OH), O 3.

R. A. Johnson and J. P. Kwan, Anal. Chem. 23:651 (1951). R. A. Johnson and B. R. Anderson, Anal. Chem. 27:20 (1955); R. A. Johnson and J. P. Kwan, ibid. 25:1017 (1953).

B. Klein, Ind. Eng. Chem., Anal. Ed. 16:536 (1944); T. V. Letnoff and J. G. Reinhold, J. Biol. Chem. 114:147 (1936).

R. J. Bertolacine and J. E. Barney, Anal. Chem. 29:281 (1957); 30:202, 498 (1958).

B. G. Stephens and F. Lindstrom, Anal. Chem. 36:1308 (1964).

J. B. Pate et al., Anal. Chem. 34: 1660 (1962); E. Lahman and K. E. Prescher, Z. Anal. Chem. 251:300 (1970); R. V. Nauman, ibid. 32:1307 (1960).

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.41

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.42

SECTION SIX

FIGURE 6.4

Possible shapes of photometric titration curves.

H = Heterometric titration SPADNS = 2-(4-Sulfophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid Thoron = 2-(2-Hydroxy-3,6-disulfo-1-naphthylazo)benzenearsonic acid Attention is especially called to the fact that a blank space in the column headed “Interferences” means merely that no information is being given; it does not mean that the titration in question is free from interferences.

6.5 6.5.1

TURBIDIMETRY AND NEPHELOMETRY Principles Turbidimetry and nephelometry involve the formation of a suspension and the measurement of the amount of radiation that passes through the sample in the forward direction (turbidimetry) or the amount of radiation scattered (nephelometry). Nephelometric measurements are made at an angle to the direction of the beam of radiation through the sample, usually 45° or 90°. These methods are applicable to colorless, opaque suspensions that do not show selective absorption, and so white light is generally used. There is no sharp division between these methods and absorptiometric methods involving true solutions, for example, procedures involving the formation of color “lakes.” Many elements can be determined turbidimetrically or, less often, nephelometrically. But as a rigid control of conditions is essential, the use of these techniques in practical analysis is restricted to the determination of elements that do not give good color reactions or that give precipitates that are difficult to separate from their mother liquors.

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0.2–0.7 mg per 100 mL 0.5–10 mg per 100 mL 0.1–0.6 mg per 20 mL 0.02–0.4 mg per 6–100 mL 5–200 mg per 1–10 mL 1–10 mg per 90 mL 0.5–100 mg per 90 mL

Bi

F−

Cu(II)

Cr2O72−

Co

Ce(III)

Cd

Ca

0.2–0.5 mg per 20 mL 1.5–20 mg per 50 mL 0.6–6 mg per 25 mL 0.5–4 mg per 10–200 mL 10–60 mg per 90 mL 5–50 mg per 35 mL 5–70 mg per 25 mL

0.5–50 mg per 75–100 mL 0.1–5 mg per 35–100 mL

As(III)

Ba

0.1–10 mg per 80 mL

Scale

Ag

Substance titrated

Chloroacetate buffer, pH 3.0 pH 3.1

pH 2.6

KI, OAc– buffer, pH 4 Citrate buffer, pH 7

0.75M H2SO4

Na3Citrate + HOAc

10% K2P2O7, pH 5.5–7.0

NH3 buffer, pH 10

NaOH, pH 12.5–13.5

Chloroacetate buffer, pH 2 Chloroacetate buffer, pH 2 Diethylamine buffer, pH 12.3 NH3 buffer, pH 10

Aq. NH3 + excess K2Ni(CN)4, pH 11.3–11.8 0.5M H2SO4 + OsO4 catalyst NH3 buffer, pH 10

Solution composition Reagent

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 1 mM Th(NO3)4

0.05 mM Th(NO3)4

Alizarin Red S SPADNS

None

None

0.1M Triethylenetetramine SO2⫺ 4 0.1M EDTA

None

(0.01M in EtOH) 0.05M HAsO2

None

None

a-Nitroso-b-naphthol

Na2S2O3 (0.025M)

None

None

KMnO4 (2–40 mM)

EDTA (1–10 mM)

Eriochrome Black T Murexide

EDTA (1–2.5 mM) + Mg–EDTA complex EDTA (0.5–10 mM)

580

520

745

575 F

390 F

350

H

525

222–228

600–620

630–650

600 F

400

Thiourea Calcon

265

630–650

320

435 F

l, nm

None

Eriochrome Black T

None

Murexide

Indicator

EDTA (0.0004M)

Ce(SO4)2 (0.0004M– 0.1M) EDTA (0.002M– 0.1M) + Mg– EDTA complex EDTA (0.001M– 0.01M) EDTA (0.01M)

EDTA (0.6–5 mM)

TABLE 6.17 Photometric Titrations of Inorganic Substances

Th-Alizarin Red S lake Th–indicator lake

Cu(II)–EDTA complex

Cu(II)–trien complex

I3−

(Continued)

Bi(III), Cl−, phosphate

Bi, Co, Fe(III), Ni, Pb

Other reducing agents

As(III), Cr(III), F−, Hg(I), I2, Sb(III), Tl(I), V(IV) Cu(II), Fe(III)

MnO⫺ 4 Co-a-nitroso-bnaphthol complex Cr2O72−

Most other metal ions

Fe(III)

Fe(III), Th

Most other metals except alkali metals

Ba, Ca, Cu(II), Hg(II), Li, Mg, Pb, Sr, Tl(I), Zn Other reducing agents

Interferences

Cd–EDTA complex

FI

FI

FI

Bi–thiourea complex

Bi–EDTA complex

FI

Ce(SO4)2 excess

Ni–murexide complex

Absorbing species

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.43

6.44

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H6TeO6

Sr

Sc

Sb(III)

SO42−

Rare earths

Pb

H2O

Ni

Mo(V)

4–11 mg per 75 mL 0.05–0.9 mg per 100 mL 10–500 mg per 50 mL

0.08–2 mg per 90 mL 1 mg–2 mg per 100 mL 1–11 mg per 90 mL 10–60 mg per 90 mL 0.5–40 mg per 3 mL 0.2–2 mg per 100 mL 0.01–40 mg per 100 mL 0.17–170 mg/ 200 mL 6–50 mg per 35 mL 3–30 mg per 80 mL

Mg

K

10–60 mg per 90 mL 0.2–1 mg per 100 mL 4–28 mg per 0.4 mL

Scale

Fe(III)

Substance titrated

0.01M ZnCl2

OAc– buffer, pH 4.2, excess EDTA OAc– buffer, pH 4.0

Aqueous solution

NH3 buffer, pH 10

pH 3.0

OAc– buffer, pH 4.6; excess EDTA H2O–MeOH– isopentanol 1M–1.5M H2SO4

HOAc + H2SO4 catalyst Chloroacetate buffer, pH 2 Pyridine buffer, pH 6

EDTA (1–10 mM)

NH3 buffer, pH 10

2.5 mM EDTA + Mg–EDTA complex 0.02M–2M NH3

Neutral 2–20 mM KBrO3 + excess KBr 0.03M EDTA

1.25 mM Ba(ClO4)2

2.5 mM La(NO3)3

EDTA (0.01–50 mM)

EDTA (1–10 mM)

Acetic anhydride

0.1M EDTA

None

EDTA (1–10 mM)

Eriochrome Black T None

Cu(NO3)2

None

Alizarin Red S Thoron

Arsenazo

None

None

None

Eriochrome Black T Aliz C

None

None

Hexadecyltrimethylammonium bromide (2 mM)

0.01M EDDHA

Salicylic acid

Indicator

Chloroacetate buffer, pH 3.5 At pH 4 add excess NaB(C6H5)4, separate precipitate and back-titrate NH3 buffer, pH 10

0.1M EDTA

Reagent

pH 2

Solution composition

TABLE 6.17 Photometric Titrations of Inorganic Substances (Continued )

250–280

630

745

296–330

520

520 F

570

240

250–260

1000

520 F

630–660

222–228

570

470

525

l, nm

H5TeO⫺6

Cu(II)–EDTA complex FI

Br3−

Ba–thoron complex

Rare-earth–indicator complex FI

Pb–EDTA complex

Acetic anhydride

Ni–EDTA complex

Zn–indicator complex

FI

Mg–EDTA complex

Fe(III)–salicylate complex Fe(III)–reagent complex Hexadecyltrimethyl ammonium tetraborate

Absorbing species

Interferences

Bi, F–, Hf, SO42− , Zr

Other substances that react with Br2

Co, Cu(II), Fe(III), Ni, Th

Most other metal ions

Ag, Al, Cu(II), Th

Bi, Co, Cu(II), Ni, Pb ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

Zr

10–100 mg per 300 mL

C6H6 saturated with NH4OAc plus a little MeOH Excess EDTA, pH 2.3

30–50 mg per 50 mL

Zn

0.3M HCl containing 1% H2O2 NH3 buffer, pH 9.5

0.5 mg per 30 mL 0.1–5 mg per 100 mL

0.02M FeCl3

Dithizone (2 mM in benzene)

6 mM pyridine-2,6dicarboxylic acid 2.5 mM EDTA

0.2 mM EDTA

Aqueous EtOH, pH 3.0

V(V)

2.5 mM EDTA

pH 2.8

0.09–180 mg per 200 mL 2–60 mg per 35 mL

Th

K benzohydroxamate

520 F

640

665

Eriochrome Black T None

436 F

422

520 F

None

Alizarin Red S Quercetin

Fe(III)–indicator complex

FI

V(V)–H2O2–reagent complex FI

Th–quercetin complex

FI

Al, Bi, Ce(IV), Sn(II,IV), Th, Ti(IV)

Other metals titrated with EDTA at pH 9.5 unless masked Co, FE(III), Ni, Pb

F–, Fe(II,III), Mn, Mo(VI), PO43−, SO42−, Ti, OAc– Nb(V), Sn(IV), Ta

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.45

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20–60 mg per 100 mL 15–150 mg per 80 mL

25 mg per 40 mL

20 mg per 100 mL 420 mg per 150 mL 1.5 mg per 150 mL 130 mg per 100 mL 80 mg per 100 mL

4–Bromophenol

Phenol A

4-Phenylphenol

NaOAc

Quinoline

Propoxycaine HCl

Procaine HCl

Oleic acid

Aniline

10–200 mg per 100 mL 10 mg per 80 mL

Scale

Al oxinate

Substance titrated 0.1M HClO4 in HOAc

Reagent

Glacial HOAc

Glacial HOAc

6M HCl

0.1M HCl in HOAc

0.1M HClO4 in HOAc

0.1M NaNO2

MeOH–glacial Neutral 0.04M KBrO3 HOAc–12M HCl– containing excess 40% aqueous KBr KBr > 90% acetone Tributylmethylammonium OH (0.1M in 80% C6H6–20% 2-PrOH) Butylamine 0.05M NaOH in absolute EtOH 6M HCl 0.1M NaNO2

MeOH–H2O–12M Neutral 0.04M KBrO3 HCl–40% aqueous containing excess KBr KBr Aqueous solution 0.1M NaOH

Glacial HOAc

Solution composition

TABLE 6.18 Photometric Titrations of Organic Substances

2-Chloroaniline

None

312

350

385

385

None None

372

None

660 F

360

None

Azo violet

325

350

450

l, nm

None

None

None

Indicator

FI

Propoxycaine diazonium Cl– Quinolinium ion

HNO2

4-C6H5C6H4O–

Basic form of azo violet

Other basic compounds

Other basic compounds

Certain other phenols

Other acidic compounds Other compounds that react with Br2 Other acidic compounds

4-BrC6H4O– Br⫺ 3

Other compounds that react with Br2

Interferences

Br−3

Al oxinate

Absorbing species

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.46

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.5.2

6.47

Sensitivity and Accuracy Nephelometric methods are inherently more sensitive than turbidimetric (or colorimetric) ones, but are seldom used because the experimental conditions are more critical. The sensitivity is sometimes increased by the use of blue, instead of the customary white, light. Because of the difference in the angle of measurement, turbidimetry is best suited for determining relatively high concentrations of suspended particles, whereas nephelometry is most suited for determining very low concentrations. The concentrations of solutions analyzed turbidimetrically usually lie between 0.05 and 0.5 mg per 100 mL. Under favorable conditions the range can be extended down to 0.02 mg per 100 mL and occasionally up to 2.0 mg per 100 mL. If a given suspension does not scatter strongly, that is, if the transmittance is greater than about 95% to 98%, turbidimetry should not be used. In this instance nephelometry would be much more sensitive since the small amount of scattered light would be measured against a black background. For both techniques the accuracy is low because of the many variables involved and is usually about ±5%.

6.5.3

Calibration Curves Calibration curves are usually empirical, since they depend on particle size as well as on concentration, and they should therefore be checked by absolute methods. In turbidimetry Beer’s law is sometimes followed over only a limited range, but the addition of a protective colloid, such as gum arabic or gelatin, often stabilizes the suspension and extends the range. The suspension that results from accurately weighing and dissolving 5 g of hydrazinium(2+) sulfate and 50 g of hexamethylenetetramine in 1 L of distilled water is defined as 4000 nephelometric turbidity units (NTU). After standing 48 h the insoluble polymer formazin, formed by the condensation reaction, develops a white turbidity. This turbidity can be prepared repeatedly with an accuracy of ±1%. The mixture can be diluted to prepare standards of any desired value.

6.5.4

Conditions The suspensions should have these characteristics: 1. 2. 3. 4. 5.

Low solubility. High rate of formation. Fair stability. High opacity. A refractive-index difference between the particle and its surrounding medium is needed if either reflection or scattering is to occur. It is sometimes advantageous to change solvents in order to increase the refractive-index differences.

Strict adherence to established conditions is essential if reproducible suspensions are to be obtained. The following factors affecting the primary and secondary particle size must be clearly defined: 1. 2. 3. 4. 5. 6. 7. 8.

Range of concentration of the substance being determined. Concentration of reagent. Rate and manner of addition of reagent. pH of solution. Temperature. Rate and extent of agitation. Addition of a protective colloid. Effect of other salts in solution.

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.48

SECTION SIX

9. Time interval before measurement. 10. Wavelength of incident light (in turbidimetry). In turbidimetry it is important to choose a wavelength at which the sample solution does not absorb strongly.

6.5.5

Applications Turbidimetry and nephelometry are used on gaseous, liquid, or even transparent solid samples. Whenever precipitates form that are difficult to filter, either due to small particle size or a gelatinous nature, they usually make ideal suspensions to be measured by light-scattering techniques, replacing gravimetric operations. A particularly valuable area of application is in air- and water-pollution studies, in which the two techniques are used to determine the clarity and to control the treatment of potable water, water-plant effluents, and other types of environmental waters. Similarly, light-scattering measurements are used to ascertain the concentration of smog, fog, smoke, and aerosols. Table 6.19 gives information on a number of the most useful procedures employing turbidimetric and nephelometric procedures. Other applications include turbidimetric titrations, the measurements of the haziness of water, studies of the efficiency of filtration processes, and the examination of smokes and gases.

6.6

FLUORESCENCE ANALYSIS The methods of fluorescence analysis outlined in this section are confined to the spectral region from 200 to 800 nm. Fluorescence methods are often more specific and more sensitive than colorimetric methods. Two types of spectra are determined in the development of a fluorometric procedure: the excitation spectrum and the emission spectrum. The excitation spectrum coincides with the absorption spectrum, and the excitation band of longest wavelength intersects the emission band. The excitation spectrum is found by measuring the intensity of the emission on exciting the substance over a wide range of radiant energy. The emission spectrum is obtained by measuring the intensity of the emission over a range of wavelengths while the substance is being irradiated with a monochromatic source. For organic compounds and metal chelates, both these measurements usually result in band spectra, and only the peak is reported in the tables cited in this section. The maximum excitation point is not necessarily chosen for analysis, since it may be too close to the emission for separation with filters in filter fluorimeters. An excitation should be chosen for which the compound under irradiation suffers the least decomposition. Much of the literature reports only the uncorrected excitation and emission maxima as given by a particular instrument. Data of this type are only approximate and require correction.6 Most of the organic compounds and metal chelates that fluoresce above 400 nm can be excited with 350- to 360-nm radiation.

6.6.1

Photoluminescence Related to Concentration Whether fluorescence (or phosphorescence), the luminescent power PL is proportional to the number of molecules in excited states. In turn, this is proportional to the radiant power absorbed by the sample. Thus PL = Φ L ( P0 − P) where ΦL = luminescence efficiency or quantum yield of luminescence P0 = radiant power incident on the sample P = radiant power emerging from the sample 6

C. E. White, M. Ho, and E. Q. Weimer, Anal. Chem. 32:438 (1960).

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(6.16)

T or N T T

T or N

T T T or N T T T or N

T or N T T

T T or N T or N

Ag As Au

Ba

Ca

Cl– K Li Na

SO2− 4 Se Sn

Te Zn

Suspension

Te K2Zn3[Fe(CN)6]2 ZnS

BaSO4 Se Arsonic compounds

CaC2O4 Ca oleate AgCl K2Na[Co(NO2)6] Li stearate NaMII(UO2)3(OAc)9

BaSO4

AgCl As Au

Reagent

BaCl2 NaPH2O2 or SnCl2 Phenylarsonic or 4-hydroxy3-nitrophenylarsonic acids NaPH2O2 K4[Fe(CN)6] H2S or Na2S

H2SO4 or Na2SO4 plus a protective colloid H2C2O4 Na oleate AgNO3 Na3[Co(NO2)6] Stearic acid Mg or Zn uranyl acetate

NaCl KPH2O2 SnCl2

Solution composition

Dilute HNO3 or H2SO4 HOAc 1-pentanol Water containing excess reagent HCl (1:200) HCl (1:1) HCl + HNO3 dilute or H2SO4 dilute HCl (1:1) 0.4M HCl Neutral solution

Alkaline

HOAc

HCl (1:200)

Dilute HNO3 HCl (1:1) HCl

References 1 Snell and Snell, Colorimetric Methods of Analysis, 3d ed., Van Nostrand, Princeton, NJ, Vol. I, 1949 and Vol. II, 1959. 2 E. B. Sandell, Colorimetric Determination of Traces of Metals, 3d ed., Interscience, New York, 1959. 3 D. F. Boltz, ed., Colorimetric Determination of Nonmetals, Interscience, New York, 1958. 4 ASTM, ASTM Methods for Chemical Analysis, 1956. 5 Steele and England, Analyst 82:595 (1957). 6 Challis and Jones, Analyst 81:703 (1956). 7 Challis, Analyst 67:186 (1942). 8 Crossley, Analyst 69:209 (1944). 9 Challis and Jones, Anal. Chim. Acta 21:58 (1959).

Technique

Element

TABLE 6.19 Turbidimetric (T) and Nephelometric (N) Procedures for the Determination of the Elements

1,4 1,4,7 9 1,4,7,8 1,2 1,2,4

Pb Te Fe Se Cu, Fe Heavy metals

Li

Br–, I– SO2− 4

1,3,6 1,2 1,2 2

1,2

Mg, Na, SO2− 4

Mg

1,2

2 5 1,2

References

Se, Te Ag, Hg, Pd, Pt, Ru, Se, Te Pb

Interferences

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.49

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.50

SECTION SIX

Applying Beer’s law to Eq. (6.16), one obtains PL = Φ L P0 (1 − e −⑀bC )

(6.17)

When expanded in a power series, this equation yields  ⑀ bC (⑀ bC ) 2  PL = Φ L P0 ⑀ bC 1 − + − L   2! 3!

(6.18)

If ⑀bC is 0.05 or less, only the first term in the series is significant and Eq. (6.18) can be written as PL = Φ L P0 ⑀ bC

(6.19)

Thus, when the concentrations are very dilute and not over 2% of the incident radiation is absorbed, there is a linear relationship between luminescent power and concentration. Of particular interest in Eq. (6.19) is the linear dependence of luminescence on the excitation power. This means sensitivity can be increased by working with high excitation powers. The b term is not the path length of the cell, but the solid volume of the beam defined by the excitation and emission slit widths together with the beam geometry. Therefore, silt widths are the critical factor and not the cell dimensions. 6.6.1.1

Problems with Photoluminescence

6.6.1.1.1 Self-Quenching. Self-quenching results when luminescing molecules collide and lose their excitation energy by radiationless transfer. Serious offenders are impurities, dissolved oxygen, and heavy atoms or paramagnetic species (aromatic substances are prime offenders). Always use “Spec-pure” solvents. 6.6.1.1.2 Absorption of Radiant Energy. Absorption either of the exciting or of the luminescent radiation reduces the luminescent signal. Remedies involve (a) diluting the sample, (b) viewing the luminescence near the front surface of the cell, and (c) using the method of standard additions for evaluating samples. 6.6.1.1.3 Self-Absorption. Attenuation of the exciting radiation as it passes through the cell can be caused by too concentrated an analyte. The remedy is to dilute the sample and note whether the luminescence increases or decreases. If the luminescence increases upon sample dilution, one is working on the high-concentration side of the luminescence maximum. This region should be avoided. 6.6.1.1.4 Excimer Formation. Formation of a complex between the excited-state molecule and another molecule in the ground state, called an excimer, causes a problem when it dissociates with the emission of luminescent radiation at longer wavelengths than the normal luminescence. Dilution helps lessen this effect. 6.6.2

Structural Factors Affecting Photoluminescence Factors that affect photoluminescence are as follows: 1. Fluorescence is expected in molecules that are aromatic or contain multiple-conjugated double bonds with a high degree of resonance stability. 2. Fluorescence is also expected in polycyclic aromatic systems. 3. Substituents, such as −−NH2, −−OH, −−F, −−OCH3, −−NHCH3, and −−N(CH3)2 groups, often enhance fluorescence.

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.51

4. On the other hand, these groups decrease or quench fluorescence completely: −−Cl, −−Br, −−I, −−NHCOCH3, −−NO2, and −−COOH. 5. Molecular rigidity enhances fluorescence. Substances fluoresce more brightly in a glassy state or viscous solution. Formation of chelates with metal ions also promotes fluorescence. However, the introduction of paramagnetic metal ions gives rise to phosphorescence but not fluorescence in metal complexes. 6. Changes in the system pH, if it affects the charge status of chromophore, may influence fluorescence.

6.6.3

Instrumentation for Fluorescence Measurement The primary filter or excitation monochromator selects specific wavelengths of radiation from the source and directs them through the sample. The resultant luminescence, usually observed at 90° to the excitation radiation, is isolated by the secondary filter or fluorescence emission monochromator and directed to the photodetector. Front surface or small-angle viewing (37°) is used for highabsorbance samples or solids. 6.6.3.1 Radiation Sources. It is desirable to use a source as powerful as possible. High-pressure xenon arc lamps are used in nearly all commercial spectrofluorometers. The xenon lamp emits an intense and relatively stable continuum of radiation that extends from 300 to 1300 nm. Low-pressure mercury vapor lamps are most frequently used in filter fluorometers. With a clear bulb of ultraviolet-transmitting material, individual mercury emission lines occur at 253.7, 296.5, 302.2, 312.6, 313.2 (doublet), 365.5 (triplet), 366.3, 404.7, 435.8, 546.1, 577.0, and 579.1 nm. Interference filters are used to select the desired mercury line. When the lamp bulb is coated with a phosphor, a more nearly continuous spectrum is emitted. 6.6.3.2 Fluorescence Measurements. Fluorescent measurements are usually made by reference to some arbitrarily chosen standard. The standard is placed in the instrument and the circuit is balanced with the reading scale at some chosen setting. Without readjusting any circuit components, the standard is replaced by known concentrations of the analyte and the fluorescence of each is recorded. Finally, the fluorescence of the solvent and cuvette alone is measured to establish the true zeroconcentration readings, if the instrument is not equipped with a zero-adjust circuit. Fluorescence quantum yield values and secondary standards are given in Table 6.20. With separate emission and excitation monochromators, the emission and excitation spectra can be ascertained. If no knowledge of the spectra is available, place a solution of the analyte into the cuvette. Select an emission wavelength that produces a fluorescent signal either visible to the eye or from the detector signal. While scanning through the spectrum with the excitation monochromator, plot the strength of the fluorescence signal. This gives an uncorrected excitation spectrum that should resemble the normal absorption spectrum obtained in the ultraviolet-visible region. Next, select a suitably strong excitation wavelength and scan the fluorescence spectrum with the emission monochromator.

6.6.4

Comparison of Luminescence and Ultraviolet-Visible Absorption Methods If applicable, luminescence is usually the method of choice for quantitative analytical purposes, especially trace analysis. The significant advantages of luminescence over ultraviolet-visible absorption methods are enumerated below: 1. Fewer luminescing species exist than absorbing species in the ultraviolet-visible region. 2. Luminescence is more selective. A pair of wavelengths, excitation and emission, characterize the process instead of one.

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.52

SECTION SIX

TABLE 6.20 Fluorescence Quantum Yield Values Compound

Solvent

QF value vs. QF standard

QF standard 9-Aminoacridine Anthracene POPOP* Quinine sulfate dihydrate

Water Ethanol Toluene 1N H2SO4

0.99 0.30 0.85 0.55

Secondary standards Acridine orange hydrochloride

Ethanol

0.54 Quinine sulfate 0.58 Anthracene

1,8-ANS† (free acid)

Ethanol

0.38 Anthracene 0.39 POPOP

1,8-ANS (magnesium salt)

Ethanol

0.29 Anthracene 0.31 POPOP

Fluorescein

0.1N NaOH

0.91 Quinine sulfate 0.94 POPOP

Fluorescein, ethyl ester

0.1N NaOH

0.99 Quinine sulfate 0.99 POPOP

Rhodamine B

Ethanol

0.69 Quinine sulfate 0.70 Anthracene

2,6-TNS‡ (potassium salt)

Ethanol

0.48 Anthracene 0.51 POPOP

* POPOP denotes p-bis[2-(5-phenyloxazoyl)]benzene. † ANS denotes anilino-8-naphthalene sulfonic acid. ‡ TNS denotes 2-p-toluidinylnaphthalene-6-sulfonate. Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

3. Luminescence is more sensitive. This is because (a) a luminescence signal is measured directly and against a very small background and (b) the signal is proportional to the intensity of the incident radiation. Greater sensitivity for weakly emitting compounds can be obtained by using more intense sources. Luminescence analyses can determine sub-part-per-billion concentrations of many substances. By contrast, in absorption spectrophotometry concentration is proportional to absorbance, which is the logarithm of the ratio between incident and transmitted radiant power. This corresponds to the measurement of a small difference between two large signals. 4. Luminescence lifetimes offer another factor for discrimination among compounds.

6.6.5

Applications Fluorometric analysis is of greatest use in the determination of concentrations that are too small to be easily determined by spectrophotometry, colorimetry, or emission spectrography; determinations on the order of 0.1 mg in 10 mL are common. In Tables 6.21 and 6.22, the excitation and emission data are given as taken from the literature and are often incomplete and only approximate. The sensitivity of a given procedure is difficult to specify precisely, because it depends on such factors as the intensity of the exciting radiation, the sensitivity of the detector, and so on. The sensitivity values given in the following tables are interpreted from statements in the original articles and are not to be taken as absolute.

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470, 580

410 370

pH 4.6

CHCl3 extraction; pH 5.7 5.6 pH 12.8 in EtOH

Pontachrome Blue Black R

8-Hydroxyquinoline

Chloramine T + nicotinamide (Quenching) Al− Alizarin Garnet R 8-Hydroxyquinoline

CN−

Benzoin

Flavonol 8-Hydroxyquinoline

8-Hydroxyquinoline

Ge

Hf In

Li

Ga

Rhodamine B

Calcein Ce(III) fluorescence

Ca Ce

F−

1-Amino-4-hydroxyanthraquinone Morin

Morin

Salicylidene-oaminophenol Benzoin

430

pH 3.3

Morin

0.1M H2SO4 CHCl3 extraction, pH 5.1 Slightly alkaline EtOH

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365–400 365

365

580

Orangeyellow Yellowgreen 460 535

470–610

436

CHCl3 extraction, pH 2.6 C6H6 extraction, 6M HCl Alkaline EtOH 365

590

470

Blue

485 355

570

630

460–540

480

520

520

630

500

590

Emission, nm

pH 4.6

365

360 260

470

0.05M NaOH 0.4M KOH 0.6M–2.9M HClO4 1M KOH

530, 570

Dilute HCl–H2C2O4 0.02M NaOH 365

365

470

Excitation, nm

pH 4.6

Conditions

Alizarin Garnet R

Reagent

Be

B

Al

Substance determined

Cr(III), Sc, Th; F−, citrate, C2O42−, tartrate Be, Sb

0.027

0.2

0.1 0.04

2

0.01 mg

0.05 mg

0.001

0.3

Mg

As(V), B, Be, Cr(VI), NO2−, silicate Al, F−, Fe, PO3− 4 , Zr Al, Be, Cu, Fe, Zr

Be, Co, Cr, Cu, Fe, Ni, PO3− 4 , Th, Zr Cu, Fe(III), Mo(VI), V(V); colored extractants Au, Fe, NO3−, Sb, Tl, W

Ca, Cr(VI), Li, rare earths, Zn Ba, Sr NO3−

0.01 0.02 0.1

Li, Cr(VI)

0.2

1

0.04

Ga

Co, Cr, Cu, Fe, Ga, Ni, V

Be, Co, Cr, Cu, Fe, NO3−, Ni, PO3− 4 , Th, Zr Fe, Th, U

F−,

Interferences

0.1

0.001

0.001

0.007

Sensitivity, mg · mL–1

TABLE 6.21 Fluorometric Methods for the Determination of Inorganic Substances References

(Continued)

Anal. Chem. 23:478 (1951)

Anal. Chem. 23:1149 (1951) Z. Anal. Chem. 138:337 (1953)

Anal. Chim. Acta 13:159 (1955); 24:413 (1961) Nature 175:167 (1955)

Anal. Chem. 27:961 (1955)

Anal. Chem. 25:960 (1953)

Anal. Chem. 29:879 (1957)

Ind. Eng. Chem., Anal. Ed. 18:179 (1946); 13:809 (1941) Anal. Chem. 31:598 (1959); 24:1467 (1952) Anal. Chem. 35:1238 (1963) Anal. Chem. Acta 41:404 (1968)

Analyst 86:62 (1961); 82:606 (1957); Anal. Chem. 19:802 (1947) J. Chem. Soc. 79:231 (1958)

Talanta 13:609 (1966)

Anal. Chem. 33:1360 (1961); Ind. Eng. Chem., Anal. Ed. 12:229 (1940) Anal. Chem. 18:530 (1946); Ind. Eng. Chem., Anal. Ed. 18:530 (1946) Anal. Chem. 27:961 (1955)

Anal. Chem. 25:960 (1953)

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.53

Tl(III)

Tl(I)

Tb Th

Sn(IV), organo

Sn(II)

Se

Sc

S2−

Ru

Mg

Li

Substance determined

Rhodamine B

1-Amino-4-hydroxyanthraquinone

Morin

2,3-Diaminonaphthalene extract 7-Amino-3-nitronaphthalenesulfonic acid Morin

Morin

5-Methyl-1,10-phenanthroline Pd complex of 5-sulfo-8-hydroxyquinoline Salicylaldehyde semicarbazone

2M KOH, 50% 1,4-dioxane Aqueous

Dibenzothiazolymethane 8-Hydroxyquinoline– sulfonic acid Bis(salicylideneethylene)diamine

2M HCl, C6H6 extraction

3.3M HCl, 0.8M KCl

pH 2.3

360

250

550, 580

230 420

415–420

Hexane

0.5M HCl 0.01M HCl in 50% EtOH

365

365

370

365

465

355

365

365

Excitation, nm

pH 10.6

pH 7; EtOAc extraction pH 2.0

Isobutylamine in dimethylformamide Reduce to Ru(III), pH 6 pH 9.2; add MgCl2 and glycine pH 6.0

Conditions

Reagent

580

430

545 520

495–500

Blue

Greenish

455

416–436

577

440

415

Emission, nm

0.1

0.01

0.001 (dialkyl) 0.1 (trialkyl) 3 0.02

0.1

0.1

0.05

0.2

1

0.0002

0.1

Sensitivity, mg · mL–1 Interferences

Only Au, Bi, Pt(IV), Sb(V) after Et2O extraction Au, Fe, Ga, Hg, Sb

Fe, nitrate, U Al, Ca, Fe, La, Zr

Isolated by hexane or ethyl acetate extraction

Fe, Ti, U, V, dithionite

Cupferron and tributyl phosphate extractions required Al, Be, F–, Fe, Ga, In, phosphate

Ag, Co, Cr(VI), Fe, Mn(VII), Pd Cyanide

Ca unless masked with EGTA Be, In, Zn

Insoluble hydroxides; Zn

TABLE 6.21 Fluorometric Methods for the Determination of Inorganic Substances (Continued)

Anal. Chim. Acta 9:393 (1953)

Anal. Chem. 26:1134 (1954) J. Am. Chem. Soc. 79:5425 (1957); Anal. Chem. 29:1426 (1957) Ind. Eng. Chem., Anal. Ed. 13:809 (1941) Talanta 12:517 (1965)

Anal. Chem. 55:1901 (1983)

Anal. Chim.Acta 15:246 (1956)

Analyst 87:558 (1962)

Zh. Anal. Khim. 22:1812 (1967)

Analyst 91:23 (1966); Jpn. Anal. 24:321 (1975)

Anal. Chem. 32:1426 (1960); Zh. Anal. Khim. 39:1658 (1984) Anal. Chem. 30:93 (1958)

Anal. Chem. 31:2083 (1959)

Clin. Chim. Acta 136:137 (1984)

Anal. Chim. Acta 37:460 (1967)

References

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.54

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Flavonol Morin

Zr 400 425

420

8-Hydroxyquinoline

Zn

Y

360 365

10M H2SO4 0.1M NaCl, pH 2.0 pH 9.5, CHCl3 extraction Acetate buffer, gum arabic 0.1M H2SO4 2M HCl, 80% EtOH

Resorcinol Rhodamine B (quenching) 8-Hydroxyquinoline

V(V) W(VI)

254 365

Conc. H3PO4 or H2SO4

Na–K–Li–F fused button

U(VI)

0.1 0.02

1

Al, Ga, Ge, Hf, Sb, Sc, Sn, Th, U; EDTA removes Zr fluorescence but not the interferences

Al, Fe, Mg, others

2.5 1

Red 570–640

Greenyellow 465 515

Ag, Au, Ca, Ce, Co, Cr, Mn, Ni, Pb, Pt, rare earths, Th Ce(IV) As, Au, Cr, F−, Fe, Mo, PO3− 4 , Tl, V Ce, La

0.001 mg

0.02

Many

0.1

Yellowgreen 560

Z. Anal. Chem. 161:406 (1958) J. Chem. Soc. Jpn. 77:1259 (1956) J. Chem. Soc. Jpn. 77:1474 (1956) Ind. Eng. Chem., Anal. Ed. 16:758 (1944) Anal. Chem. 23:1149 (1951) Anal. Chim. Acta 16:346 (1957); Zh. Anal. Khim. 28:1331 (1973)

Anal. Chem. 25:322 (1953); 28:1651 (1956)

Anal. Chem. 19:646 (1947) ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.55

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Atabrine Azaindoles Benz[c]acridine Benz[a]anthracene 1,2-Benzanthracene Benzanthrone Benzil Benzo[b]chrysene 11-H-Benzo[a]fluorene

Ascorbic acid

6.56

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Na 1,2-Naphthoquinone4-sulfonate Caffeine Na benzoate

Ninhydrin

F3CCOOH EtOH solution Pentane Pentane

0.05M H2SO4 Water, pH 10 F3CCOOH Pentane

pH 7.5

Water, pH 14 Water, pH 7–9 Pentane Water, pH 7 0.3M KOH Petroleum ether

Amobarbital Anilines Anthracene Anthranilic acid Arginine Aromatic polycyclic hydrocarbons

Water, pH 11 15 min at 100°C

pH 3.9

pH 7

365 290, 299 295, 380 284 280, 340 370, 420 350 283 317

365

265 280, 291 420 300 305, 390 Various

280, 370 330, 342 300

295 365

436

365

358 280 285 285 285 365

F3CCOOH or MeOH Water, pH 1 Water, pH 1 Water, pH 1 Water, pH 1

lex, nm

291 330 365

Conditions Pentane 12 h in dark; then pH 10 1 h at 100°C; then pH 6.6

Water, pH 7 F3CCOOH Water, pH 11

Benzoic acid

K3[Fe(CN)6], pH 6; then alkaline ascorbate 1-Anilinonaphthalene9-sulfonic acid D-1-Ribitylamino-2-amino-4,5dimethylbenzene in HOAc

Resorcinol + HCl o-Aminobenzaldehyde in 0.2M NaOH

Reagent

Aminopterin 1-Aminopyrene 4-Aminosalicylic acid

4-Aminobenzoic acid 2-Aminophenol

Alloxan

Albumin

Acridine Adenine Adenosine Adenosine triphosphate Adenylic acid Adrenalin

Acenaphthene Acetoacetic acid Acetol

Compound

TABLE 6.22 Fluorescence Spectroscopy of Some Organic Compounds

540 310, 347 480 382 390, 410 550 600 398 340

Blue-white

410 344, 361 430 405 495 Various

460 415 405

345 Greenish yellow

520

475 375 395 395 295 Yellow-green

341 440 440

lem, nm

Anal. Chem. 23:540 (1951)

J. Biol. Chem. 154:597 (1944)

Arch. Biochem. Biophys. 68:1 (1957) Nature 183:1053 (1959) Anal. Chem. 32:810, 1436 (1960); J. Chem. Soc. 1946:1017; 1949: 1683; 65:1540 (1943) Z. Anal. Chem. 139:263 (1953)

J. Pharm. Exptl. Therap. 120:20 (1957) Ibid.

Arch. Biochem. Biophys. 68:1 (1957) Ind. Eng. Chem., Anal. Ed. 12:403 (1940)

Anal. Chem. 22:822 (1950)

Biochem. J. 56:xxxi (1954)

Ann. Chim. (Paris) 5:642 (1950) Arch. Biochem. Biophys. 68:1 (1957) Ibid. Ibid. Ibid. Anal. Chem. 30:1063 (1958)

Z. Physiol. Chem. 286:145 (1951) Anal. Chem. 22:902 (1950)

References

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Ninhydrin

Salicyloyl hydrazide

285 280 390

pH 1 pH 1 Ethanol solution, heat 1 h; then pH 10 0.3M KOH

305, 390

285

280 308 370 370, 335 390, 410 282 280

406

405

405 280 305, 390 365

365

0.005M H2SO4

Pentane pH 7

5,12-Dihydronaphthacene 3,4-Dihydroxyphenylacetic acid 3,4-Dihydroxyphenylalanine 3,4-Dihydroxyphenylethylamine 3,4-Dihydroxyphenylserine 4-Dimethylaminobenzaldehyde Dimethylguanidine

30 min at 60°C

30 min at 60°C

10% sulfuric acid Ethanol solution 0.3M KOH pH 7.5

Ammoniacal solution

Pentane Pentane Pentane

Ninhydrin Na 1,2-naphthoquinone4-sulfonate 3,5-Diaminobenzoic acid in 0.6M HClO4 3,5-Diaminobenzoic acid in 0.6M HClO4

Thionyl chloride, then NH3, then 76% H2SO4

Pentane Water, pH 1

305 291 355 250, 300 320

315

Water, pH 1 Water, pH 7 N,N-Dimethylformamide

285 365 329 280 363 362

70% H2SO4 Benzene solution Pentane F3CCOOH Pentane EtOH solution

Dibenzo[a,e]anthracene Dibenzo[b,k]chrysene Dibenzo[a,e]pyrene 3,4,8,9-Dibenzopyrene

Deoxyribose

Deoxyribonucleic acid

Coproporphyrin Coumarin Creatine Cysteine

Citric acid

Benzoic acid 3,4-Benzopyrene Benzo[e]pyrene Benzoquinoline Benzoxanthane 3-Benzyl-4-methyl7-hydroxycoumarin Bromolysergic acid diethylamide Brucine Carbazole Chlortetracycline Chrysene Cinchonine

495

320 470

325

325

340 330

381 428 401 480, 510

520

520

650 352 495 Blue-white

440

500 359 445 260, 380 420

460

385 390, 480 389 425 418 448

Nature 183:1053 (1959) (Continued)

Ibid. Ibid.; Anal. Chem. 31:296 (1959)

Ibid.

Ibid.

Arch. Biochem. Biophys. 68:1 (1957)

Ibid.

J. Biol. Chem. 233:184, 483 (1958)

Rec. Trav. Chim. 74:556 (1955) J. Am. Chem. Soc. 81:1348 (1959) Nature 183:1053 (1959) Z. Anal. Chem. 139:263 (1959)

J. Pharm. Exptl. Therap. 120:26 (1957) Anal. Chem. 21:811 (1949)

J. Pharm. Exptl. Therap. 120:26 (1957)

J. Am. Chem. Soc. 81:1348 (1959)

Anal. Chem. 32:819,1436 (1960)

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.57

Water, pH 7 Ethanol solution; 1 h at 80˚C; then pH 10 Ethanol solution; 1 h at 80˚C; then pH 10 pH 12 pH 7 Ethanol solution pH 2–10 pH 7 pH 11 pH 7 pH 7 pH 7 pH 1 pH 7

Salicyloyl hydrazide Salicyloyl hydrazide

3-Hydroxyanthranilic acid o-Hydroxybenzaldehyde

p-Hydroxybenzaldehyde

3-Hydroxybenzoic acid 4-Hydroxycinnamic acid 7-Hydroxycoumarin 5-Hydroxyindole 5-Hydroxyindoleacetic acid 3-Hydroxykynurenine (or 5-) 4-Hydroxymandelic acid 4-Hydroxyphenylacetic acid 4-Hydroxyphenylpyruvic acid 4-Hydroxyphenylserine 5-Hydroxytryptophan

Water, pH 7 pH 1

Homovanillic acid Hydroxyamphetamine

314 350 325 295 300 365 300 280 290 290 295

390

340 390

270 275

270 360 290

354 300 490 365 315 405 295, 335 305, 390 285 250, 312 300, 365

Pentane Pentane Water, pH 7–11 Water, pH 7 Water, pH 7 Cold 85% H2SO4; 1 h Water, pH 7 0.3M KOH Water, pH 1 Water, pH 11 Water, pH 1 70% H2SO4

328 295 275 370, 425 435

lex, nm

Pentane Water, pH 7 or 0.005M H2SO4 Water, pH 2 Isooctane solution

Conditions

Water, pH 7

o-Phthalaldehyde

Ninhydrin

Destroy fluorescence with 0.04% KMnO4

Reagent

Hippuric acid Histamine Homogentisic acid

Ethacridine 6-Ethoxy-1,2-dihydro-2,2,4trimethylquinoline Fluoranthrene Fluorene Fluorescein Folic acid Gentisic acid Gibberellic acid Griseofulvin Guanidinium compounds Guanine Guthion Harmine

1,4-Diphenylbutadiene Epinephrine

Compound

TABLE 6.22 Fluorescence Spectroscopy of Some Organic Compounds (Continued )

430 440 441 330 355 460 380 310 345 320 340

470

430 470

315 300

370 450 340

464 321 515 450 440 465 450 495 365 380 400

370 335 320 515 480

lem, nm

Anal. Chem. 30:1361 (1958) Arch. Biochem. Biophys. 68:1 (1957) J. Am. Chem. Soc. 81:1348 (1959) Arch. Biochem. Biophys. 68:1 (1957) Ibid.; Science 125:442 (1957) Arch. Biochem. Biophys. 68:1 (1957) Ibid. Ibid. Ibid. Ibid. Science 125:442 (1957)

Anal. Chem. 31:296 (1959)

Federation Proc. 18:444 (1950) Arch. Biochem. Biophys. 68:1 (1957) Ibid. J. Pharm. Exptl. Therap. 120:27 (1957) Ibid. Ibid.

Nature 184:364 (1957) Nature 183:1053 (1959) Arch. Biochem. Biophys. 68:1 (1957) J. Agr. Food Chem. 6:32 (1958) J. Pharm. Exptl. Therap. 120:26 (1957)

Arch. Biochem. Biophys. 68:1 (1957)

Anal. Chem. 28:376 (1956)

References

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.58

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Penicillin

2-Nitrophenol

2-Nitronaphthalene

Nerve gases (sarin, soman, or tabun) Nicotinamide

2-Naphthol

7-Methyldibenzopyrene 4-Methyl-7-diethylaminocoumarin Methylguanidine 4-Methyl-7-hydroxycarbostyril 4-Methyl-7-hydroxycoumarin 2-Methylphenanthrene 3-Methylphenanthrene 1-Methylpyrene 4-Methylpyrene Morphine Naphthacene Naphthaleneacetic acid 1-Naphthol

1M HCl

Benzene solution

pH 4.6

CNBr at 80°C for 8 min, pH 7 60% oleum, then dilute and add Zn dust Zn + HCl; then benzoic acid at 160ºC 2-Methoxy-6-chloro-9(b-aminoethyl)aminoacridine

1M NaOH

Indole + NaBO3

0.3M KOH Ethanol solution Ethanol solution Pentane Pentane Pentane Pentane 0.005M H2SO4

Pentane Ethanol solution

pH 11, water 0.1M NaOH in 20% ethanol 0.1M NaOH in 20% ethanol Let stand 1 min

Ninhydrin

Br2 water, then 2-aminobenzaldehyde in 0.15M NaOH

2-Naphthol

Kynurenic acid Lysergic acid diethylamide Malic acid

4-Methyl-7-aminocarbostyril 9-Methylanthracene 3-Methylcholanthrene 5-Methylcytosine

pH 7, water NaOH–ethanol

CNBr

Indoleacetic acid Isoniazid pH 7 Water, pH 7 Water, pH 9 92% H2SO4, 30 min at 90°C Ethanol solution Pentane Pentane Phosphate buffer

pH 7

Indole(s)

365

365

365

540

Green-yellow

450

470

480

365 365

426

495 412 442 357 368 394 386 365 480, 515 327 480

467 456

407 410 392 420

405 465 440 Green

345 405

355

365

305, 390 338 325 257 292 336 338 270–290 290, 310 230, 282 365

460 375

365 382 297 365

325 325 315 365

285 300

280

(Continued)

Ind. Eng. Chem., Anal. Ed. 12:403 (1940) J. Biol. Chem. 164:195 (1948)

Science 114:16 (1951); 116:462 (1952) Anal. Chem. 23:717 (1951)

Anal. Chem. 29:276 (1957)

Ibid.

J. Agr. Food Chem. 6:22 (1958) Anal. Chem. 30:96 (1958)

Nature 183:1053 (1959) J. Am. Chem. Soc. 81:1348 (1959) Ibid.

J. Am. Chem. Soc. 81:1348 (1959)

J. Biol. Chem. 233:483 (1958)

J. Am. Chem. Soc. 81:1348 (1959)

Anal. Chem. 21:1375 (1949)

Ibid.; Arch. Biochem. Biophys. 68:1 (1957) Science 125:442 (1957) Am. Rev. Respiratory Diseases 81:485 (1960) Arch. Biochem. Biophys. 68:1 (1957) Science 125:364 (1959) ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.59

pH 10 3M HCl Water Water Water, pH 13 Conc. H2SO4, 1 h at 130ºC Pentane

Salicyloyl hydrazide Serotonin

6.60

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p-Terphenyl

Resorcinol

295 290 366 470

Water, pH 1 Water, pH 11

Rutin Salicylic acid

Skatole Streptomycin Succinic acid

350 295

Water, pH 1 Water Water, pH 7

Reserpine Resorcinol Riboflavin

284

430 310

300 265 270, 370

360 250, 350

285

280 330 330 436 365

Water, pH 1 Water, pH 1

Water Pentane Water, pH 7 H2SO4

Proteins Pyrene Pyridoxal Pyruvaldehyde Pyruvic acid

275

Quinidine Quinine

Water, pH 11

Procaine

Water, pH 11

Pentane Methanol

215, 260 360 270 281 248, 292

315 252 265

265

lex, nm

Quinacrine

Water Pentane

Phenylalanine o-Phenylenepyrene Phenylephrine Picene Piperonylbutoxide

Chromotropic acid Diphosphopyridine nucleotide, reduced + enzymic coupling

pH 13 Pentane pH 13

Pentothal Phenanthrene Phenobarbital

Conditions pH 13

Reagent

Pentobarbital

Compound

TABLE 6.22 Fluorescence Spectroscopy of Some Organic Compounds (Continued )

338

340 370 445 Green

425 550

520 435

375 315 520

460 450

500

313, 350 382 385 540 460

345

282 506 305 398 318

530 362 440

449

lem, nm

Plant Physiol. 23:443 (1948)

Anal. Chem. 27:1178 (1955); 28:1017 (1956) Science 105:48 (1947) J. Pharm. Exptl. Therap. 120:26 (1957) Anal. Chem. 31:296 (1959) J. Pharm. Exptl. Therap. 117:82 (1956) Arch. Biochem. Biophys. 68:1 (1957) Science 125:442 (1957)

J. Pharm. Exptl. Therap. 120:26 (1957) J. Lab. Clin. Med. 36:478 (1950) J. Pharm. Exptl. Therap. 120:26 (1957) Ibid.

Arch. Biochem. Biophys. 68:1 (1957) Anal. Chem. 22:899 (1950) Biochem. J. 64:56P (1956)

J. Agr. Food Chem. 6:32 (1958) J. Pharm. Exptl. Therap. 120:26 (1957) Biochem. J. 76:381 (1960)

J. Pharm. Exptl. Therap. 120:26 (1957) Biochem J. 65:476 (1957)

J. Pharm. Exptl. Therap. 129:26 (1957) Ibid.

References

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

Water, pH 1

Pentane Pentane Water, pH 7 Water, pH 11 Water, pH 1 Water, pH 7 or 0.005M H2SO4 Water, pH 1 1-Butanol Water, pH 7 Methanol Water, pH 1 20% H2SO4 Water, pH 11

Tribenzo[a,e,i]pyrene Triphenylene Tryptamine Tryptophan Tyramine Tyrosine

Uric acid Vitamin A Vitamin B12 Warfarin Xanthine Xanthopterin Xanthurenic acid 2,6-Xylenol 3,4-Xylenol Yohimbine

Hexane–ethanol

Tocopherol

pH 7.5 Phosphate buffer pH 7

Na 1,2-naphthoquinone-4-sulfonate Br2 water, then o-aminobenzaldehyde in 0.15M NaOH

Thioglycolic acid Thymidine and Thymine

pH 7

Thymol

Alkaline K4[Fe(CN)6]

Thiamine

325 340 275 290, 341 315 365 350 275 280 270

384 288 290 285 275 275

295

265

365 365

365

370 490 305 385 435 460, 520 460 305 310 360

448 357 360 365 310 310

340

300

Blue-white 420

450

J. Pharm. Exptl. Therap. 120:26 (1957)

Arch. Biochem. Biophys. 68:1 (1957) Ibid. Ibid. J. Agr. Food Chem. 6:32 (1958) Arch. Biochem. Biophys. 68:1 (1957) Analyst 72:383 (1947) Arch. Biochem. Biophys. 68:1 (1957)

Arch. Biochem. Biophys. 68:1 (1957) Ibid. Ibid. Ibid.; Biochem. J. 65:476 (1957)

J. Pharm. Exptl. Therap. 120:26 (1957) Arch. Biochem. Biophys. 84:116 (1959)

Anal. Chem. 27:1178 (1955); 29:1017 (1956) Z. Anal. Chem. 139:263 (1953) J. Biol. Chem. 233:483 (1958)

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.61

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.62

SECTION SIX

Bibliography Lakowicz, J. R., Principles of Fluorescence Spectroscopy, Plenum, New York, 1983. Rendell, D., Fluorescence and Phosphorescence, Wiley, New York, 1987. Schulman, S. G., ed., Molecular Luminescence Spectroscopy, Wiley, New York, 1985, Vol. 1; 1988, Vol. 2. Wehry, E. L., ed., Modern Fluorescence Spectroscopy, Plenum, New York, 1981.

6.7

PHOSPHORIMETRY Instrumentation for phosphorimetry is identical to that described for fluorescence measurements (Sec. 6.6) with the addition of a radiation interrupter and provision for immersion of the sample in a Dewar flask for liquid-nitrogen temperatures. Time resolution is considerably improved by the use of a microsecond-duration pulsed source in place of the older rotating can chopper or set of slotted disks with equally spaced ports. Phosphorescence decay times down to milliseconds can be observed and recorded with pulsed radiation. The sample cuvette is a small Dewar flask made of fused silica and silvered, except in the region where the optical path traverses the Dewar. The solvent frequently used is a mixture of diethyl either, isopentane, and ethanol (often given the acronym EPA) in a volume ratio of 5:5:2. Another solvent mixture is isopentane and methylcyclohexane (1:4). When these mixtures are cooled to liquid-nitrogen temperatures, they give a clear transparent glass. The phosphorescence spectroscopy of some organic compounds is given in Table 6.23. Some substances give phosphorescence at room temperatures when adsorbed on a solid.7

6.8

FLOW-INJECTION ANALYSIS Flow-injection analysis (FIA) is based on the introduction of a precisely defined volume of sample (perhaps 30 mL) as a “plug” into a continuously flowing carrier or reagent stream (1 mL · min−1) in a narrow-bore (0.5-mm) nonwetting tubing. These typical conditions produce laminar flow in which molecules of fluid flow in streamlines parallel to the walls of the tubing with a parabolic velocity gradient between the center streamline, which flows at twice the average linear velocity, and the wall, at which the velocity is zero. Sample introduction is via a syringe or valve, the latter actuated by a microprocessor. The result is a sample plug bracketed by carrier. It is a nonsegmented stream in contrast to continuous-flow analysis. The absence of air segmentation leads to a higher sample throughput (30 to 300 h−1). There is no need to introduce and remove air bubbles, and an expensive high-quality pump is not necessary. FIA has been described as HPLC without the column and without the pressure. The carrier stream is merged with a reagent stream to bring about a chemical reaction between sample and reagent. The total stream then flows through a detector. Experimental conditions are held constant for both standards and samples in terms of residence time, temperature, and dispersion. Precise pumping and injection lead to precise fluid flow, the fundamental feature of FIA that makes it a useful technique. The sample concentration is evaluated against appropriate standards treated identically. Sample volumes are in the range of 20 to 200 mL, but normally are 30 mL. The flow of reagents is in the range of 0.5 to 9 mL · min−1. Typical FIA flowrates are in the range of 0.5 to 2.0 mL · min−1; they can be easily varied by shifting pump tubes. A wide range of detectors has been described, among which are atomic absorption spectrometry, induction coupled plasmas, 7 T. Vo-Dinh, Room Temperature Phosphorimetry for Chemical Analysis, Wiley, New York, 1984; R. J. Hurtubise, Solid Surface Luminescence Analysis, Dekker, New York, 1981; E. B. Asafu-Adajaye and S. Y. Yue, Anal. Chem. 58:539 (1986); S. M. Ramasamy, Y. P. Senthilnathan, and R. J. Hurtubise, ibid. 58:612 (1986).

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ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY

6.63

TABLE 6.23 Phosphorescence Spectroscopy of Some Organic Compounds (EPA, diethyl ether, isopentane, and ethanol (5:5:2) volume ratio)

Compound Acenaphthene 3-Acetylpyridine Adenine Adenosine p-Aminobenzoic acid 2-Aminofluorene 6-Amino-6-methylmercaptopurine 2-Amino-4-methylpyrimidine 2-Amino-5-nitrobenzothiazole 2-Amino-5-nitrobiphenyl 3-L-Aminotyrosine-2HCl Anthracene Aspirin Atropine 8-Azaguanine Benzaldehyde 1,2-Benzanthracene Benzimidazole Benzocaine 1,2-Benzofluorene Benzoic acid 3,4-Benzopyrene Benzyl alcohol 6-Benzylaminopurine Biphenyl 6-Bromopurine Brucine Caffeine Carbazole 2-Chloro-4-aminobenzoic acid p-Chlorophenol o-Chlorophenoxyacetic acid p-Chlorophenoxyacetic acid 6-Chloropurine Chlorpromazine · HCl Chlorotetracycline Cocaine · HCl Codeine Cytidine Desoxypyridoxine · HCl Diacetylsulfanilamide 2,6-Diaminopurine 2,6-Diaminopurine sulfate

Solvent Ethanol Ethanol Water–methanol (9 :1) Ethanol Ethanol Ethanol Water–methanol (9:1) Ethanol EPA EPA Ethanol Ethanol EPA Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol EPA Ethanol Ethanol Water–methanol (9:1) Ethanol Water–methanol (9:1) Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol Water–methanol (9:1) Ethanol Ethanol Ethanol Ethanol Water–methanol (9:1) Ethanol Ethanol Water–methanol (9:1) Ethanol

Lifetime, s

Excitation wavelength, nm

Emission wavelength, nm

0.5 2.9

300 395 278

515 525 406

280 305 380 321

422 425 590 456

302 375 380 286 300 240

2.8

282 254 310 280 310 315 240 325 219 286

438 515 520 398 462 380 410 442 433 510 406 430 502 400 508 393 413

1.0 0.5

270 273

385 420

0.9 2.0 7.8 1.0 5mg/L

2−7

Selectivity coefficients: H+, 0.06; NH+4 , 0.004; Li+, 0.0005; Na+, 0.003; K+, 0.02; Rb+, 0.01; Cs+, 0.003; Mg2+, 8 × 10−6; Ca2+, 0.0002; Sr2+, 0.03

Solid

1−(5 × 10−6)

2−12

[S2−] ≤ 107M; [I−] < 2 × 10−4[Br−]; [CN−] < 8 × 10−5 [Br−]; [OH−] < 3 × 104 [Br−]; [Cl−] < 400 [Br−]

Cadmium

Solid

1−10−7

3–7

[Ag+], [Hg2+], [Cu2+] ≤ 10−7M; high levels of Pb2+, Fe3+ Selectivity coefficients: Fe2+, 214; Tl+, 126; Mn2+, 2.5; Al, 0.8; Ni, 0.03; Cu2+, 0.02; Zn, 4 × 10−4; Ca, 4 × 10−4; Mg, 1.7 × 10−4

Calcium

Neutral PVC membrane

1−(5 × 10−6)

6−8

Selectivity coefficients: Zn, 10−4; Fe2+, 0.005; Pb2+, 10; Mg2+, 10−4; (Na, K, NH4+), 0.003; Cu2+, 0.0025; Li+, 0.081; Ba2+, 0.010; Sr2+, 0.017; (I−, ClO−4 ), < 10−3M; H+, 0.0062

Carbon dioxide

Gas

(3 × 10−2)−10−5

[Pb2+]

Iodide

Solid

1−(2 ×

Lead

Solid

1−10−7

Lithium

Liquid

0.7−70 ng/L

Nitrate

Liquid

1−(5 × 10−6)

3–10

Selectivity coefficients: I−, 20; Br−, 0.1; Cl−, 0.004; NO−2 , 2− − − 0.04; SO2− 4 , 0.000 03; CO3 , 0.0002; ClO4 , 1000; F , 0.000 06; ClO−3 , 2; CN−, 0.02; HCO−3 , 0.02; OAc−, 0.006; H2HPO−4 , 0.003; HPO2− 4 , 0.000 08

Nitrite

Gas

0.02−(5 × 10−8)

Citrate buffer

Volatile organic acids; SO2 must be destroyed; Cr(VI), CO2 interferes

Perchlorate

Liquid

0.1−10−5

3–10

Selectivity coefficients: I−, 0.012; NO−3 , 0.0015; Br−, 0.000 56; F−, 0.000 25; Cl−, 0.000 22; OH−, 1.0; OAc−, 0.000 51; HCO3− , 0.000 35; SO2− 4 , 0.000 16

Potassium

Liquid

1−10−5

3–10

Selectivity coefficients: Cs+, 1.0; NH+4 , 0.03; H+, 0.01; Ag+, 0.001; Na+, 0.0002; Li+, 0.0001; Mg, 0.001; Cu2+, 0.002

Silver/sulfide

Solid

1−10−7 Ag+ or S2−

2–9 (Ag+) 13–14 (S2−)

[Hg2+] ≤ 10−7M as silver sensor None as sulfide sensor if ascorbic acid is added to remove O2

Selectivity coefficients: H+, 1; NH+4 , 0.05; Na, 0.05; K, 0.007; Rb, 0.004; Cs, 0.003; Mg, 0.0002; Ca, 0.0006

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.45

TABLE 14.12 Ion-Selective Electrode Characteristics (Continued) Electrode

Type

Measuring range, M

Preferred pH range

Interferences

1−10−6

9–10

Selectivity coefficients: Li+, 0.002; K+, 0.001; Rb+, NH+4 , 0.000 03; H+, 100; Cs+, 0.0015; Tl+, 0.0002; Ag+, 350; (C2H5)4N+, 0.005 Selectivity coefficients: H+, 0.5; Li+, 0.04; K+, 0.5; NH+4 , 0.2; Cs+, 0.002; Ca2+, 0.002; Mg2+, 0.0008

Ag2S

≥10−14

13

[CN−] < 500 [S2−]

Sulfur dioxide

Gas

10−2−10−6

0–2

Volatile acids; Cl2, NO2 must be destroyed with N2H4

Tetrafluoroborate

Liquid

0.1−10−5

3–10

Selectivity coefficients: NO−3 , 0.1; Br−, 0.04; OAc−, 0.004; − − HCO−3 , 0.004; Cl−, 0.001; SO2− 4 , 0.001; OH , 0.001; I , 20; F−, 0.001

Thiocyanate

Solid

1−(5 × 10−6)

2–12

[OH−] < 100 [SCN−]; [Br−] < 0.003 [SCN−]; [Cl−] < 20 [SCN−]; [NH3] < 0.13 [SCN−]; [S2O23−] < 0.01 [SCN−]; [CN−] < 0.007 [SCN−]; [I−], [S2−] ≤ 10−7M; Bi and transitional-metal ions form weak SCN− complexes; Au(I) and Hg(II) must be absent

Sodium

Glass

Neutral ion carrier Sulfide

14.4

POTENTIOMETRIC TITRATIONS Potentiometric titrations involve following the changes in the cell emf brought about by the addition of a titrant of an accurately known concentration to the test solution. The method is applicable to any titrimetric reaction for which an indicator electrode is available to follow the activity (concentration) of at least one of the substances involved. Rapid changes in cell emf signal the equivalence point of the titration. A general advantage of potentiometric titrations is that no visual error can be made in the detection of the end point, and a high degree of precision can be reached. Especially in the titrations of colored or turbid systems, or of those for which there are no suitable visual indicators, the potentiometric methods have great importance. Precision is limited by the sharpness of the potential change at the end point and by the accuracy with which the volume of titrant can be delivered. Generally, solutions more dilute than 10−3M do not give satisfactory end points. This is a limitation of potentiometric titrations. Requirements for reference electrodes are greatly relaxed. Accuracy is increased because measured potentials are used to detect rapid changes in activity that occur at the equivalence point. This rate of emf change is usually considerably greater than the response slope, which limits precision in direct potentiometry. Furthermore, it is the change in emf versus titration volume rather than the absolute value of emf that is of interest. Thus, the influence of liquid-junction potentials and activity coefficients is minimized. The equivalence point may be calculated or it can be located by inspection from the inflection point of the titration curve. It is the point that corresponds to the maximum rate of change of cell emf per unit volume of titrant added (usually 0.05 or 0.1 mL).

14.4.1 Potentiometric Precipitation Titrations Extensive work has been done on potentiometric methods of detecting the end points of precipitation titrations because relatively few chemical indicators for such titrations are available and because most of those are not suitable for the titration of mixtures. Potentiometric precipitation titrations are carried out with a silver, mercury, or platinum indicator electrode, or an ion-selective electrode (Sec. 14.3), together with an appropriate reference electrode. The end point is typically located by

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ELECTROANALYTICAL METHODS

14.46

SECTION FOURTEEN

finding the point of maximum potential change for a given increment of titrant (the inflection point of the titration curve). Although this method is often theoretically incorrect, as in the case of unsymmetrical precipitates, the error involved is usually slight. Graphical methods are also extensively used, especially in the titration of mixtures. Titration to a calculated or predetermined end-point potential is also of use, but it must be remembered that changes in ionic strength, in liquid-junction potentials, or in the surface of the indicator electrode may render this method unreliable unless care is taken to compensate for their effects. The accuracy of the titration increases with increasing concentration of the species being determined and also with increasing insolubility of the precipitate. If we take 60 mV as a minimum ∆E for which an accurate titration is possible, and the case in which the ratio of cation to anion is 1, then the solubility product for the precipitate when titrating solutions of a monovalent ion should be no greater than 2.5 × 10−5 when titrating 10−2M solutions 2.5 × 10−9 when titrating 10−4M solutions 2.5 × 10−13 when titrating 10−6M solutions Table 14.13 outlines procedures for the execution of a number of potentiometric precipitation titrations, selected on the basis of their reliability and usefulness. The reader should also consult the material in Sec. 3.4.

14.4.2 Potentiometric Oxidation–Reduction Titrations As one proceeds down in Table 14.14, the oxidizing agents decrease in strength and the reducing agents increase in strength. In general, if two half-reactions are represented by the following equations, (Oxidizing agent)1 + ne− = (reducing agent)1

(14.41)

(Oxidizing agent)2 + ne− = (reducing agent)2

(14.42)

and the second equation occurs lower in the table than first equation, then a reaction may occur between (oxidant)1 and (reductant)2, whereas no reaction is possible between (oxidant)2 and (reductant)1. If two or more reactions between two substances are possible, usually the reaction involving half-reactions that are farthest apart in Table 14.14 will occur first. The reader is also referred to the discussion in Sec. 3.5.1. No predictions concerning the rate of the reaction are possible. Potentiometric redox titrations are carried out using some “noble”-metal electrode, the potential of which is measured versus a suitable reference electrode. The noble-metal indicator electrode theoretically serves simply to transfer electrons from dissolved species in solution to the external circuit. Usually platinum or gold is used as an indicator electrode. It should always be remembered that both platinum and gold can become oxidized by strong oxidants. This oxidation sometimes obscures end points or at least causes steady potentials to be obtained rather slowly. Certain difficulties are also observed with platinum electrodes in strongly reducing solutions such as chromium(II) solutions. The reduction of hydrogen ion is catalyzed at platinum surfaces, causing the observed potentials to be less reducing than they should be. Mercury electrodes do not have the latter disadvantage. The general technique of redox titrations is essentially the same as that for other types of titrations. One difference arises from the fact that the indicator electrode responds to the ratio of the dominant oxidation–reduction couple and not to a single ion. Usually the end point is located as the point of maximum potential change for a specific volume increment. Sometimes potential changes are so great that accurate results may be obtained by approaching the end point dropwise and establishing the drop that causes a large potential change. The end point may also be established by other methods such as titrating to a calculated or, better, a predetermined potential. Successive titrations can be handled more easily than would be the case with visual end points. Table 14.15 gives information about a few of the most useful or interesting potentiometric redox titrations. Most of the procedures outlined in Table 3.24 are adaptable to potentiometric titrations.

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BaSO4, 9.96

Dithiooxamide

AgNO3

NaB(C6H5)4

≥ 0.01M La(NO3)3

Cs+

F−

AgNO3

CN−

CrO2− 4

0.05M (C6H5)4As+, Cl−

ClO−4

Co(II)

Ag+

Cl−

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. F− ISE

Cs PVC ISE (crown ethers)

Ag wire, Ag2S ISE

CuS ISE

Ag wire, CN ISE

Perchlorate ISE

or

Same as for Ca2+

Cd2+ Ag wire, Cl− ISE

Ag−Ag2Ox, CaOx, Ca2+

C2O2− 4

Ca2+

Ag+

Hg (as above)

Hg2(NO3)2

Ag wire, Br− or Ag2S ISE Hg pool or Hgplated Au wire

C2O2− 4

Hg2(NO3)2

≥ 0.001M AgNO3

Pb2+ ISE

Na2SO4, then Pb(ClO4)2

Br−

BaCrO4, 9.93

Pt

K2CrO4

Ba2+

Ag wire, Ag2S ISE F− ISE

AgNO3 La(NO3)3; NaF

AsO3− 4

LaF3, 16.2

Ag2CrO4, 11.95

Co dithiooxamide

AgCN, 15.92

AgCl, 9.75

CdC2O4, 7.04

CaC2O4, 8.4

Hg2C2O4, 12.7

Hg2Br2, 22.24

AgBr, 12.30

Ag3AsO4, 22.0 LaAsO4

AgB(C6H5)4

Ag metal, Ag2S ISE

(of excess NaB(C6H5)4

Amines

AlF3

F− ISE

Ag+

Ag wire, A2S ISE

Na diethyldithiocarbamate (NaDDC)

NaF; La(NO3)3

AgI, 16.08; AgBr, 12.3; AgCl, 9.75 Ag diethyldithiocarbamate

Precipitate formed formed, and pKsp

Ag wire, Ag2S ISE

Indicator electrode

KI, KBr, KCl

Titrant

Al3+

Ag+

Analyte

TABLE 14.13 Potentiometric Precipitation Titrations

≤0.01M titrant; dilute solution 1: 1 with methanol. (Continued)

Sodium and divalent ions have little effect; potassium, rubidium, and ammonium ions must be absent.

Solution 1:1 with methanol.

Ammoniacal solution; pH 8.1–8.4.

pH ≥ 9. The complexometric Ag(CN)−2 end point is sharper.

End-point break is 150–200 mV. By operating at 2°C useful limit is extended to 0.1 g/L of perchlorate. Iodide, periodate, and permanganate must be absent.

0.01M HNO3; in mixtures with Br− and I−, also add 5% (w/v) Ba(NO3)2 to decrease adsorption.

Add excess oxalate and back-titrate.

Third class electrode.

May be applied to determination of Ca, Cd, Pb, or Sr by adding excess oxalate and back-titration.

Reagent solution must be acid to prevent hydrolysis.

0.01M HNO3.

Solvent: 30% aqueous EtOH. Titrate at 70°C. End-point break is small unless air is excluded. Add excess Na2SO4, then Pb(ClO4)2; back-titrate excess Pb2+.

Maintain pH at 9–11 (with NaOH) during titration. Back-titrate excess La(NO3)3 with NaF.

To sample solution add excess NaB(C6H5)4, filter, wash precipitate with 50% aqueous acetone, and titrate excess reagent in filtrate and washings.

Add excess NaF, back-titrate excess with La(NO3)3.

0.01M HNO3. Bi, Cd, Cu, Fe, Pb, and Zn may be masked with EDTA at pH 4–5.5. Solution 50% ethanol.

Supporting electrolyte, procedure

ELECTROANALYTICAL METHODS

14.47

Ag+ La(NO3)3; KF

Na2MoO4

NaB(C6H5)4

Pb(NO3)2

PO3− 4

Pb2+

Rb+

ReO−4

14.48

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Pb(NO3)2

K4[Fe(CN)6]

Tungstate

Zn2+ Pt

Pb ISE

NaF; La(NO3)3

Th(IV)

ISE

Ba2+ PVC ISE Pb ISE

Ag wire, Ag+ ISE

Ag2S ISE or Pb ISE

F ISE

Pb ISE

K ISE

Pb ISE

Nitrate ISE

Pb ISE

F ISE

F−

Sr2+

BaCl2 Pb(ClO4)2

Na diethyldithiocarbamate

Ni2+

SO2− 4

[(C6H5)2 Tl]2SO4

AgNO3

Pb(NO3)2

MoO2− 4

NO−3

SCN−

≥0.001M AgNO3

Mercaptan

NaF

CuS or Ag2S ISE Ag wire, Ag+ ISE F− ISE

NaF

Li+

AgNO3 or Pb(NO3)2

Ag2S ISE

NaF

La3+

S2−

F ISE

NaB(C6H5)4; Ag+

K+

Rare earths

AgI ISE Ag2S ISE

≥ 0.001M KI Thioacetamide

Hg2+ K ISE

Hg

Ag wire, Ag+ or I− ISE Hg

Indicator electrode

KCl, KBr, or KI

HgCl2

Ag+

Titrant

Hg2+ 2

I−

Analyte

TABLE 14.13 Potentiometric Precipitation Titrations (Continued)

K2Zn3[Fe(CN)6]2

PbWO4, 6.36

ThF4, 7.23

BaSO4, 9.96 PbSO4, 7.79

AgSCN, 12.00

Ag2S, 49.2; PbS, 27.9

(Rare earth)F3

PbReO4

RbB(C6H5)4

PbMoO4, 13.00

Ag3PO4, 15.84 LaPO4, 22.43

PbMoO4, 13.0

LiF, 2.42

LaF3

KB(C6H5)4, 7.65

HgI2

Hg2Cl2, 17.88; Hg2Br2, 22.24; Hg2I2, 28.35

HgI2

AgI, 16.08

Precipitate formed formed, and pKsp

pH 2–3; add 3–4 drops 1% K3[Fe(CN)6]. Many interferences eliminated by EDTA.

Interference from molybdate and perrhenate.

Add excess NaF; back-titrate with La3+.

See method for Ca2+.

BaCl2 excess; back-titrate with EDTA Medium: 50% 2-propanol, acetone, or methanol.

0.01M HNO3 containing 5% Ba(NO3)2.

Interference from Cs, K, and ammonium ions.

Medium: 50% methanol.

Add excess La3+; back-titrate with F−.

Medium: 50% ethanol, pH 4−6.

Medium: 0.1M K2SO4 plus 0.05M H2SO4.

Solvent 1:1 methanol.

Add excess reagent; wash precipitate; titrate filtrate and washings with Ag+.

Direct or add excess Ag+ and back-titrate with I−. 0.033M EDTA, 0.66M NaOH, 0.4% gelatin.

Br− and Cl− do not interfere if less concentrated than I−.

0.01M HNO3.

Supporting electrolyte, procedure

ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

TABLE 14.14 Potentials of Selected Half-Reactions at 25°C A summary of oxidation–reduction half-reactions arranged in order of decreasing oxidation strength and useful for selecting reagent systems. Half-reaction F2(g) + 2H+ + 2e− = 2HF O3 + H2O + 2e− = O2 + 2OH− O3 + 2H+ + 2e− = O2 + H2O Ag2+ + e− = Ag+ 2− − S2O2− 8 + 2e = 2SO4 HN3 + 3H+ + 2e− = NH+4 + N2 H2O2 + 2H+ + 2e− = 2H2O Ce4+ + e− = Ce3+ MnO−4 + 4H+ + 3e− = MnO2(c) + 2H2O 2HClO + 2H+ + 2e− = Cl2 + 2H2O 2HBrO + 2H+ + 2e− = Br2 + 2H2O H5IO6 + H+ + 2e− = IO−3 + 3H2O NiO2 + 4H+ + 2e− = Ni2+ + 2H2O Bi2O4(bismuthate) + 4H+ + 2e− = 2BiO+ + 2H2O MnO−4 + 8H+ + 5e− = Mn2+ + 4H2O 2BrO−3 + 12H+ + 10e− = Br2 + 6H2O PbO2 + 4H+ + 2e− = Pb2+ + 2H2O + − 3+ Cr2O2− 7 + 14H + 6e = 2Cr + 7H2O Cl2 + 2e− = 2Cl− 2HNO2 + 4H+ + 4e− = N2O + 3H2O N2H+5 + 3H+ + 2e− = 2NH+4 MnO2 + 4H+ + 2e− = Mn2+ + 2H2O O2 + 4H+ + 4e− = 2H2O ClO−4 + 2H+ + 2e− = ClO−3 + H2O 2IO−3 + 12H+ + 10e− = I2 + 3H2O N2O4 + 2H+ + 2e− = 2HNO3 2ICl−2 + 2e− = 4Cl− + I2 Br2(1q) + 2e− = 2Br− N2O4 + 4H+ + 4e− = 2NO + 2H2O HNO2 + H+ + e− = NO + H2O NO−3 + 4H+ + 3e− = NO + 2H2O NO−3 + 3H+ + 2e− = HNO2 + H2O 2Hg2+ + 2e− = Hg2+ 2 Cu2+ + I− + e− = CuI + OsO4(c) + 8H + 8e− = Os + 4H2O Ag+ + e− = Ag − Hg2+ 2 + 2e = 2Hg Fe3+ + e− = Fe2+ H2SeO3 + 4H+ + 4e− = Se + 3H2O HN3 + 11H+ + 8e− = 2NH+4 O2 + 2H+ + 2e− = H2O2 Ag2SO4 + 2e− = 2Ag + SO2− 4 Cu2+ + Br− + e− = CuBr(c) Au(SCN)−4 + 3e− = Au + 4SCN− 2HgCl2 + 2e− = Hg2Cl2(c) + 2Cl− Sb2O5 + 6H+ + 4e− = 2SbO+ + 3H2O H3AsO4 + 2H+ + 2e− = HAsO2 + 2H2O TeOOH+ + 3H+ + 4e− = Te + 2H2O Cu2+ + Cl− + e− = CuCl(c) I−2 + 2e− = 3I−

E 0, volts 3.053 1.246 2.075 1.980 1.96 1.96 1.763 1.72 1.70 1.630 1.604 1.603 1.593 1.59 1.51 1.478 1.468 1.36 1.3583 1.297 1.275 1.23 1.229 1.201 1.195 1.07 1.07 1.065 1.039 0.996 0.957 0.94 0.911 0.861 0.84 0.7991 0.7960 0.771 0.739 0.695 0.695 0.654 0.654 0.636 0.63 0.605 0.560 0.559 0.559 0.536 (Continued)

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14.49

ELECTROANALYTICAL METHODS

14.50

SECTION FOURTEEN

TABLE 14.14 Potentials of Selected Half-Reactions at 25°C (Continued) Half-reaction 2e−

2I−

= I2 + Cu+ + e− = Cu 4H2SO3 + 4H+ + 6e− = S4O2− 6 + 6H2O Ag2CrO4 + 2e− = 2Ag + CrO2− 4 2H2SO3 + 2H+ + 4e− = S2O2− 3 + 3H2O + + − 4+ UO2 + 4H + e = U + 2H2O 4− − Fe(CN)3− 6 + e = Fe(CN)6 Cu2+ + 2e− = Cu VO2+ + 2H+ + e− = V3+ + H2O BiO+ + 2H+ + 3e− = Bi + H2O + − 4+ UO2+ 2 + 4H + 2e = U + 2H2O Hg2Cl2(c) + 2e− = 2Hg + 2Cl− AgCl + e− = Ag + Cl− SbO+ + 2H+ + 3e− = Sb + H2O − − CuCl2− 3 + e = Cu + 3Cl + + 2e− = H SO + H O SO2− + 4H 4 2 3 2 Sn4+ + 2e− = Sn2+ + − S + 2H + 2e = H2S Hg2Br2(c) + 2e− = 2Hg + 2Br− CuCl + e− = Cu + Cl− TiO2+ + 2H+ + e− = Ti3+ + H2O 2− − S4O2− 6 + 2e = 2S2O3 AgBr + e− = Ag + Br− HCOOH + 2H+ + 2e− = HCHO + H2O CuBr + e− = Cu + Br− 2H+ + 2e− = H2 Hg2I2 + 2e− = 2Hg + 2I− Pb2+ + 2e− = Pb Sn2+ + 2e− = Sn AgI + e− = Ag + I− N2 + 5H+ + 4e− = N2H+5 V3+ + e− = V2+ Ni2+ + 2e− = Ni Co2+ + 2e− = Co Ag(CN)−2 + e− = Ag + 2CN− PbSO4 + 2e− = Pb + SO2− 4 Cd2+ + 2e− = Cd Cr3+ + e− = Cr2+ Fe2+ + 2e− = Fe H3PO3 + 2H+ + 2e− = HPH2O2 + H2O 2CO2 + 2H+ + 2e− = H2C2O4 U4+ + e− = U3+ Zn2+ + 2e− = Zn Mn2+ + 2e− = Mn Al3+ + 3e− = Al Mg2+ + 2e− = Mg Na+ + e− = Na K+ + e− = K Li+ + e− = Li 3N2 + 2H+ + 2e− = 2HN3

E 0, volts 0.536 0.53 0.507 0.449 0.400 0.38 0.361 0.340 0.337 0.32 0.27 0.2676 0.2223 0.212 0.178 0.158 0.15 0.144 0.1392 0.121 0.100 0.08 0.0711 0.056 0.033 0.0000 −0.0405 −0.125 −0.136 −0.1522 −0.225 −0.255 −0.257 −0.277 −0.31 −0.3505 −0.4025 −0.424 −0.44 −0.499 −0.49 −0.52 −0.7626 −1.18 −1.67 −2.356 −2.714 −2.925 −3.045 −3.10

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

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ELECTROANALYTICAL METHODS

TABLE 14.15 Potentiometric Redox Titrations Substance determined

Reagent

End-point potential, V vs SCE

Titration conditions and remarks

As(III)

Ce(IV) KMnO4 KBrO3

∆E max. ∆E max. ∆E max.

4M HCl; ICl catalyst. Na2CO3 medium. Medium: 10% HCl.

Au(III)

Ascorbic acid

∆E max.

pH 1.6–3; 50°C. Cu(II), Fe(III), Hg(II), and 0.1M Cl− do not interfere.

Br−

KMnO4

∆E max.

10 mL sample + 5 mL 10% KCN + 10 mL conc. H2SO4 + 100 mL water. Equiv. wt. is Br/2.

Oxalate

KMnO4 Ce(IV)

∆E max. ∆E max.

0.2M–1M H2SO4; 70°C. Add 20 mL conc. HCl + 10 mL 0.005M ICl per 70 mL sample.

Ce(III)

K3[Fe(CN)6] or KMnO4

0.04

Add sample to enough 4M K2CO3 to ensure 1.5M K2CO3 at end point. Titrate in absence of air.

Ce(IV)

Fe(II) H2C2O4

Cr(VI)

As(III) Ti(III)

∆E max. ∆E max.

Medium: 20% H2SO4. Medium: 10% H2SO4; exclude air.

Fe(II)

KMnO4

1.09

Ce(IV)

0.95

Medium: 0.2M H2SO4; H3PO4 may be added to improve break. Medium: 1M H2SO4. End point depends on acid used.

Fe(CN)3− 6

Ce(IV)

I−

KMnO4

1.0

Medium: 0.1M–0.25M H2SO4. Br− and Cl− may be present.

Mn(II)

KMnO4

0.53 (pH 6.0)

Fe(II)

1.3; 0.7

Add acid solution of sample to 250 mL fresh saturated Na4P2O7 solution, adjust pH to 6–7, and titrate. Equiv. wt. is Mn/4. Titration is specific except for vanadium; vanadium does not interfere if pH is 3–3.5 although end-point break is smaller. To 50 mL sample containing 2M–5M HNO3, add small portions of AgO until solution is black; dilute to 150 mL with 1M H2SO4 and titrate. First break = Ag(II); second break = Mn (as MnO2). Equiv. wt. is Mn/2.

MnO−4

Fe(II)

1.09

NO−2

KMnO4

H2O2

Ce(IV)

Medium: 0.5M–3M HOAc or HCl.

Sb(III)

KBrO3

Medium: 3M (5%) HCl.

Sb(V)

KBrO3

Sulfite

KMnO4

Sn(II)

Iron(III)

Vanadium

KMnO4

Medium: 1M H2SO4. Add 20 mL conc. HCl + 10 mL 0.005M ICl per 70 mL sample.

Medium: >1M HCl or H2SO4.

Medium: 0.2M H2SO4; add excess standard Fe(II) and back-titrate with standard KMnO4. Add nitrite sample slowly to 10% excess KMnO4 + 0.75M H2SO4; then add excess KI and titrate with KMnO4.

0.3–0.5; ∆E max.

To sample in 5% HCl add excess Ti(III), then immediately three drops 3% CuSO4 solution, stir, and titrate with KBrO3. First break = excess Ti(III); second break = Sb. No interference from As(V). Add excess of KMnO4 to alkaline sulfite; add H2SO4 to give 0.5M and excess KI, then titrate excess with KMnO4. Medium: 10% HCl, 75°C.

0.05 0.44 1.1

V(II) → V(III). V(III) → V(IV). V(IV) → V(V) done at 70°C.

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ELECTROANALYTICAL METHODS

14.52

SECTION FOURTEEN

14.4.2.1 Potentiometric Titrations at Constant Current. A number of redox couples normally used in titrimetry, such as thiosulfate–tetrathionate and dichromate (VI)-chromium(III), are irreversible and thus establish steady potentials at a platinum electrode rather slowly if the ordinary zero-current method of measurement is used. Steady potentials are obtained more rapidly and larger potential breaks are obtained by the use of polarized indicator electrodes. Interpretations of the titration curves obtained are best discussed on the basis of current–potential curves (see Sec. 14.5). Potentiometric titrations at constant current may be divided into two classes, depending on whether one or two indicator electrodes are used. In the former case the potential of a single polarized platinum indicator electrode is measured versus a reference electrode. The indicator electrode may be polarized either anodically or cathodically, depending on the situation. Alternatively, two platinum electrodes may be used, in which case one will act as an anode and the other as a cathode. The potential between these two platinum electrodes is measured. If thiosulfate were titrated with iodine, some idea of the various possible titration curves could be established by knowing that the iodide–iodine couple is reversible while the thiosulfate–tetrathionate couple is irreversible. The three possible curves are obtained with (1) two similar polarized platinum electrodes, (2) one platinum electrode polarized anodically, and (3) one platinum electrode polarized cathodically. In case (1), before the equivalence point, the cathode would reduce oxygen (air) or water, while the anode would oxidize iodide ion. Thus a fairly large difference in potential between the two platinum electrodes would exist. After the equivalence point, excess iodine would be present and would be reduced at the cathode. The difference in the potential between the two electrodes would therefore decrease rapidly around the end point. In case (2), the single electrode would oxidize iodide ion both before and after the equivalence point. Only a small potential change would be noticed when excess iodine was added. The results in this case would be much worse than in standard zero-current potentiometry. If the single electrode is polarized cathodically [case (3)], it will first reduce oxygen or water, but after the equivalence point will reduce iodine and thereby undergo a substantial potential shift. Despite the advantages of using one polarized electrode for cases in which electrode potentials are established slowly, caution is in order because of the possibility of error due to the fact that the point of greatest potential change does not correspond to the equivalence point. This error is proportional to the size of the current, and thus small currents (0.1 g Ag, add Hg(NO3), solution (=0.1 wt of Ag). 0.1–0.5 g Ag in Ag–Cu alloys. Dissolve sample in 20 mL 1:1 HNO3, dilute to 100 mL, add 6 g NaNO3. 0.5 g Ag as AgNO3. Dissolve in 200 mL H2O, add 1M KOH solution to complete precipitation of Ag, redissolve precipitate by addition of 10% KCN solution. 0.06–0.6 g Ag in Ag solder. Dissolve sample in 6 mL 1:1 HNO3, boil, cool, dilute to 150 mL, add 8 mL NH3. Pt

Pt

Pt

Pt, −0.24 V

Pt

Pt

Pt

Pt

Pt, −0.40 V

Pt

Pt

Pt

Pt

Pt

Pt

Pt, +0.1 V

Pt

Anode

Pt

Cathode

Bi, Cd, Cu, Hg, Pb, Pt, Sb, Sn, NO−3 Heavy metals, CN−, Fe, Ga, In, Zn

2 A · dm−2, 45 min.

Cu, Ag, Be, Hg, Sb

Cu, Ag, Bi, Cd, Hg, Pb, Pt, Sb, Se, Sn, Te

Hg

Au, Bi, Cd, Co, Cu, Hg, Ni, Zn

Cu, Hg (As, Au, Bi, Cd, Pb, Pt, Sb, Se, Sn, Te) As, Hg, Se, Te

Codeposit or interference

2–3 A, 20 min.

C.p., 20 min, 50°C. As codeposits as Cu3As2 with known Cu.

2 A ⋅ dm−2, 1.5 h. As codeposits as Cu3As2 with known Cu.

C.p., 25 min; O2 bubbled through solution to keep Cu oxidized and prevent colloidal Ag. C.p.

2 A, 20 min.

2 A ⋅ dm−2, 1 h.

2 A, 20 min, 95°C; Ag codeposits with known Hg.

Current, time, temperature

Conditions of electrolysis

The abbreviation C.p. refers to controlled-potential electrolysis. Electrode potentials are referred to the saturated calomel electrode (SCE). Table 14.25 should be consulted for directions regarding the treatment of the deposit at the conclusion of the electrolysis.

TABLE 14.26 Electrogravimetric Methods for the Elements

As(V), Cd, Sn, Zn

As(V)

Cu and baser metals

Cd, Cu, Zn

Bi, Cd, Co, Cr, 0.5 g Cu, Fe, Mn, Ni, Pb, Zn Fe

Zn

Separation from

ELECTROANALYTICAL METHODS

14.96

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Bi Dissolve 0.1 g Bi in 5 mL HNO3, add 10 mL HClO4, fume, dilute to 200 mL, add 5 mL saturated N2H4 ⋅ H2SO4 solution. Dissolve 0.25 g Bi in 3 mL HNO3 or 5 mL HCl, dilute to 100 mL, add 3 g NaOH plus 2 g NaHtartrate plus 1 g NH2OH · HCl (plus 3 g KCN if Cu present). Dissolve 0.1 to 0.4 g Bi in 10 mL 1: 2 HNO3 (or 5 mL HCl plus minimum HNO3 if Sb or Sn present), dilute to 200 mL, add 1 g urea plus 12 g Na2tartrate plus 2 g N2H4 · 2HCl plus 1 g succinic acid; adjust to pH 5.9. 0.1−0.3 g Bi in Bi−Pb, Sn alloys. Dissolve 0.4 g sample in 1−2 mL HNO3, add 10 mL HCl, boil, add 5 mL more HCl, dilute to 100 mL, add 5 g oxalic acid plus 0.5 g N2H4 · 2HCl. Br 30–300 mg Br as NaBr in 100 mL containing 2.7 g NaOAc, adjust to pH 4.7 with HOAc. Cd Dissolve 0.25 g Cd in 10 mL 2.5M H2SO4 or 5 mL HClO4, dilute to 200 mL, add 10 mL 0.1% gelatin. 0.1–0.4 g Cd in metal alloys. Dissolve sample in 3 mL H2O plus 5 mL HClO4

Dissolve 0.1 g Au in aqua regia, evaporated to dryness twice with least excess HCl, dilute to 100 mL, add 1 to 2 g KCN. 50 mg Au in Au–Ni, Au–Ag, Cd, Cu, Zn alloys. Dissolve sample in aqua regia, evaporate twice to dryness with least excess HCl, dilute to 120 mL, filter off any AgCl, add 2 mL HCl plus 2 g NH4OAc.

Pt

Pt

Ag, –0.22 V

Pt

Pt, −0.40 V

Pt, −0.15 V then up to −0.30 V

Pt

Cu/Pt

Pt

Pt

Pt, −0.75 V then −0.90 V

Pt, −0.40 for 15 min,

Pt

Pt

Pt

Pt

Pt

Pt

C.p.

3 A, 40 min.

C.p., AgBr deposited on anode.

C.p., 85°C.

C.p.

C.p., 75°C.

1 A, 1 h.

0.7 V applied to cell (0.1 A initially), 100 min, 60°C; pass N2 or CO2 through solution to displace Cl2 formed.

0.3–0.5 A, 30 min, 30–50°C.

Ag, Bi, Cu, Pb (can be separated from

Cu, Ag, Au, Bi, Hg, Pb, Pt, Sb, Sn

Cl, I

Ag, Cu, Hg (can be separated from Bi at −0.30 V)

Ag, Cu, Pb

Ag, As, Cd, Cu, Hg, Pb, Sb, Sn

Nitrate

Cu, Pd, Pt, Ag, Bi, Cd, Co, Hg, Ni, Zn

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(Continued)

Zn can be deposited at –1.45 V.

0.5 g NiSO4, 0.5 g ZnSO4

Pb, Sn and baser metals

0.1 g Al, Cd, 0.04 g Fe, Mg, Mn, 0.1 g Pb, Sb, 0.2 g Sn, Zn

0.25 g Cu, Sb

Ag, Cd, 0.06 g Cu, Pd, 0.06 g Pt, Zn

Fe

ELECTROANALYTICAL METHODS

14.97

Dissolve 0.1–0.4 g Cu in 10 mL 1 : 2 HNO3 (if Sb or Sn present, dissolve in 5 mL HCl plus minimum HNO3), dilute to 200 mL, add 1 g urea plus 12 g Na2 tartrate plus 2 g N2H4 · 2HCl plus 1 g succinic acid; adjust pH to 5.9. Cu in brass, bronze, white metals. Dissolve in 5 mL HNO3 plus 5 mL HCl, add 10 mL H3PO4, evaporate to remove HCl and HNO3, dilute to 120 mL. 3−30 mg Cu in Fe. Dissolve sample in 1.5 mL H2SO4, dilute to 150 mL, add 0.2 g N2H4 · H2SO4.

Cu Cu in brass, bronze, and Cu–Sn alloys. Dissolve 1 g sample in 20 mL 1: 1 HNO3 without heating, add 10 mL H3PO4, boil 2 min, dilute to 200 mL, add 2 drops 0.1M HCl. Dissolve 0.3 g Cu in 12 mL HNO3, boil, dilute to 150 mL, add 10 mL NH3 plus 2 g N2H4.

Co Dissolve CoSO4 in 100 mL H2O containing 15 mL NH3 plus 3 g NH4Cl plus 0.1 g NH2OH · HCl.

30–150 mg Cl in 100 mL containing 2.7 g NaOAc, adjust to pH 4.7 with HOAc.

Cl

plus 5 mL HNO3, evaporate almost to dryness, take up in 75 mL H2O, add 5 g Na2tartrate plus 1.5 g N2H4 ⋅ 2HCl plus 20 mL 0.1% gelatin. Adjust to pH 4.5−5.0 with NH3, dilute to 175 mL, add 5−10 mg standard Cu solution. HOAc–OAc− buffer, pH 4

Cd

Preparation of solution

Pt

Pt

Pt

Pt

Pt

Pt, −0.73 V

Pt, −0.30 V

Pt, −0.35 V

Pt, −0.40 V

Pt

Ag, −0.25 V

Pt

14.98

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C.p.

C.p.

C.p., purge O2 by bubbling N2 through solution.

2 A, 1.5 h.

2–5 A ⋅ dm−2, 45 min.

C.p., AgCl deposited on anode.

C.p.

Current, time, temperature

Conditions of electrolysis Anode

Pt

Pt

Pt

Pt, −0.80 V

then −1.00 to −1.15 V

Cathode

TABLE 14.26 Electrogravimetric Methods for the Elements (Continued)

Ag, Bi

Ag, Hg (both separated from Cu at −0.24 with O2 present). Ag, Hg

Ag, Bi

Cu, Ni, Zn, Pd, Tl

Br, I

Cd at −0.60 V)

Codeposit or interference

10 g Fe

Cd, Cr, Fe, Mn, Ni, Pb, Sb, Sn, Zn

0.1 g Al, 0.4 g Bi, Cd, 0.04 g Fe, Mg, Mn, 0.1 g Pb, 0.2 g Sn, Zn

Cd, Zn

Sb, Sn, Zn

Zn

Separation from

ELECTROANALYTICAL METHODS

Pb 0.1–0.3 g Pb in metallurgical material. Dissolve in 3 mL H2O plus 5 mL HNO3 plus 5 mL HClO4. Evaporate almost to dryness, add 75 mL H2O, 5 g Na2tartrate plus 1.5 g N2H4 ⋅ 2HCl plus 20 mL 0.1% gelatin. Adjust pH to 4.5–5.0 with NH3. 60 mg Pb as Pb(NO3)2 in 100 mL containing 5 to 9 mL HNO3.

Ni Fume sample (0.1–0.3 g Ni) with 8 mL H2SO4, cool, dilute, neutralize with NH3, cool, add 35 mL NH3, dilute to 200 mL. To dissolved sample (0.15 g Ni) in 100 mL, add 5 g Na2SO4 plus 30 mL NH3 plus 3 g Na2SO3. Dilute to 200 mL. Ammoniacal tartrate plus Na2SO3.

Mn 0.2 Mn in 100 mL water containing 5 mL 77% HCOOH plus 1 g NaOOCH.

In 0.02 g In in salt; dissolve in 100 mL water, add 0.04 g Cu (standard solution). Make alkaline with NH3, then add HCOOH to discharge blue color and redissolve precipitate.

Hg 0.3 g Hg dissolved in 10 mL HNO3 or H2SO4. Add 90 mL H2O.

Ga 0.2 g Ga. Dissolve in 100 mL H2O, add 10 mL NaOH plus 5 g (NH4)2SO4 plus 0.04 g standard Cu.

Fe 0.05 g Fe. Dissolve in 100 mL H2O containing 1 mL H2SO4, add 5 g ammonium oxalate.

Pt

Pt

Pt

Pt, −1.10 V Pt

Pt

Cu/Pt, −0.95 V

Cu/Pt, −0.60 V

Pt

Pt dish

Pt

Pt

Pt

Pt

Cu/Pt

Pt

Pt

Au

Pt

Pt

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C.p. Use cathode precoated with Cu.

C.p.

0.5 A, 3 h. If Co is present, add 2 g NaHSO3 to ensure its quantitative codeposition. C.p., 90°C.

1.4 A · dm−2, 1.5 h unstirred. Hydrated Mn oxide deposits on anode.

4−5 A, In codeposits with known Cu.

1 A, 45 min.

4−5 A, 70°C. Ga codeposits with known Cu.

6 A, 30 min, deposition quantitative at pH 4−9.

>25 mg Fe, Mn, Tl

Ag, Bi, Cu, Hg

Cu, Co, Fe, In, Pd, Tl

Ag, As, Co, Cu, Mo, Zn, nitrate

Pb, Tl

Co, Cu, Fe, Ni, Pd, Tl, Zn

Ag, Au, Bi, Cd, Cu, Pb, Pt, Sb, Sn

Cu, Co, Fe, In, Ni, Pd, Tl, Zn

Co, Cu, Mn, Ni

(Continued)

2 g NH4 salts, Bi, Cu, Sb

Ag, As, Au, Bi, Cd, Cu, Hg, Pb, Pt, Sb, Tl

5 A, 15 min, 45°C.

3.5 A · dm−2, 1 h. CuTe codeposits with Cu from Te(IV) solution.

C.p.

C.p., 45°C.

2 A · dm−2.

Tl

Zn

Te(VI), Zn

Cd, Zn

Zn

(Continued)

ELECTROANALYTICAL METHODS

14.101

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Zn 0.4 g Zn as Zn(SO4). Dissolve in 50 mL H2O, add NH3 or NaOH to incipient precipitation, then 1 g KCN plus 20 mL NH3, dilute to 125 mL. 0.15 g Zn as ZnSO4. Dissolve in 50 mL H2O, add 1 g KNa tartrate, then KOH to just redissolve precipitate. Dissolve 0.1–0.35 g Zn in 10 mL HCl plus 5 mL H2O, add 10 mL 12M NaOH, cool, add 14 g NaCl, dilute to 500 mL. Add 14.3 g Na2 tartrate plus 1 g succinic acid plus 2 g urea, plus 3 g N2H4 · 2HCl; adjust to pH 9 with NH3.

Preparation of solution Pt

Pt

Pt

Cu–Pt

Cu–Pt, C.p.

Bi, Cu, Fe, Pb, Sn

Bi, Cu, Fe, Pb, Sn

−1.40 V until current becomes constant (60 to 100 mA, 40 min), then −1.50 V for 20 min.

Ag, Au, Bi, Cd, Co, Cu, Hg, Ni

Codeposit or Interference

0.3 A, 45 min.

3 A, 30 min.

Current, time, temperature

Conditions of electrolysis Anode

Cu–Pt

Cathode

TABLE 14.26 Electrogravimetric Methods for the Elements (Continued)

Al, Mn, Co, Ni, Sb

0.15 g Ni as NiSO4, Co, Sb

Fe

Separation from

ELECTROANALYTICAL METHODS

14.102

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.103

TABLE 14.27 Determinations by Internal Electrolysis Element

Cathode solution

Ag Ag

HNO3 Ammoniacal solution plus Na2SO3

Ag Bi Bi Cd Co Cu Cu Cu Cu + Bi Cu + Bi

H2SO4 plus N2H4 ⋅H2SO4 H2SO4 HNO3 HOAc–NaOAc buffer plus NH4Cl plus N2H4 ⋅ HCl HOAc–NaOAc buffer H2SO4 plus N2H4 ⋅ H2SO4 H2SO4 H2SO4 plus HNO3 HNO3 plus tartrate buffer plus NH2OH HNO3 plus HF plus tartate buffer

Hg Hg In Ni

HNO3 H2SO4 Tartrate buffer, pH 3.6 Acetate buffer

Pb Sb Sb

HCl plus 0.1% gelatin, or acetate buffer HCl H2SO4 plus tartrate buffer

Sn

HCl

Zn

HOAc−acetate buffer

Anode system Cu metal plus Cu(NO3)2 Cu metal plus Cu(NH3)2+ 4 , ammoniacal solution Cu metal plus CuSO4 Pt metal in V(II)–V(III) Zn metal plus ZnCl2 Zn metal plus ZnCl2 Mg metal plus NH4Cl and HCl Fe metal plus FeSO4 Zn metal plus ZnCl2 Mg metal plus MgCl2 and HCl Pb metal plus Pb(NO3)2 Pb metal plus Pb(NO3)2 Zn metal plus Zn(NO3)2 Cu metal plus CuSo4 Zn metal plus ZnCl2 Mg metal plus MgSO4 and (NH4)2SO4 or HCl and NH4Cl Zn metal plus Zn(NO3)2 Zn metal plus ZnCl2 Mg metal plus MgSO4 or (NH4)2SO4 Zn metal plus ZnCl2 or Mg metal plus MgCl2 Mg metal plus NH4Cl and HCl

Separation from Bi, Cu, Pb Cu Cu, Fe, Ni, Zn

Zn

Fe Fe, Zn Pb, Sb Sb, Sn, Te Zn Cu, Zn Zn

Zn

The anode need not always be constructed of the material that constitutes the matrix of the sample. For selective reduction of several trace constituents in zinc, for example, four separate samples would be dissolved for the separation of traces of silver, copper, lead, and cadmium. In the first, an attackable anode of copper would permit the complete removal of silver. Similarly, a lead anode would make it possible to remove silver plus copper; a cadmium electrode would remove silver, copper, and lead; a zinc anode would remove all four elements. The amount of deposit is generally limited to quantities not exceeding 25 mg. With larger quantities the deposit is apt to be spongy and there is danger that some of the metal ions may diffuse to the anode. Little attention is required during an electrolysis except to flush the anolyte compartment once or twice. Average running time is 30 min per sample. Table 14.27 states the experimental conditions for the internal electrolysis of several metals.

14.7 14.7.1

ELECTROGRAPHY Principle Electrography is used mainly to identify anions and cations capable of being released from specimen surfaces by controlled electrolysis into paper or other porous media and to map the distribution of these ions.

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ELECTROANALYTICAL METHODS

14.104

SECTION FOURTEEN

FIGURE 14.19 Schematic arrangement of equipment and electrical circuit for electrographic analysis.

Basically, the electrographic arrangement is simple. It consists of two metal surfaces between which is sandwiched a layer of absorbent paper (or for better rendition, gelatin-coated paper) moistened with electrolyte. The working electrode may be a flat square electrode for flat surfaces, a long narrow electrode for use on metal ribbons, or sponge rubber covered with aluminum foil for uneven sample surfaces. Pressure is applied to the surface to ensure intimate contact. A general laboratory circuit is shown in Fig. 14.19. If, as is most common, the specimen is the anode, its ions move into the paper where they react, either with the ions of the electrolyte or with an added reagent, to produce an identifiable color product. When anions in thin conducting films, such as chloride or sulfide, are to be identified, the specimen is made cathodic. The current source may be a 12-V storage battery or a 7.5-V C battery. The flat plate in most cases may be aluminum or stainless steel. The second electrode connection may be a flexible cord or cords terminating in any of a variety of clips and probes suitable for making contact on either flat or irregularly shaped specimens. In general, 50 mg of most metals will produce brilliantly colored products if the reaction is confined to an area of 1 cm2. These conditions require a current of 15 mA and an exposure time of 10 s. The electrographic method can be applied for the inspection of lacquer coating and of plated metals for pinholes and cracks in their surface. It can be used for many alloy identifications, such as the differentiation of lead-containing brass from ordinary brass, nickel in steel, and the distribution of metal constituents within an alloy. In the biological field the method is applicable to the localization of constituents that are normally present within the tissue in an ionic state. Portable field kits have found extensive use in inspection and sorting work in the stockroom and in mineralogical field work. Analogous to the anodic oxidation transfer is the cathodic reduction of certain anions of tarnish or corrosion films on metals. These are often tied up as basic insoluble salts and are not detectable in simple contact printing.

14.7.2 Procedure A general outline for producing an electrographic print is as follows: 1. Select the printing condition. a. Standard pad. Use a hardened filter paper similar to Whatman 50 or S&S 576, and a thick, soft backing paper such as Eastman Kodak blotting paper. Gelatin paper or imbibition paper similar to Eastman Kodak transfer paper is sometimes used instead of filter paper. b. Reagent papers. Prepare by immersing hardened filter paper in the first specified solution rapidly and uniformly. Follow by drying. Then immerse in the second solution uniformly, wash well with running water, and dry. If gelatin paper is used, immerse and wash for longer periods of time. The preparation of fixed reagent papers along with the color of reaction products for 12 metals is described in Table 14.28; analogous reagent papers for three anions and printing conditions are described in Table 14.29. 2. Immerse the printing medium in electrolyte. Table 14.30 lists some electrolytes and their uses.

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black blue brown bright

Dk Fl Gn Gy

dark fluorescence green gray

Lt Or Rd Vi

light orange red violet

Wh Wk Yl

white weak yellow

A

A *

Bk

Yl * *

KI‡ A§

A A

Bk

* *

A† a A

Bk

Ag

A

* Dark spot due to reduction of Ag or Mo. † Wash print with dilute HOAc. ‡ Bleach print with H2SO3. § Wash print with ammoniacal ammonium citrate solution. ¶ Fluorescent reagent; examine under ultraviolet light.

Zinc xanthate

Zinc sulfide

2% solution in EtOH 0.25M Zn(OAc)2, then 0.25M K4Fe(CN)6 0.25M Zn(OAc)2, then 0.25M Na2S 0.1M Zn(OAc)2, then 0.1M K xanthate

0.25M Cd(OAc)2, then 0.25M Na2S 1% solution in EtOH 1% solution in EtOH 0.5% solution in EtOH

Preparation

2% Na sulfantimoniate; then dilute HCl 1% solution in EtOH 1% solution in acetone

Reagent paper

Antimony sulfide (orange) a-Benzoinoxime Cd diethyldithiocarbamate Cadmium sulfide (yellow) Cinchonine Dimethylglyoxime Morin¶ (yellow-white) Salicylic acid Zn2[Fe(CN)6]

Electrolyte

Fl

Br Fl

Al

Bi

Bn

Wk Fl

Or

Bn

Bn

Yl

Wk Fl

Cd

Gy Gn

Gn

Bn

Or

Co

Yl

Bk

Gn Yl Rd Br

Lt Yl

Bn

Bk

Gn Lt Bn

Bk

Cu

Bn

Vi Bl

Bn

Fe

Vi

*

Bn

* * *

*

* *

*

Mn

Color of reaction product with

Electrolyte abbreviations are A, 0.5M Na2CO3 + 0.5M NaN3, 3 : 1 (v/v); or 0.5M Na2CO3 + 0.5M NaCl, 3 : 1 (v/v) where Co or Fe predominate; and a, 0.25M ammonium citrate.

Bk Bl Bn Br

Prints are to be prepared by using anodic specimens and current densities of 25 mA⋅ cm −2 for 10–30 s. The following abbreviations are used for the colors of the papers and of the reaction products:

TABLE 14.28 Preparation of Fixed Reagent Papers for Metals

Or

Lt Gn

Or Rd

Ni

Pb

Bn

Wk Fl

Yl

Bn

Bn

Wk Fl

Sn

Fl

Br Fl

Zn

ELECTROANALYTICAL METHODS

14.105

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Ag2CrO4

PbCO3

Ba rhodizonate

Cl−

S2−

SO2− 4

Anion

Reagent paper 0.1M AgNO3, then 0.1M Na2CrO4 0.1M Pb(OAc)2, then 0.1M Na2CO3 0.5M Na rhodizonate, then 0.25M Ba(OAc)2

Preparations

Orangered

White

Red

Paper color

Standard plus veiling

Standard plus veiling Standard

Pad

TABLE 14.29 Preparation of Fixed Reagent Papers and Their Reaction with Anions

0.5M NH4OAc

0.5M Na2CO3

0.5M Mg(OAc)2

Electrolyte

10–20

10–25

5–10

Current density, mA⋅ cm −2

60–120

10–60

10–60

Time, s

Printing conditions (cathodic reduction)

Orange-red is bleached

Black to brown

Red is bleached

Print color

ELECTROANALYTICAL METHODS

14.106

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.107

TABLE 14.30 Composition of Electrolytes for Electrography Electrolyte

Composition

A

0.5M Na2CO3 plus 0.5M NaNO3, 3 : 1 v/v 0.5M Na2CO3 plus 0.5M NaCl, 3 : 1 v/v

Fixing electrolyte for latent prints If iron or cobalt predominates

Principal use

B

0.5M Na2CO3 plus 0.5M Na2SO4, 2 : 1 v/v

Fixing electrolyte to remove interference of lead by masking

C

0.5M Ba(OAc)2

Solution of iron(II) alloys, fixation of chromate ion

D

0.5M NaK tartrate

E

0.5M NaNO3

Solution of iron(II) alloys where reactions of iron are to be masked For tin

3. Drain excess electrolyte from the printing medium, blot, and place between the test specimen and the other electrode. Apply pressure to ensure adequate contact. 4. Connect the circuit and apply the voltage needed for the length of time specified in the tables. 5. Release the pressure, wash in running water, and inspect the print. If a colored product is not obtained directly by the migration of ions from the specimen surface into the printing medium, develop the print with other reagents as directed in the tables. Direct electrographic metal prints and confirming tests for seven common metals are described in Table 14.31. In Table 14.32 the schematic examination of a single print of a pure metal surface for nine elements is outlined. Table 14.33 summarizes the electrographic tests for metals in aluminum, copper, and iron alloys. The table also contains the printing conditions recommended in these electrographic tests. The development procedures are described in detail in Table 14.34. Finally, the electrographic reactions of minerals is summarized in Table 14.35. A brief discussion of electrography may be found in Ref. 12. A thorough treatment of electrography by these same authors is given in Ref. 13. 12 H. W. Hermance and H. V. Wadlow, in Standard Methods of Chemical Analysis, 6th ed., F. J. Welchor, ed., Van Nostrand, New York, 1966, Vol. 3, Pt. A, pp. 500–520. 13 H. W. Hermance and H. V. Wadlow, in Physical Methods in Chemical Analysis, G. Berl, ed., Academic, New York, 1951, Vol. II, pp. 155–228.

TABLE 14.31 Direct Electrographic Metal Prints and Confirming Tests Color of confirming test Fuming with

Metal

Color with electrolyte A* and standard pad

NH3

HCl

Heat and light

Ag Co Cr Cu Fe Mo Ni

Colorless Dirty brown Yellow (CrO2− 4 ) Greenish blue Brown Deep blue-violet Light green

Brown Yellow Deep blue Brown Gray Light violet

Light blue Yellow Green-yellow Orange-yellow Gray Green

Brown to black HCl blue deepens Yellow Green-blue Brown Gray Light green

* See Table 14.30.

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ELECTROANALYTICAL METHODS

14.108

SECTION FOURTEEN

TABLE 14.32 Schematic Examination of a Single Print of a Pure Metal Surface for Nine Common Elements Prepare a print using the standard pad, electrolyte A, and a current density of 25 mA ⋅ cm−2 for 10 s. Hold the print over concentrated aqueous NH3 until it is thoroughly permeated. Classify by color. If group II or III metals are present, cut the print into at least three parts; use two of these for the group treatments and the other for confirmatory tests. In the following outline the symbol of each element is set in boldface type at the point where the element may be positively identified. Group I (strong colored) Clear yellow: Cr (as CrO42−); confirm with 1,5diphenylcarbohydrazide. Light brown: Fe [as Fe(OH)3]; confirm with K4[Fe(CN)6]. Group II Immerse in warm photographic developer (e.g., Eastman D-76), wash well with 1% Na2CO3, then H2O. Gray to black: Ag; confirm with Cr2O72−, giving red Ag2CrO4. Blot and immerse print in 1% K ethylxanthate for 60 s, wash thoroughly and note color. Bright yellow: Cu, confirm with a-benzoinoxime, giving green color. Orange: Ni, confirm with dimethylglyoxime (red color). If results are negative, proceed with group III.

14.8

Group II (weakly colored) Blue-green: Cu, Ni.

Group III (colorless) Cd, Pb, Sn, Zn.

Gray after exposure to light: Ag.

Group III Immerse in solution containing 1 g Na2S plus 2 g NH4OAc in 100 mL H2O, wash in suction apparatus with 1% HOAc until all sulfide is removed. Yellow: Cd; colorless: Zn; brown: Sn; brown-black: Pb. If print is colorless, spot with 5% Pb(NO3)2 or 2% AgNO3. Black: Zn. If print is suspected to contain Pb or Sn, immerse in solution containing 5 g NaOH plus 5 g Na2S plus 1 g S in 25 mL H2O, then wash in suction apparatus with several small portions of this reagent, and then wash with H2O. Brown color disappears: Sn. Brown to black color remains: Pb, confirm with I− or CrO42−.

COULOMETRIC METHODS The major advantage of coulometric methods accrues from the fact that standardized reagent solutions are unnecessary. Titration with electrons eliminates the laborious preparation of primary standard materials. The analyst needs only to obtain a good-quality electrical supply and timer plus small platinum electrodes (occasionally an attackable silver electrode). It is essential that every electron passing through the generating circuit be effective in producing the desired reagent. There must be no side reaction, nothing to reduce the current efficiency from 100%. With the exception of the very different means employed for adding the reagent and for measuring the quantity of reagent used, the coulometric titration method differs but little in either theory or practice from conventional titration methods. The measurement of current and time, and hence coulombs, can be made with extremely high accuracy and precision. The method is applicable in the range from milligram quantities down to microgram quantities. Sensitivity is usually limited only by problems of end-point detection. The minimum amount of material that can be titrated successfully is determined not by difficulties in regulating and measuring small electrolysis current but rather by difficulties encountered in connection with the location of the equivalence point and by side reactions resulting from impurities in the reagents or in the sample. The end-point detection system need not be different from that used in volumetric titrimetry. It may be photometric, potentiometric, or amperometric, to name the most widely used approaches.

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.109

TABLE 14.33 Electrographic Tests for Metals in Aluminum, Copper, and Iron Alloys This table summarizes the printing conditions recommended in electrographic tests for some common constituents in various alloys. The first column lists the alloys in alphabetical order, the second lists the elements to be detected. The third column lists the elements that interfere in the procedure, but that are not normally present. The “electrolyte” column refers to Table 14.30. The last column gives the code number used in Table 14.34 to identify the recommended procedure for the development and interpretation of the print obtained. Printing conditions Metal sought

Alloy

Interferences

Pad

Electrolyte

Current density, mA⋅ cm−2

Time, s

Development procedure number

a std std std

D A A A

20 20 to 30 15 15 to 20

60 60–120 60 60

5 6 7 11

60 60–120

1 6

Al-based

Cu Fe Mn Zn

Cu-based Al bronzes

Al Fe

Be, Bi, Fe, Pb, Sn

std std

A A

20 to 25 20 to 30

Be alloys

Be

Al, Bi, Pb, Sn

std

A

20 to 25

60

3

Brasses and bronzes

Fe Pb Sn Zn

Bi

std std b std

A A E A

20 to 30 15 to 20 15 to 20 15 to 20

60–120 60 60 60

6 9 10 11

Constantin, Ni brass, Ni coinage

Al Mn Ni Zn

Be, Bi, Fe, Pb, Sn Same Same Same

std std c std

A B A A

20 to 25 20 to 25 20 to 25 15 to 20

30–60 60 60 30

2 7 8 11

Mn bronzes, manganin

Mn Ni

Pb Pb

std c

B A

20 to 25 20 to 25

60 30–60

7 8

Fe-based steels

Cr Cu Ni

std a d

C D C

15 20 15

30–60 60–120 60

4 5 4A

a

Cd diethyldithiocarbamate paper. Phosphormolybdate paper plus veiling. Dimethylglyoxime paper. d Dimethylglyoxime paper plus veiling. b c

The error due to impurities in the supporting electrolyte can be eliminated by pretitration, and in most cases errors due to impurities in the reagents used in preparing the sample for titration can be eliminated by blank titrations. The preparation, storage, and standardization of standard solutions are eliminated. Reagents difficult to use or unstable reagents present no problems. They are produced in situ and immediately consumed. Coulometric titration is best suited for small samples. For larger samples, either the time must be increased to inconvenient lengths, or the electrolysis current must be increased to the point at which side reactions are difficult to prevent, so that the necessary 100% current efficiency is lost. At the other extreme, the smallness of samples is limited by one’s ability to control and measure very small currents (less than 100 mA) and to measure short time intervals. The usual difficulties in handling very small samples also apply.

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ELECTROANALYTICAL METHODS

14.110

SECTION FOURTEEN

TABLE 14.34 Electrographic Tests for Metals in Aluminum, Copper, and Iron Alloys: Development Procedures The tests described in this table are applicable to the prints obtained by the procedures outlined in Table 14.33. The first column of this table gives the code numbers that appeared in the last column of Table 14.33; the second column gives the symbol of the element tested for; the third column summarizes the procedure for developing the print; and the fourth column describes the appearance of a positive test and also contains various supplementary remarks. Procedure number

Metal sought

1

Al

Immerse in 1M KCN contg. 1% aq NH3, wash with H2O, using suction apparatus, suck dry. With the suction continued, add dropwise to the print on the pad a satd. soln. of either alizarin or morin in 50% EtOH contg. 5% aq NH3. Wash with 50% EtOH and dil. aq NH3.

Al is indicated by bright red color with alizarin or brilliant yellow-green fluorescence under UV light with morin

2

Al

Wash print in suction apparatus with 0.5M NH4Cl– 0.5M KCN soln. contg. 10% aq NH3, wash with H2O, suck dry, and treat with alizarin or morin as in preceding method.

See preceding method.

3

Be

Immerse in 1M KCN until blue-green color (Cu) has disappeared. Wash with H2O in suction apparatus. Add a satd. soln. of alizarin in 50% EtOH dropwise on the suction pad, sucking through the paper each time. Wash with dil. aq NH3 until the background color is decreased sufficiently to permit recognition of Be lake.

Be is indicated by the lavender color of its alizarin lake, which persists on washing with aq NH3.

4

Cr

Wash the print with H2O in suction apparatus to remove reddish ferric acetate.

Cr is indicated by the presence of yellow BaCrO4.

4A

Cr, Ni

5

Cu

Procedure

Interpretation and remarks

Cr and Ni can be detected simultaneously by using two papers in the sandwich. The one in contact with the specimen removes CrO42− as yellow BaCrO4 (see preceding method), but allows Fe and Ni to pass through to the second paper. This is impregnated with dimethylglyoxime, which ppts. Ni as the red complex. Prepare the printing pad as follows: Immerse the dimethylglyoxime paper and the backing paper in the electrolyte, blot fairly dry, and place on the cathode plate. Immerse the top printing paper in the electrolyte, blot, place on the pad, and print immediately. This prevents excessive soln. and diffusion of dimethylglyoxime into the top sheet. Separate the printing sheets and wash each with Cr is indicated by a yellow color H2O in suction apparatus. on the top print, as in preceding method. Ni is indicated by a red color on the second print. Wash on suction apparatus alternately with H2O and 0.25M KNaTart soln. until color is no longer lost.

0.1–0.3% Cu can be detected by appearance of a light brown color. 0.05% Cu can be detected by first etching the metal in 10% HCl 15–30 min, washing, and blotting dry without rubbing to conc. alloyed Cu on the surface.

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.111

TABLE 14.34 Electrographic Tests for Metals in Aluminum, Copper, and Iron Alloys: Development Procedures (Continued) Procedure number

Metal sought

6

Fe

Wash on suction apparatus with 5% aq NH3–0.5M KNO3 until the blue color (Cu) has entirely disappeared, then wash with H2O. Add dropwise 0.2M K4[Fe(CN)6] contg. 5% HOAc while continuing suction; wash with H2O.

Fe is indicated by the blue color of prussian blue. A pinkish color indicates incomplete removal of Cu; this may usually be dis counted if the Fe content ≥1%, especially if a red filter is used to view the print. In doubtful cases print again and wash more thoroughly.

7

Mn

Immerse in 1M KCN contg. 1% aq NH3, wash with 1% aq NH3 on suction apparatus, and add 5% H2O2 dropwise to print on suction pad. Repeat H2O2 treatment several times, sucking dry each time, wash with H2O, suck dry, and immerse in 1% aq benzidine acetate soln.

Mn is indicated by a green-blue color (benzidine oxdn. product) changing slowly to yellow-brown. Pb is masked by SO2− 4 .

8

Ni

Wash dimethylglyoxime print with 0.2M NH4Cl contg. 10% aq NH3 to remove Cu and Mn.

Ni is indicated by the red color of its dimethylglyoxime complex.

9

Pb

Immerse 2–3 min in 0.2M K2Cr2O7 + 5% HOAc with agitation. Wash with 5% HOAc on suction pad until all Cr2O−7 color is gone from area outside print.

Pb is indicated by the yellow color of PbCrO4, not removed by washing with HOAc.

10

Sn

Use a top veiling sheet of S. & S. No. 576 paper to prevent contact of the phosphomolybdate paper with the surface of the metal. (All metals above Ag in the electromotive series will red. this paper.) Thus only Sn2+ can effect the redn. The paper is light-sensitive and should be freshly prepared and preserved away from light and metals. Prepare paper by impregnation with a soln. contg. 5 g NH4 phosphomolybdate + a little NH3 in 100 mL H2O, dry, immerse in 5% HNO3 in subdued light, wash thoroughly with H2O, dry in darkness, and preserve under compression away from light.

11

Sn, Zn

Procedure

Interpretation and remarks

Immerse print in 2% KOH until the yellow background color is bleached; wash with H2O.

Sn is indicated by the blue color resulting from redn. of phosphomolybdate by Sn2+. Excess rgt. is removed by washing with dil. alk., which gives a light-stable print.

Immerse in fresh 1M KCN–1M Na2S soln. 2–3 min with agitation. Using suction apparatus, wash with 0.25M KCN–0.25M NH4Cl–5% aq NH3, then with 5% Na2Sx (to remove Sn), then H2O, then 2% HOAc. Suck dry, blot, and immerse in 0.5M Pb(OAc)2 (or use spotting or streaking technique if a residual color is obtained).

If Bi, Pb, etc., are absent, and if washings are properly made, the print should bleach to white. Small amounts of Pb give an off-white to yellowish brown color. Sn is still detectable if spotting is used so that the increase in color can be noticed. Zn is indicated by the brown spot of PbS or increased color developing on the bleached original print area.

Source: L. Meites, ed., Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963.

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ELECTROANALYTICAL METHODS

14.112

SECTION FOURTEEN

TABLE 14.35 Electrographic Reactions of Minerals This table summarizes electrographic methods for the detection of various important constituents in a number of common minerals. The symbol “CR” (cathodic reduction) in the third column signifies that the print is made with the specimen as the cathode; otherwise the specimen is understood to be the anode. The abbreviations used to denote the various colors in the last column are defined in the introduction to Table 14.28. Printing conditions Conducting mineral

Element sought

Electrolyte

Voltage, V

Time, s

Reagent for development

Print color

Bismuth (native)

Bi

HCl (1 : 20)

4

30

KI-cinchonine

Or

Bornite

Cu

aq NH3

4

15

Rubeanic acid

Dk Gn

Breithauptite

As Sb

HCl (1 : 1) H2Tart + H3PO4

4 4

30 30

SnCl2 + HCl Methyltrioxyfluorone

Bn Rd

Chalcopyrite

Fe S

HCl (1 : 20) 5% NaOH (CR)

4–8 4

30 30

Chromotropic acid SbCl3 + HCl

Gn Or

Chalcosine

Cu

dil. aq NH3

4

5

Chloanthite

Ni

aq NH3

8

15

Dimethylglyoxime

Rd

Cobaltite

As Co S

aq NH3 + H2O2 (5 : 1) aq NH3

8 4–8

30 30

AgNO3 a-Nitroso-b-naphthol NaN3 + I2

Bn Bn

Copper (gray)

Ag As Cu S Sb

NHO3 (1 : 4) HCl (1 : 1) aq NH3 5% NaOH (CR) 10% H2Tart + H3PO4

8–12 8–12 4–8 8–12 8–12

60–180 60 15 30 60

Redg. agent SnCl2 + HCl Rubeanic acid SbCl3 + HCl Methyltrioxyfluorone

Bk Bn Dk Gn Or Rd

Covellite

S

5% NaOH (CR)

4

15

SbCl2 + HCl

Or

Danaite

As

aq NH3 + H2O2 (5 : 1)

4–8

30

AgNO3

Bn

Danaite

Co S

aq NH3

8–12

60

Rubeanic acid NaN3 + I2

Yl-Bn

Galena

Pb S

HOAc 5% NaOH (CR)

4 4

30 15

KI + SnCl2 SbCl3 + HCl

Yl-Or Or

Rubeanic acid

Dk Gn

4

15

Dimethylglyoxime

Rd

12–16 16

60 180

K4[Fe(CN)6] Chromotropic acid

Bl Rd-Bn

5% KCN aq NH3 HCl (1 : 20) aq NH3 5% NaOH (CR)

8 8 8 8 8

30 30 30 15 15

(Direct print) a-Benzoinoxime Chromotropic acid Dimethylglyoxime SbCl3 + HCl

Yl-Or Gn Gn Rd Or

As Fe

HCl (1 : 1) HCl or HNO3 (1 : 20)

4 4

30 30

SnCl2 + HCl K4[Fe(CN)6]

Bn Bl

Magnetite

Fe

HCl or HNO3 (1 : 10)

4–8

30

K4[Fe(CN)6]

Bl

Marcasite

Fe S

HCl or HNO3 (1 : 20) 5% NaOH (CR)

4 4

20 15

K4[Fe(CN)6] SbCl3 + HCl

Bl Or

Millerite

Ni S

aq NH3 5% NaOH (CR)

4 4

10 15

Dimethylglyoxime SbCl3 + HCl

Rd Or

Mispickel

Fe

HCl or HNO3 (1 : 20)

4–8

30

K4[Fe(CN)6]

Bl

Nickeline

Ni

aq NH3

4

10

Dimethylglyoxime

Rd

Pentlandite

Fe Ni S

HCl or HNO3 (1 : 20) aq NH3 5% NaOH (CR)

4 4 4

30 15 15

K4[Fe(CN)6] Dimethylglyoxime SbCl3 + HCl

Bl Rd Or

Gersdorffite

Ni

aq NH3

Ilmenite

Fe Ti

HCl or HNO3 (1 : 10) 25% H3SO4 + H3PO4

Linnaeite

Co Cu Fe Ni S

Lollingite

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.113

TABLE 14.35 Electrographic Reactions of Minerals (Continued) Printing conditions Conducting mineral

Element sought

Electrolyte

Pyrites

Voltage, V

Time, s

Reagent for development

Print color

See Marcasite above

Pyrrhotine

See Marcasite above

Rammelsbergite

As

aq NH3 + H2O2 (5 : 1)

4

30

AgNO3

Bn

Safflorite

Fe

HCl or HNO3 (1 : 20)

8

60

K4[Fe(CN)6]

Bl

8

30

a-Nitroso-b-naphthol

Bn

4–8 8 8

30 30 60

SnCl2 + HCl SbCl3 + HCl Methyltrioxyfluorone

Bn Or Rd

Smaltite

Co

dil. aq NH3

Ullmannite

As S Sb

HCl (1 : 1) 5% NaOH (CR) H2Tart + HNO3

14.8.1 Controlled-Potential Coulometry 14.8.1.1 General Principles. Controlled-potential coulometry employs the same equipment as used in controlled-potential electrolysis with the addition of a coulometer. A mercury pool is often used for reduction processes; oxidations can be performed at a cylindrical platinum electrode. The potential of the working electrode is controlled within 1 to 5 mV of the control value by a potentiostat. Current-potential diagrams must be determined for analyte system and for any possible interfering system. The necessary data can be obtained in two ways: 1. Set the potentiostat to one cathode-reference potential after another in sequence, allowing only enough time at each setting for the current indicator to balance. 2. Perform the coulometric analysis in the usual manner. Periodically adjust the potential to a value that stops the current flow. Note the net charge transferred up to each adjustment point; a plot of number of coulombs versus the potential provides a coulogram. A controlled-potential coulometric electrolysis is like a first-order reaction, with the concentration and the current decaying exponentially with time during the electrolysis [Eq. (14.50)] and eventually attaining the residual current of the supporting electrolyte. The concentration limits vary from about 2 meq down to about 0.05 meq (set by the magnitude of the residual current). Advantages of controlled-potential coulometry are these: (1) No indicator-electrode system is necessary. (2) It proceeds virtually unattended with automatic instruments. (3) Optimum conditions for successive reactions are easily obtained. As in voltammetry, it is necessary to prereduce the supporting electrolyte in a coulometric reduction. To do this step, add the sample and deaerate the system before the reduction is started. Standards should be run under the same conditions that will be used with the samples. In order to achieve 100% current efficiency, the generating current must not exceed the diffusion current (even with stirring). As the desired constituent is removed from solution by reaction at the electrode surface, the diffusion current decreases from a relatively large value at the start of the determination to essentially zero at the equivalence point. Thus the potential of the working electrode must be controlled at some value on the diffusion current plateau in order to achieve the necessary 100% current efficiency. In many ways coulometric titrations at controlled potentials are very similar to electrogravimetric determinations at controlled potentials, the major difference being that the desired constituent is estimated from the number of coulombs in the former method and from the weight of the deposit in the latter method. The coulometric method is more versatile, since it can be applied to determinations in which the electrolysis product is a gas, a species in solution, or an amalgam, none of which can be weighed readily (see Table 14.36).

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0.1M NaOAc, 0.1M HOAc; +0.16 ± 0.05 V; Pt,Ag–SCE–Pt 0.1M KCl; PES −1.0 V, scan −1.0 to −0.3 V; Pt,Hg or Ag,Hg–SCE; anodic stripping, 0.02 V per second Acetate buffer, pH 5; +0.25 V; Pt,Ag−SCE−Pt 1M pyridine, 0.3M HCl, 0.2M N2H4 ⋅H2SO4, pH 7; PESE, PES −0.95 V, −1.20; Hg–SCE–Ag 1M NH3, 1M NH4Cl; PES −1.10 V, −1.45 V; Hg–SCE || Pt 0.5M H2SO4 containing NaI; +0.1 V; Pt−SCE || Pt 0.1M NH3, 0.1M NH4Cl; PESE, −0.75 V; Hg–SCE || Pt 0.1M HCl (or HClO4) or 0.5M citrate buffer, pH 5.5; PESE −0.1 V (or −0.5 V); Hg–SCE || Pt 0.5M H2SO4; PESE, −0.3 to −0.5 V; Hg–SCE || Pt (or C) 2.4M HClO4; −0.2 V; Pt–SCE || Pt In slags and Cu–Au alloys, potentiometrically

Br−

0.2M HClO4, 0.008M Ce(III); scan +0.8 to +0.2 V; Au–SCE 0.025M acetate buffer, pH 5; +0.22 V; Pt–SCE || Pt,Ag

Fe(III)

Fe(CN)3− 6

Fe(II)

0.1M HCl (or HClO4); PES −0.9 V, −0.1 V; Hg–Ag,AgCl || Pt 1M H2SO4; +0.9 to +1.0 V; Pt–SCE || Pt In slags and Cu–Au alloys, potentiometrically

Eu(III)

Cr(VI) Cu(II)

Cl− Co(II)

Cd(II)

0.4M Na2tart, 0.1M NaHtart, 0.2M NaCl; PESE −0.7 V, PES −0.24 V, then −0.35 V; Hg–SCE–Ag

Ag, F−, Hg, PO3− 4 , Pd, Pt, Pu, Ru Cl−

SO2− 4 , >0.016M HNO3

Zn

Br−, CN−, Fe(III), >1M HNO3, Pu(III), Ru

2.4M HClO4; +0.15 V; Pt–SCE–Pt 1M HCl; PES +0.75 V, then +0.45 V, Pt–SCE || Pt; deposited Au may be anodically stripped at +1.20 V

In slags and Cu–Au alloys, potentiometrically

Interferences

Supporting electrolyte; working electrode potential; electrodes; notes

Bi(III)

Ag(I) Au(III)

Substance determined

Abbreviations: PES, preelectrolyze sample solution at potential stated; PESE, preelectrolyze supporting electrolyte alone at potential stated, add sample and electrolyze at potential stated (if different).

TABLE 14.36 Controlled-Potential Coulometric Methods

Au, Cl−, Cu, NO−3 , Np, Pb, SO2− 4

Al, Ca, Ce, traces Fe, Cd, La, Si, Y, Yb

Cu, Ni

See Br− Ni(II)

I− at pH 5 in presence of 5% Ba(NO3)2

Cu(II)

Ag, OAc−, F−, Hg(II), PO3− 4 , Pb(II), Pd(II), SO2− 4 , strong oxidants

No interference

Z. Anal. Chem. 169:102 (1959)

Anal. Chem. 24:986 (1952) Anal. Chim. Acta 63:129 (1973) Anal Chem. 31:1095 (1959)

Anal. Chem. 31:492 (1959) Anal. Chem. 30:487 (1958) Anal. Chim. Acta 63:129 (1973) Anal. Chem. 31:1095 (1959)

Anal. Chem. 30:487 (1958) Anal. Chem. 27:1116 (1955) Anal. Chem. 27:1116 (1955)

Anal. Chem. 25:274 (1953) Anal. Chim. Acta 13:281 (1955) Anal. Chem. 28:404 (1956)

Anal. Chem. 25:1393 (1953)

Anal. Chem. 25:274 (1953)

Anal. Chim. Acta 63:129 (1973) J. Am. Chem. Soc. 67:1916 (1945)

Anal. Chem. 30:487 (1958) J. Electroanal. Chem. 3:112 (1962)

References

ELECTROANALYTICAL METHODS

14.114

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3M NaBr, 0.3M HCl; PESE −0.8 V, PES −0.4 V [to Sn(II)], −0.7 V; Hg–SCE || Pt 1M NaOH; −0.6 V (to Te metal); Hg–SCE || Pt

Sn(IV)

Te2−

1M NH4Cl, NH3, pH 8; −1.65 V; Hg–SCE || Pt

Oxidation at 0.73 V vs. AgCl of Pu(III); reduction at 0.33 V Pu(IV) to Pu(III); mixed H2SO4 and HNO3; Au electrode 0.03M H2SO4, 0.03M bathophenanthroline; reduction Pu(IV) to (III) and Fe(III) to (II) at +0.30 V; stand 30 min, then oxidize Pu to (IV) at +0.66 V 0.2M HCl; −0.20 V to Rh(0); Pt cathode Sb(V) to (III) at −0.21 V and (III) to (0), Sb(Hg) at −0.35 V; 0.4M tartaric acid, 6M HCl 0.2M KNO3; +0.38 V (H2O), +0.28 V (MeOH); Ag–SCE–Pt 1M NH4Cl, NH3, pH 8; −0.4 V; Hg–SCE || Pt

Se(IV)

Se2−

SCN−

Rh(III) Sb

Pu(IV)

Pb(II)

Os(VI)

0.1M KCl or KNO3; PES −1.0 V; scan −0.7 to −0.2 V; Pt,Hg–SCE 1M H3Cit, 0.1M Al2(SO4)3, KOH, pH 4.5; PES [to Pu(III)], −0.07 V; Hg–SCE || Pt

1M pyridine, 0.3M HCl, 0.2M N2H4 ⋅ H2SO4, pH 7; PESE −0.95 V; Hg–SCE–Ag; Co may be subsequently determined in same solution >3M HCl; −0.3 V; Hg–SCE || Pt 0.1M–10M NaOH; −0.35 V [to Os(IV)], then −1.0 V [to Os(II)]; Hg–SCE || Pt 0.5M KCl; −0.50 V; Hg–SCE–Ag

Ni(II)

Ir(IV) Mn(II) Mo(VI)

In(III)

0.5M acetate buffer, pH 5; PESE, +0.32 V; Ag–SCE || Pt 1M HClO4, 1M NaI (12 : 1 ratio I− :In); −0.615 V for In(III) to In(Hg) 0.2M HCl; +0.25 V to Ir(III); Pt cathode 0.25M Na4P4O7 at pH 2; to Mn(III) at +1.10 V 0.3M HCl, NaOAc, pH 1.5−2.0; −0.4; Hg–SCE–Ag

Fe(CN)4− 6

Tl

As(III), Ce(III), Sb(III), Tl(I)

Anions giving insoluble Ag salts In HClO4, Ag(I) reduced 0.0 V and Cd(II) at −0.63

Rh As, Fe, Ni, Pb, Sn, U

Fe

Cu, little Fe, Hg, Pb, Pd, U(VI)

Cd

Cd

20–200 mg Co

Rh(III) Cr(III), Cl− Cr(III)

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(Continued)

Chem. Abst. 51:16201a (1957) J. Am. Chem. Soc 70:4115 (1948) J. Am. Chem. Soc 71:196 (1949) Anal. Chim. Acta 22:577 (1960) J. Am. Chem. Soc. 70:4115 (1948)

Anal. Chem. 43:602 (1971) Anal. Chem. 34:499 (1962)

Anal. Chem. 43:603 (1971)

J. Am Chem. Soc. 79:4631 (1957) J. Am. Chem. Soc. 67:1916 (1945) Anal. Chim. Acta 11:574 (1954) U.S. Atomic Energy Commission Report No. HW-58491 (1958) Talanta 19:1321 (1972)

Anal. Chem. 43:602 (1971) Anal. Chem. 41:758 (1969) Chem. Abst. 52:12668b (1958) Anal. Chim. Acta 13:281 (1955)

Z. Anal. Chem. 179:342 (1961) Anal. Chem. 43:607 (1971)

ELECTROANALYTICAL METHODS

14.115

Zn(II)

Yb(III)

V(V)

U(VI)

U(IV)

Tl(I)

Ti(IV)

Te(IV)

Substance determined

0.1M Et4NBr in MeOH containing known amount Eu(III), −1.20 V; Hg–Ag,AgBr || Ag. Determine Yb by difference (reaction induced by Eu) 2M NH3, 1M (NH4)3Cit; PES −1.1 V, change Hg, then −1.45 V; −0.5 to −1.0 V; Hg–SCE || Pt

Oxidation at 0.73 V vs. AgCl; Au electrode; H2SO4 and HNO3 1M H3Cit, 0.1M Al2(SO4)3, KOH, pH 4.5; PES −0.2, then −0.6 V; Hg–Ag,AgCl || Pt 0.5M H2SO4; PES + 0.175 V, −0.2 V; Hg–Ag,AgCl || Pt Reduction to V(IV); media effects studied

9M H2SO4; Ti(IV) to (III) at −0.20 V; reoxidation at +0.22 V allows both oxidation states to be determined 1M H2SO4; PESE +1.38 V, then +1.34 V; Pt–SCE–Pt 1M HClO4 (or HNO3 or H2SO4 plus 3M H3PO4), sulfamic acid; +1.4 V; Pt–Ag,AgCl || Pt

0.5M H3Cit, pH 1.6; −0.65 V; Hg–SCE || Pt

0.5M NH4Cl, NH3, pH 9.4; −0.9 V; Hg–SCE || Pt

Supporting electrolyte; working electrode potential; electrodes; notes

TABLE 14.36 Controlled-Potential Coulometric Methods (Continued)

Co

Interferences studied

As(III), Br−, CN−, Ce(III), Cl−, Cu(I), Fe(II), Hg(I), I−, Mn(II), Mo(III), Ru, Ti(III), V(IV)

As(III), Bi(II), Cu(II), Mo(VI), Se(VI), Te(IV)

Interferences

Al, Ce, Cu, Fe, HCl, Hg(II), HNO3 Cr, Cu, Mo, Sb

Co(II), mg amounts Cr(III), Th

Fe, Nb, V, W, Zr

No interference References

Anal. Chim. Acta 2:456 (1959)

Anal. Chem. 31:492 (1959) Analyst (London) 98:553 (1973) Anal. Chem. 32:1417 (1960)

Anal. Chem. 31:10 (1959)

Talanta 19:1321 (1972)

Anal. Chem. 33:1016 (1961)

Anal. Chem. 28:1101 (1956)

J. Am. Chem. Soc. 71:196 (1949) J. Am. Chem. Soc. 71:196 (1949) Anal. Chem. 43:747 (1971)

ELECTROANALYTICAL METHODS

14.116

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14.117

14.8.1.2 Application Examples. In a mixture of uranium and chromium, both metals can be prereduced at −0.15 V to uranium(III) and chromium(II). Now, if electrolysis is carried out at −0.55 V, only uranium(III) is oxidized to uranium(IV), not with 100% current efficiency but completely. Chromium(II) is then determined by oxidation to chromium(III) at −0.15 V. Mixtures of two reversible oxidation–reduction states are handled as follows for vanadium(IV)– vanadium(V). At 0.7 V versus SCE, vanadium(IV) is oxidized to vanadium(V); the reduction of vanadium(V) will occur quantitatively at 0.3 V versus SCE. In a mixture of vanadium(IV) and vanadium(V), control the anode potential at 0.75 V and measure the number of coulombs involved in the oxidation of vanadium(IV). Reverse the working-electrode potential, control the cathode potential at 0.3 V versus SCE, and measure the coulombs required for the reduction of original and generated vanadium(V). The difference in number of coulombs between the cathodic reduction and the anodic oxidation gives the original vanadium(V) concentration. Table 14.36 summarizes the conditions that have been used and the results that have been obtained in determining various inorganic species by controlled-potential coulometric analysis.

14.8.2 Constant-Current Coulometry Chemical reagents are generated within the supporting electrolyte in constant-current coulometry (often called coulometric titrations). A constant current is maintained through the electrochemical cell throughout the reaction period. The quantity of unknown present is given by the number of coulombs (measuring the product of current and time) of electricity used. The use of chemical intermediates not only facilitates the determination of many substances that can be determined by primary processes, but makes the coulometric method applicable to many determinations that cannot be carried out at all by primary electrode processes. The problem is to find electrode reactions that proceed with 100% current efficiency and suitable end-point detection systems. 14.8.2.1 Primary Coulometric Titrations. Only electrodes of silver metal, mercury, or mercury amalgams, or electrodes coated with silver-silver halide, are suitable sources of the electrogenerated species, For example, the silver ions generated at a silver anode will react with mercaptans dissolved in a mixture of aqueous methanol and benzene to which aqueous ammonia and ammonium nitrate are added to buffer the solution and supply the supporting electrolyte. The end point is determined amperometrically; excess silver ions will generate a signal at a platinum indicator electrode. Before the mercaptan sample is added, free silver ion is generated to a predetermined amperometric (current) signal. The sample is added and the generation continued until the same amperometric signal is attained again. Chloride ion in biological samples is determined in a similar manner. Combustion in an oxygen flask precedes the titration step for nonionic halides in organic compounds. 14.8.2.2 Secondary Coulometric Titrations. Secondary coulometric titrations are the most frequently used coulometric technique. These conditions must be met: 1. An active intermediate ion must be generated from an oxidation–reduction buffer (titrant precursor) added in excess to the supporting electrolyte. 2. The intermediate must be generated with 100% efficiency. 3. The intermediate must react rapidly and stoichiometrically with the substance being determined. 4. The standard potential of the titrant precursor must lie between the potential “window” of the unknown redox system and the potential at which the supporting electrolyte or another sample constituent undergoes a direct electrode reaction. 5. An end-point detection system must be available to indicate when the coulometric generation should be terminated. An example will aid the discussion. Consider the coulometric titration of iron(II) to iron(III). The direct coulometric method will not succeed with 100% efficiency unless the potential is carefully

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controlled. When a finite current is forced to flow through the electrochemical cell, the current transported by the iron(II) ions soon falls below that demanded by the imposed current flow. However, if excess cerium(III) ions are added as the titrant precursor, they will begin and continue to transport the current. At the anode the cerium(III) ions will be oxidized to cerium(IV) ions, which will immediately react with unoxidized iron(II) ions. The reaction is stoichiometric and cerium(III) is reformed. The total coulombs ultimately required will be the sum needed for the direct oxidation of iron(II) and for the indirect oxidation via cerium(IV). Because there is an inexhaustible supply of cerium(III), the anode potential is stabilized at a value less positive than the oxidation of water, which would destroy the coulometric efficiency required. The end point is signaled either potentiometrically with a platinum reference-electrode pair or spectrophotometrically at the wavelength where the first excess of unused cerium(IV) absorbs strongly. 14.8.2.3 Instrumentation. The instrumentation required consists of an operational amplifier to force a constant current through the generator cell and some means to measure the electrogeneration time. A manual circuit can be assembled easily. The current source can be a heavy-duty dry cell (6 V) or several B batteries connected in series with an adjustable rheostat (to control the current level), a precision resistor with a potentiometer connected across its terminals (to measure the current), and the generator electrodes. If electrolytic products generated at the counterelectrode interfere with the reactions at the working electrode, the counterelectrode must be isolated from the remainder of the electrochemical cell by a porous glass frit or another type of salt bridge. The solution in the electrochemical cell is stirred throughout the titration. 14.8.2.4 Applications. Secondary coulometric methods enable uncommon, but useful, titrants such as bromine, chlorine, chromium(II), copper(I), silver(II), titanium(III), and uranium(III and V) to be generated in situ. Ordinarily these solutions would be difficult or impossible to prepare and store as standard solutions. Even electrolytic generation of hydroxyl ion offers the advantages of preparing very small amounts for the determination of very dilute acid solutions, such as would result from adsorption of acidic gases, and in a carbonate-free condition. Coulometric reagents and their precursors are listed in Table 14.37. In an aqueous solution the strongest oxidant is silver(II) (E 0 ≈ 2 V) and the strongest reductant is uranium(III) (E 0 = −0.63 V). The electrogeneration of these titrants requires that the kinetics of their reactions with water and hydrogen ion, respectively, be slow. Internally generated halogens, particularly bromine, have widespread applications, especially in organic analysis. Bromine can be easily generated with 100% current efficiency from a solution consisting of dilute acid and 0.2M sodium bromide. Sodium and lithium bromides are quite soluble in various organic solvents in which bromination can be conducted. Bromine is generated at the platinum anode, while hydrogen is generated at a platinum cathode that is isolated in a fritted glass tube. Iodine can similarly be generated from iodide solutions ranging from strongly acid to approximately pH 8 with 100% current efficiency. Strict control of reagent concentration is required for the generation of chlorine. The coulometric Karl Fischer titration allows the determination of microgram amounts of water in organic liquids and of moisture in gases. Iodine is generated from an iodide salt in anhydrous methanol plus amine solvents that contain sulfur dioxide. Azo dyestuffs can be titrated with titanium(III) generated externally and delivered to the hot dye solution via a capillary delivery tube. External generation guarantees that an optimum set of generation and reaction conditions prevail for each step. A double-arm electrolytic cell with separate anode and cathode delivery tubes (one to the sample and other to waste) is used for external generation. The coulometric titrations listed in Tables 14.38a and 14.38b do not include all examples available in the literature. If several reagents are applicable, only the most common and convenient reagents are listed. Not all organic compounds have been listed; however, many determinations can be selected by finding a compound with the same functional group. Table 14.37 should be consulted for details of reagent generation. The titration conditions are given in detail if they differ from those specified for generation of the reagent. The precursor is often not listed unless its concentration varies from the usual generation conditions. Detailed titration conditions are not given for those cases in which consultation of the original reference is desirable. Inert atmospheres should be used for all strongly reducing systems and will be specified only for titrations in which their use might not be readily apparent.

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14.119

TABLE 14.37 Coulometric Reagents and Their Precursors Unless otherwise indicated, precursor concentrations are 0.05M–0.1M and generator electrode (platinum except as otherwise indicated) areas or 2–5 cm2 are employed with generating currents up to about 50 mA. Variations from these conditions may usually be calculated by estimating the ratio of current density to precursor concentrations as 0.5 mA⋅cm−2 ⋅ mmol −1 ⋅L−1. In some cases, 100% current efficiency can be obtained only by using current densities smaller than this criterion would indicate; such cases are indicated by the appearance of a current density (mA⋅cm−2) in the second column (represented by the symbol i/A). Nitrogen atmospheres must be used in work with reducing agents as strongly reducing as Fe(II). For a general reference, see J. J. Lingane, Electroanalytical Chemistry, 2d ed., Interscience, New York, 1958. Reagent Ag(I) Ag(II) Br2 Br − BrO − Ca(II) Ce(IV) Cl2 Cl− Cu Cu(I) Fe(II) Fe(CN)63− Fe(CN)4− 6 Fe(EDTA)2− + H

Hg2(II) Hg(II) HSCH2COO− H2EDTA2− H(EDTA)3− I− I3− Karl Fischer reagent Mn(III) OH−

Precursor; solution composition; conditions Ag anode; 0.5M HClO4 or 0.2M NH3 + 0.05M NH4NO3 0.1M AgNO3; 5M HNO3; Au anode, 0°C, i/A = 2 to 25 mA⋅ cm− 2 0.2M Br−; H2SO4, HClO4, or HCl (pH < 5) Ion-exchange membrane 1M Br−; Na2B4O7 buffer, pH 8–8.5 (avoid NH3 in reagents) Ion-exchange membrane Saturated Ce2(SO4)3; >3M H2SO4; i/A = 1 − 10 0.1M–2M Cl−; HCl, H2SO4, or HClO4 (pH < 1) Ion-exchange membrane Saturated CuI; 1M NaI plus HOAc or KH phthalate buffer, pH 3.5 Cu(II); 1M–3M HCl; CuCl32− is reductant Fe(III); 1M H2SO4 plus 0.1M H3PO4; for potentiometric end point [Fe (III)/unknown] 2 H2O; 1M Na2SO4 H2O; 0.05M LiClO4 in acetonitrile plus hydroquinone (0.1 g in 100 mL) HOAc; 0.1M NaClO4 in acetic anhydride–HOAc (6 :1); Hg anode Hg or Hg-plated Au anode; 0.5M NaClO4 plus 0.02M HClO4 Hg or Hg-plated Au anode; phosphate buffer, pH 9–12; internal generation only Hg(SCH2COO)2;HOAc–NaOAc or NH3–NH4OAc buffer (pH 5–10); Hg cathode, N2 atmosphere Ion-exchange membrane 0.02M HgNH3(EDTA)2−; 0.05M NH4NO3 plus NH3 to pH 8.5; Hg cathode, N2 atmosphere, i/A 0.2M Mn(II); >1.8M H2SO4; N2 atmosphere, i/A = 1 to 4 H2O; 1M Na2SO4 Internal generation: H2O; 0.05M KBr; Ag anode, i/A < 5

References Anal. Chem. 26:622 (1954) Anal. Chim. Acta 18:245 (1958) J. Am. Chem. Soc. 70:1047 (1948) Anal. Chem. 32:1240 (1960) Anal. Chem. 28:440 (1956) Anal. Chem. 32:1240 (1960) Anal. Chim. Acta 16:165 (1957) Anal. Chem. 22:889 (1950) Anal. Chem. 32:1240 (1960) Anal. Chem. 28:1510 (1956) J. Am. Chem. Soc. 71:2340 (1949) Anal. Chem. 24:1057 (1952) Anal. Chim. Acta 13:184 (1955) Anal. Chim. Acta 11:475 (1954) Anal. Chem. 28:520 (1956) Anal. Chem. 23:941 (1951); 27:1475 (1955) Anal. Chem. 28:916 (1956) Anal. Chim. Acta 21:468 (1959) Anal. Chem. 28:797, 799 (1956) Anal. Chem. 30:65, 1064 (1958) Anal. Chem. 32: 524 (1960) Anal. Chem. 32:1240 (1960) Anal. Chem. 28:443 (1956) Anal. Chem. 32:1240 (1960) Anal. Chem. 22:332 (1950) Anal. Chem. 31:215 (1959) Anal. Chim. Acta 21:536 (1959); 12:382, 390 (1955) Anal. Chem. 23:938, 941 (1951) Anal. Chim. Acta 11:283 (1954) (Continued)

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SECTION FOURTEEN

TABLE 14.37 Coulometric Reagents and Their Precursors (Continued) Reagent S2O42− Sn(II) Ti(III) U(V) U(IV) V(IV)

Precursor; solution composition; conditions 0.01M HSO−3 ; phthalic acid buffer, pH 3–5; Hg cathode, i/A = 1 0.2M SnCl4; 3M–4M NaBr plus 0.2M HCl; do not clean Pt cathode, i/A = 10 to 85 0.6M TiOSO4; 6M–8M H2SO4 or >7M HCl; i/A < 3; remove O2 UO2Cl2 or UO2(ClO4)2; HCl to pH 1.5–2.5; i/A < 2.5 at U(VI) = 0.1M; >0.03M nitrate interferes UO2SO4; 0.25M H2SO4; i/A < 2 at U(VI) = 0.1M NaVO3; 0.5M–3M H2SO4

References Talanta 1:110 (1958) Anal. Chim. Acta 20:463 (1959) Anal. Chim. Acta 15:465 (1956); Anal. Chem. 27:741 (1955) Anal. Chem. 28:1876 (1956) Anal. Chem. 27:1750 (1955) Anal. Chem. 31:1460 (1959)

14.8.2.5 Corrosion or Tarnish Films. The thickness of corrosion or tarnish films can be measured coulometrically. The specimen is made the cathode and the film is reduced with a constant known current to the metal. By following the cathode potential, the end point is taken as the point of inflection of the voltage–time curve. Anodic dissolution is used to determine the successive coatings on a metal surface. For example, the thickness of a tin undercoating and a copper–tin surface layer on iron can be measured because the two coatings exhibit individual step potentials. From the known current i, expressed in milliamperes, and the elapsed time t, in seconds, the film thickness d, in nanometers, can be calculated from the known film area A, in cm2, and the film density r, according to the equation d=

10 4 Mit AnFr

(14.51)

where M is the molecular weight of the tarnish film and F is the faraday.

14.9

CONDUCTANCE METHODS One of the oldest and in many ways simplest of the electrochemical methodologies is the measurement of electrolytic conductance. Practical applications are of three types: direct analysis, stream monitoring, and titration.

14.9.1

Electrolytic Conductivity Solutions of electrolytes conduct an electric current because the ions migrate under the influence of a potential gradient applied to two electrodes immersed in the solution. The positive ions (cations) are attracted to the negative electrode (cathode) while the negative ions (anions) are attracted to the positive electrode (anode). The flow of current depends upon the magnitude of the applied potential and the resistance of the solution between the electrodes, as expressed by Ohm’s law. The reciprocal of the resistance 1/R is called the conductance S and expressed in siemens (or reciprocal ohms, or mhos, older terms but not SI nomenclature). The conductance is directly proportional to the cross-sectional area A of the electrodes and inversely proportional to the distance d between them: 1 A = S =k R d

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(14.52)

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ELECTROANALYTICAL METHODS

TABLE 14.38a

No.

14.121

Procedures for Coulometric Titrations: Inorganic Substances

Substance determined

Reagents; titration conditions; end point; notes

1

Ag(I)

2

Al(III)

OH−; 48% EtOH, 0.05M Na2SO4, 0.006M oxine; Eglass-SCE

Anal. Chim. Acta 19:272 (1958)

3

As(III)

Br2; 0.1M–1M H2SO4; EPt-SCE, iPt-Pt (0.2 V)

J. Am. Chem. Soc. 70:1047 (1948)

Ce(IV); H2SO4, OsO4 catalyst; A320,360,375 I2; HCO3−, pH 8; EPt-SCE, iPt-Pt (0.15 V), A342

Anal. Chem. 28:515 (1956) Anal. Chem. 22:332 (1950)

Au(III)

Cu(I); 1M−2M HCl, 0.04M CuSO4; EAu-SCE Hg(EDTA)2−; measure Hg(II) displaced HSCH2COO−; NH3–NH4OAc buffer, pH 7.5; E Hg–Hg,Hg2SO4, iHg-Hg (0.15V) Sn(II); 4M NaBr, 0.3M HCl; EAu-SCE, iAu-Au (0.15 V)

Anal. Chim. Acta 19:394 (1958) Talanta 26:445 (1979) Anal. Chem. 32:524 (1960)

Br2(BrO3−)

Cu(I); 1M NaBr, 0.02M CuSO4, 0.3M HClO4; iPt-Pt (0.15 V) Sn(II); NaBr, HCl; EPt-SCE (end point at +0.38 V), iPt-Pt (0.15 V); pretitrate Br2

Br−

Ag(I); 0.5M HClO4, use 75% acetone for Br− 3M H2SO4, 0.1M H3PO4; EPt-SCE

Anal. Chem. 28:916 (1956)

122

1-Octene

Br2; see no. 89

122a

Olefins

Br2; 20–1000 ppm vapor, 82 mL HOAc, 15 mL H2O, 3 mL ethylene glycol, 2 g KBr

Anal. Chem. 34:418 (1962)

123

Oleic acid

Br2; 0.5M HBr, 85% HOAc; iPt-Pt (0.3 V); styrene does not interfere

Chem. Listy 52:1899 (1958)

124 125

Cl2; 1.2M HCl, 80% HOAc; iPt-Pt (0.35 V); styrene titrates Oxalic acid

Ce(IV)–Fe(II); see no. 121

126

Ag(II): HNO3; iPt-Pt (0.075 V); preoxidize indicator electrodes; insert after visual end point

Anal. Chim. Acta 18:245 (1958)

127

Mn(III); H2SO4; E Pt–Hg,Hg2SO4, 1,10-phenanthroline iron(II)

Anal. Chim. Acta 12:382, 390 (1955)

128

OH −; see no. 81

129 130

1-(or 2-)Pentene 2-Pentene, 4methyl (cis and trans)

Br2; see no. 89; includes 2,3,3- (or 2,4,4,-)trimethyl substituents Br2; see no. 88

(Continued)

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SECTION FOURTEEN

TABLE 14.38b

No. 131

Procedures for Coulometric Titrations: Organic Substances (Continued)

Substance determined o-Phenylenediamine

Reagents; titration conditions; end point; notes H+;

References

see no. 120

132

Phenol

Br2; pH 0.5; iPt-Pt (0.2–0.3 V); see no. 146

133

Phenol, p-amino-

Ce(IV); see no. 114 H+; see no. 120; p-aminophenol also titrates

134 135

Phenol, 4-nitro-

H2; see no. 83

136

Phenol, 4methylamino-

Ce(IV); see no. 114

137

Propanoic acid

OH−; see no. 81

138

Pyridine

H+; see no. 87

139

Pyrocatechol

Br2; pH 4–5; iPt-Pt (0.3 V)

140

8-Quinolinol

Br2; 0.2M NaBr, 0.001M–0.0001M HCl; iPt-Pt (0.25 V); 0.4–2 mg oxine per titration

Anal. Chem. 22:1565 (1950)

141

Quinone

Sn(II)-Br2; 0.2M HCl, 3M KBr, 0.2M SnCl4; iPt-Pt (0.15 V); generate excess Sn(II), back-titrate with Br2

Anal. Chim. Acta 20:463 (1959)

142

Resorcinol

Br2; see no. 139

143

Salicylic acid

Br2-Cu(I); 0.3M HCl, 0.05M CuSO4, 0.1M KBr; iPt-Pt (0.25 V); generate excess Br2, wait 2 min, then generate Cu(I); see no. 84

144

Styrene

Cl2; see no. 124

145

Thioethers; thiophenes

Br2; see no. 58

146

Thiodiglycol

Br2, 50% HOAc; iPt-Pt (0.3 V), EPt-SCE

Anal. Chem. 19:197 (1947)

146a

Thiols

Cu(II); MeOH

Talanta 27:989 (1980)

147

o-(p-)Toluidine

H+; see no. 120

148

Triethylamine

H+; see no. 87

149

Urea, thio-

Hg(II); 0.03M H2SO4; 0.1M K2SO4; iHg-Hg (0.01–0.03 V); remove O2 with N2 Ag(I); add excess saturated AgBr in concentrated NH3; warm to 70°C until no odor of NH3; add HClO4, titrate Br−; EAg-SCE; protect from light

150

Z. Anal. Chem. 161:348 (1958) Bull. Chem. Soc. Jpn. 26:394 (1953)

14.9.2 Instrumentation 14.9.2.1 Conductance Cells. A conductivity cell with large electrodes very close together has a low cell constant. If the distance is increased and/or the area of the electrodes is decreased, the cell constant increases. For example, a cell with a constant of 10 cm −1 would have electrodes perhaps 0.5 cm2 in area and spaced 5 cm apart. This arrangement would be suitable for measuring the conductance of 0.005% to 2.0% sulfuric acid solutions the specific conductance of which ranges from about 0.000 44 to 0.176 S ⋅ cm−1. The resistance readings would range from 22 700 Ω for the dilute acid to 57 Ω for the 2% acid solution.

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14.127

TABLE 14.39 Standard Solutions for Calibrating Conductivity Vessels The values of conductivity κ are corrected for the conductivity of the water used. The cell constant q of a conductivity cell can be obtained from the equation q=

kRRsolv Rsolv − R

where R is the resistance measured when the cell is filled with a solution of the composition stated in the table below, and Rsolv is the resistance when the cell is filled with solvent at the same temperature. Grams KCl per kilogram solution (in vacuo) 71.135 2 7.419 13 0.745 263*

Conductivity in ohm − 1 ⋅cm − 1 at 0°C 0.065 144 0.007 1344 0.000 773 26

18°C 0.097 790 0.011 1612 0.001 219 92

25°C 0.111 287 0.012 8497 0.001 408 08

* Virtually 0.0100M. Data from Jones and Bradshaw, J. Am. Chem. Soc. 55:1780 (1933). The original data have been converted from (int. ohm)−1 cm−1. Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

The dip-type cell is simplest to use whenever the liquid to be tested is in an open container. Whatever the configuration of the cell, the test solution must completely cover the electrodes. Pipette cells permit measurements with as little as 0.01 mL of solution. A pair of individual square platinum electrodes on glass wands is useful in conductometric titrations. 14.9.2.2 Conductivity Meters. In the classical mode of conductance measurements, resistance measurements are made using some variation of a Wheatstone bridge. To balance the capacitive effects in the conductance cell, the bridge circuit must also contain a variable capacitance (8 to 200 pF) in parallel with the balancing resistor. A built-in generator provides bridge current at frequencies of 100, 1000, and 3000 Hz. A lower frequency is preferred when the measured resistance is high and a higher frequency when the measured resistance is low. Use of an alternating current eliminates the effects of faradaic processes; that is, the deposition potential is not exceeded. The cell constant of the conductivity cell should be selected to maintain the measured resistance between 100 Ω and 1.1 MΩ. Any smooth metal surface can serve as an electrode at an operating frequency of 3000 Hz. Stainless steel electrodes are frequently used for industrial on-line applications. The Wheatstone bridge can be replaced by operational amplifier circuitry, as shown in Fig. 14.20. For conductance measurements, the cell is connected in place of R1; the result is a current i = E/R1, which is proportional to the conductance of the cell. For resistance measurements, a fixed resistance R1 is used in the input current-generating circuit and the cell is connected in place of R2. The synchronous detector FIGURE 14.20 Operational amplifier used for automatically rejects the current component resulting resistance and conductance measurements. from capacitative coupling. (From Shugar and Dean, 1990.) Differential conductivity measurements permit the measurement of differences in conductivity even when the total conductance is high. Also, a small discrete conductance change can be measured in the presence of a steadily changing conductance, as in gradient elution chromatography. A second cell monitoring the gradient can provide a flat baseline—except when a discrete change occurs due to elution of the sample. If the two

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cells are held at the same temperature, then even if that temperature changes, very precise measurement of differences in conductance can be made. A second example of difference conductance measurements involves the monitoring of a flowing stream to which a reagent or contaminant is added. If conductance changes are measured by two cells—one upstream and one downstream from the point at which the additive enters the streams— then only differences in conditions caused by the additive will appear at the meter output, and changes in temperature and concentration of the stream will not affect the measurement.

14.9.3

Direct Analysis Direct analysis involves the measurement of concentration of various electrolytes as a function of conductivity. Table 14.40 lists the conductivity of common industrial solutions at 25°C. The use of such data is restricted to solutions of a single solute at a specified temperature. When plotted from the tabulated data, the curves are markedly nonlinear at higher concentrations at which ionization may be incomplete and activity coefficients cannot be neglected. Maxima and minima are observed with many materials. Conductance increases with temperature at the rate of 0.024 per degree Celsius (at 25°C) for salts such as NaCl. The direct method of analysis is valuable for many industrial purposes such as pickling bath concentration, rinsing after plating and cleaning operations, caustic degreasing baths, fruit peeling baths, concentration of process liquids, strengths of concentrated acids and oleum, and waters of various types and origins. A widespread application of a conductance monitor is in water-purification systems. Conductance liquid chromatography detectors are very important in monitoring solvent composition in gradient elution involving change of ionic strength or buffer characteristics. The restriction to a single solute can be removed if the system is well defined and the calibration made with similar mixtures.

14.9.4 Conductometric Titrations In conductometric titrations the variation of the electrical conductivity of a solution during the course of a titration is followed. This technique is of wide applicability and good precision. Since the property measured bears a direct (not logarithmic) relation to the concentration, the titration curves (volume-corrected) consist of intersecting straight lines. A conductometric titration is devised so that the ionic species to be determined can be replaced by another ionic species of significantly different conductance. The end point is obtained by the intersection of two straight lines that are drawn through a suitable number of points (usually four for each linear branch) obtained by measurement of the conductivity after each addition of titrant. The titrant should be at least 10 times as concentrated as the analyte in order to keep the volume change small. If necessary a correction may be applied. All conductance readings are multiplied by the ratio V +v V where V is the initial volume and v is the volume of titrant added up to the particular conductance reading. The major applications are to acid–base titrimetry. Titrant concentrations can be as low as 0.0001M. Under optimum conditions the end point can be located with a relative error of approximately 0.5%. Typical acid–base titrations will be considered. Limiting equivalent ionic conductances in aqueous solutions at 25°C are given in Table 14.41. 14.9.4.1 Strong Acid Titrated With Strong Base. Consider the titration of a 0.001M solution of HCl with 0.1M NaOH. During the formation of water, the highly conducting hydronium ion (l+ = 350)

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4.2 7.4 15.5 30.6 63 114 209 368 640 1120 1730 1620

0.0001 0.0003 0.001 0.003 0.01 0.03 0.1 0.3 1 3 10 30

1.2 1.9 3.9 6.8 12 20 39 55

CO2 11.7 35.0 116 340 1140 3390 0.0111 0.0322 0.103 0.283 0.709 0.732

HCl

290 630 1490 2420 5100 0.0117 0.0347 0.118 0.390

HF 6.8 20 67 199 657 1950 6380 0.0189 0.0600 0.172 0.498 0.861

HNO3

342 890 2250 4820 0.0105 0.0230 0.0607 0.182

H3PO4 8.8 26.1 85.6 251 805 2180 6350 0.0158 0.0485 0.141 0.432 0.822

H2SO4

* Nitric acid possesses a maximum at 30.7%, and minimum at 97.6%. Sulfuric acid possess maxima at 31.4%, 92.0%, and 102.9% (oleum); and minima at 86.0% and 100.0%.

Acetic acid

% by weight

Conductivity at 25°C*

6.6 14 27 49 84 150 275 465 810 1110 1120 210

NH3

Values listed are in units of mS⋅cm −1 until the italicized value, which begins units of S⋅ cm −1. 1% by weight is equivalent to 10 000 ppm, etc. Underlined values indicate that conductivity passes through a maximum between the two listed concentrations.

TABLE 14.40 Conductivity of Common Industrial Solutions at 25°C

2.2 6.5 21.4 64 210 617 1990 5690 0.0176 0.0486 0.140

NaCl

6.2 18.4 61.1 182 603 1780 5820 0.0169 0.0532 0.144 0.358 0.292

NaOH

2.2 6.5 21.3 64 208 612 1930 5550 0.0170 0.0462

Sea salt

3600 7900 0.0170 0.0327 0.0610

SO2

ELECTROANALYTICAL METHODS

14.129

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ELECTROANALYTICAL METHODS

14.130

SECTION FOURTEEN

TABLE 14.41 Limiting Equivalent Ionic Conductances in Aqueous Solutions In mho ⋅cm2 ⋅ equiv-1. Temperature, °C Ion

0

18

25

Inorganic cations Ag+ Al3+ Ba2+ Be2+ Ca2+ Cd2+ Ce3+ Co2+ Co(NH3)63+ Co(en)33+ Cr3+ Cs+ Cu2+ D+ (deuterium) Dy3+ Er3+ Eu3+ Fe2+ Fe3+ Gd3+ H+ Hg2+ 2 Hg2+ Ho3+ K+ La3+ Li+ Mg2+ Mn2+ NH4+ N2H5+ (hydrazinium 1+) Na+ Nd3+ Ni2+ Pb2+ Pr3+ Ra2+ Rb+ Sc3+ Sm3+ Sr2+ Tl+ Tm3+ UO22+ Y3+ Yb3+ Zn2+

33 29 33.6

54.5

30.8 28

51 45.1

28

45

44 28

68 45.3 213.7

54.3

28

45.3

224.1

315.8

40.3 35.0 19.1 28.5 27 40.3

64.6 59.2 33.4 46 44.5 64

25.85

43.5

28 37.5

45 60.5

33 43.5

56.6 67.5

31 43.3

51 66

28

45.0

61.9 61 63.9 45 59.5 54 70 53 100 74.7 67 77.3 56.6 65.7 66.0 67.9 53.5 69 67.4 350.1 68.7 63.6 66.3 73.50 69.6 38.69 53.06 53.5 73.7 59 50.11 69.6 50 71 69.6 66.8 77.8 64.7 65.8 59.46 74.9 65.5 32 62 65.2 53.5

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

TABLE 14.41 Limiting Equivalent Ionic Conductances in Aqueous Solutions (Continued) Temperature, °C Ion

0

18

25

Inorganic anions Au(CN)2− Au(CN)4− B(C6H5)4− Br− Br3− BrO3− Cl− ClO2− ClO3− ClO4− CN− CO32− Co(CN)63− CrO42− F− Fe(CN)64− Fe(CN)63− H2AsO4− HCO3− HF2− HPO42− H2PO4− HS− HSO3− HSO4− H2SbO4− I− IO3− IO4− MnO4− MoO42− N3− Ni(CN)42− NO2− NO3− NH2SO3− (sulfamate) OCN− (cyanate) OH− PF6− PO3F2 − PO43− P2O74− P3O93− P3O105− ReO4− SCN − (thiocyanate) SeCN− SeO42− SO32−

43.1

67.6

31.0 41.4

49.0 65.5

36 37.3

55.0 59.1

36

60.5

42

72 46.6

40 27

42.0 21.0

28 57

36

66.5 33.9 49 53

44 40.2

59 61.7

117.8

54.8 175.8

41.7

46.5 56.6 65

50 36 21 78.4 43 55.8 76.35 52 64.6 67.9 78 72 98.9 85 55.4 111 101 34 44.5 54.4 57 36 65 50 50 31 76.9 41.0 54.5 62.8 74.5 69.5 54.5 72 71.42 48.6 64.6 199.2 56.9 63.3 69.0 81.4 83.6 109 54.9 66.5 64.7 75.7 79.9 (Continued)

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14.131

ELECTROANALYTICAL METHODS

14.132

SECTION FOURTEEN

TABLE 14.41 Limiting Equivalent Ionic Conductances in Aqueous Solutions (Continued) Temperature, °C Ion

0

18

25

Inorganic anions SO42− S2O32− S2O42− S2O62− S2O82− WO42−

41

68.3

34

35

59

80.0 85.0 66.5 93 86 69.4

Organic cations Decylpyridinium Diethylammonium Dimethylammonium Dipropylammonium Dodecylammonium Ethylammonium Ethyltrimethylammonium Isobutylammonium Methylammonium Piperidinium Propylammonium Tetrabutylammonium Tetraethylammonium Tetramethylammonium Tetrapropylammonium Trimethylammonium Triethylsulfonium Trimethylammonium Trimethylsulfonium Tripropylammonium

29.5 42.0 51.5 30.1 23.8 47.2 40.5 38.0 58.3 37.2 40.8 19.1 33.0 45.3 23.5 34.3 36.1 46.6 51.4 26.1 Organic anions

Acetate Benzoate Bromobenzoate Butanoate Chloroacetate Chlorobenzoate Citrate(3−) Cyanoacetate Cyclohexanecarboxylate Cyclopropane-1, 1-dicarboxylate Decylsulfonate Dichloroacetate Diethylbarbituate(2−) Dihydrogen citrate 3,5-Dinitrobenzoate Dodecylsulfonate Ethylsulfonate Fluorobenzoate Formate

20

34

47

41 32.4 30 32.6 39.7 33 70.2 41.8 28.7 53.4 26 38.3 26.3 30 28.3 24 39.6 33 54.6

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.133

TABLE 14.41 Limiting Equivalent Ionic Conductances in Aqueous Solutions (Continued) Temperature, °C Ion

0

18

25

Organic anions Hydrogen oxalate(1−) Lactate Methylsulfonate Octylsulfonate Phenylacetate Propanoate Propylsulfonate Salicylate Succinate(2−) Tartrate(2−) Trichloroacetate

40.2 38.8 48.8 29 30.6 35.8 37.1 36 58.8 55 36.6

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

is replaced by a less highly conducting sodium ion (l+ = 50). l is the limiting equivalent ionic conduction, l+ or l−, for cations or anions, respectively. The conductivity falls linearly, as shown in curve 1 of Fig. 14.21, reaching a minimum if the solution consists of only NaCl. Unused NaOH and previously formed NaCl constitute the conductance of the rising branch of the titration curve. The conductance of the solution at any point on the descending branch of the titration curve is given by the expression 1 1 = (C l + CNa l Na + CCl l Cl ) R 1000Θ H H

(14.54)

This equation can be expressed in terms of the initial concentration of HCl, Ci, and the fraction of the acid titrated, f: CH = Ci(1 − f ),

CNa = Ci f,

CCl = Ci

(14.55)

Substituting these values into Eq. (14.53), Ci 1 = [l + l Cl + f (l Na − l H )] R 1000Θ H

(14.56)

The term within the parentheses in Eq. (14.56) expresses the steepness of the drop in conductivity up to the end point. 14.9.4.2 Incompletely Dissociated Acids (or Bases). Titration of incompletely dissociated acids (or bases) is somewhat more difficult. Initially the solution has a low conductivity, which is due to

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ELECTROANALYTICAL METHODS

14.134

SECTION FOURTEEN

the few ionized ions present. As neutralization proceeds, the common ion formed (anion for a weak acid) represses the dissociation so that an initial fall in conductivity may occur. Consequently, the shape of the initial portion of these titration curves will vary with the strength of the weak acid (or base) and its concentration, as indicated in Figs. 14.21 and 14.22. Pronounced hydrolysis in the vicinity of the end point makes it necessary to select the experimental points for the construction of the two branches considerably removed from the end point. Sometimes no linear region is obtained preceding the end point. A clever stratagem overcomes the problem. For example, in the titration of a weak acid, sufficient aqueous ammonia is added to neutralize about 80% of the weak acid. Then the titration is carried out with standard NaOH. After the FIGURE 14.21 Acids of different strengths titrated with either sodium hydroxide or aqueous ammoremaining weak acid has been neutralized, the titrania. The numbered curves are (1) HCl, (2) an acid of tion involves the reaction of the ammonium ion pKa = 3, (3) an acid of pKa = 5, and (4) acids whose (formed during the neutralization of the weak acid) pKa = 7 or greater. (From Shugar and Dean, 1990.) with NaOH to form ammonia, water, and sodium ions. The conductance falls owing to the replacement of the ammonium ion (l+ = 73) by the sodium ion (l+ = 50). When the replacement is complete, the conductivity increases abruptly due to the unused NaOH. 14.9.4.3 Mixtures of Different Acid Strength. A mixture of a strong and a weak acid can be determined in one titration, as illustrated in Fig. 14.23 for the titration of the first and the second protons in oxalic acid. The curve would be similar for the titration of a mixture of HCl and acetic acid (or other carboxylic acid). A more distinct second end point is obtained when the titrant is aqueous ammonia (or propyl amine). Weak acids can be titrated only if the product of the ionization constant and the acid concentration exceeds 10−11. The conductometric titration technique is also useful in the titration of the conjugate base of a weakly ionized acid, and vice versa. This extends the technique to organic salts such as acetates, benzoates, nicotinates, and so on. One caveat is that the ionization constant of the displaced acid or base divided by the original salt concentration must not exceed 5 × 10−3. Conductometric titration methods are given in Table 14.42.

FIGURE 14.22 Effect of concentration upon the shape of the titration curve for an acid whose pKa = 4.8. (From Shugar and Dean, 1990.)

FIGURE 14.23 Titration of a mixture of a strong acid and a weak acid; the specific example involves the two protons of oxalic acid. (From Shugar and Dean, 1990.)

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ELECTROANALYTICAL METHODS

ELECTROANALYTICAL METHODS

14.135

TABLE 14.42 Conductometric Titration Methods Substance titrated

Concentration range

Reagent

Remarks

Acids (pKa ≤ 2)

0.1M–0.001M

NaOH

CO2 absent.

Acids (pKa 2 to 6)

0.1M–10−4M

Aqueous NH3

CO2 absent.

Acids (pKa > 7)

0.1M–10−4M

LiOH

CO2 absent. Titrate water-insoluble acids in 75% EtOH.

Ag(I)

0.1M–10−4M 0.1M–0.001M

NaCl or LiCl Na2CrO4 or Li2C2O4

Neutral or dilute acid solution. Neutral solution. Titrate in H2SO4. Three breaks appear in titration curve. Analysis based on reagent consumed between first (neutralization of excess H+) and third breaks (AlO−2 formed).

Al(III)

10 −2M–10−3M

NaOH

As(III)

10−4M–10−5M

I2 in EtOH

Titrate in dilute NaHCO3 solution.

Ba(II)

0.1M–10−3M

Li2SO4 or Li2CrO4

30% EtOH. Wait several minutes for stable readings after each addition of reagent.

Bases, strong

0.1M–10−3M

HCl

CO2 absent.

Bases, weak

0.1M–10−4M

HOAc

CO2 absent.

HCl

CO2 absent. Titrate water-insoluble bases in 75% EtOH.

Bases, very weak

10−3M

Bases, salts of weak acids

10−2M–10−5M

HCl or Cl3CCOOH

CO2 absent.

Be(II)

10−2M–10−4M

NaOH

Cl− or NO−3 solution, CO2 absent. Two breaks appear: first is for neutralization of excess H+.

0.1M–10−5M

AgNO3

Neutral or slight acid solution. Add up to 90% EtOH for titration of very dilute solution.

(C6H5)4AsCl

pH 7; 30-s pause between increments of titrant.

AgNO3 Hg(ClO4)2

Neutral solution. Two breaks in titration curve; second [formation of Ag(CN)−2 ] gives more precise results. Neutral solution. Two breaks in titration curve corresponding to Hg(CN)2− 4 and Hg(CN)2. Using first break, CN− may be determined in presence of Cl−.

Br− ClO4−

0.05M

CN−

0.1M–10−3M

0.01M

CNO−

0.1M–0.01M

CO32−

10−2M–10−4M

Cd(II)

0.1M 0.1M

AgNO3

Neutral solution.

HCl

In OH− solution, three breaks occur: neutralization − of OH−, CO2− 3 , and HCO3 . Analysis based on third break most precise.

Li4[Fe(CN)6] EDTA

Pb(II) does not interfere. Acetate buffer, pH 5.

Cl−

0.1M–10−5M

AgNO3

Add up to 90% EtOH for titration of very dilute solutions.

Co(II)

0.01M 0.01M 0.1M–10−3M

EDTA Li3[Fe(CN)6] AlCl3

Acetate buffer, pH 5. Dilute acid solution. Titrate in 30%–50% EtOH containing 50-fold excess for NaCl. Precipitate is Na3AlCl6.

K2Cr2O7 AgNO3

Titrate in 0.1M H2SO4. Neutral solution.

F− Fe(II) Fe(CN)63−

0.01M–10−3M 0.1M–0.01M

(Continued)

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ELECTROANALYTICAL METHODS

14.136

SECTION FOURTEEN

TABLE 14.42 Conductometric Titration Methods (Continued) Substance titrated Fe(CN)64−

Concentration range 0.1M–0.01M

Reagent

Remarks

Pb(NO3)2

H+

Neutral or dilute acid solution. See Acids above

I−

0.1M–10−4M

IO3−

0.01M–10−4M

K+ Mg(II)

AgNO3

Titration in presence of 2% NH3 prevents interfer ence from Cl− and small amounts of Br−.

HCl

Neutral solution containing small excess of Kl and Na2S2O3.

5 × 10−3M

NaB(C6H5)4

pH 5–10.

0.1M–0.01M 0.1M–0.01M

NaOH EDTA

NH3 buffer, pH 10. Dilute acid solution.

NO3−

0.05M

Nitron in HOAc

Ni(II)

0.01M

DimethylglyNH3 buffer. Preferable to add excess DMG and oxime in EtOH back-titrate with standard Ni(II).

OH −

See Bases above

Pb(II)

0.1M–10−4M

Peroxide

Li2C2O4 or Na2CrO4 EDTA

Acetate buffer, pH 5.

0.1M–10−3M

KMnO4

0.05M H2SO4 solution.

PO43−

0.1M–10−3M

BiOCl4

0.3M HClO4 solution. As absent. Small amounts of most metals do not interfere.

SCN −

0.1M–10−5M

AgNO3

0.1M–10−3M

Hg(ClO4)2

Neutral or slightly acid solution. Add up to 90% EtOH for titration of very dilute solutions. Neutral solution. Second break [formation of Hg(SCN)2] gives more precise results.

0.1M–10−4M

Ba(OAc)2 in 1% HOAc

20% EtOH. Large amounts of nitrate interfere.

0.01M

SO42−

Neutral solution.

S2O32−

0.1M–0.01M

Pb(NO3)2

Neutral solution.

Se(IV)

0.01M–10−2M as selenite

AgNO3 or Pb(NO3)2

Neutral solution.

Se(VI)

0.01M–10−2M as selenate

BaCl2 or Pb(NO3)2

50% EtOH solution.

Sr(II)

0.01M

EDTA

NH3 buffer, pH 10.

Tl(I)

10−3M

5× 0.02M 0.1M–10−2M

Na2CrO4 NaB(C6H5)4 KSCN

Neutral solution. Neutral solution. Neutral solution.

U(IV)

10−2M–10−3M

KMnO4

0.2M to 0.5M H2SO4 solution.

V(III)

10−2M–10−3M

KMnO4

Dilute H2SO4 solution.

V(V)

0.01M VO3− 4

AgNO3

Neutral solution.

V(V)

0.01M–0.005M VO43− 0.01M VO3−

Co(NH3)6Cl3

Neutral solution. Precipitate has composition [Co(NH3)6][V2O7]3. Neutral solution. Precipitate has composition [Co(NH3)6][VO3]3.

10−2M–10−3M

NaOH

Zn(II)

Co(NH3)6Cl3

Solution should contain H+ equivalent to Zn(II) present. Two breaks in titration curve: first for H+, second for precipitation of Zn(OH)2.

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 15

THERMAL ANALYSIS 15.1 INTRODUCTION 15.2 DIFFERENTIAL SCANNING CALORIMETRY AND DIFFERENTIAL THERMAL ANALYSIS 15.2.1 Instrumentation for DSC 15.2.2 Instrumentation for DTA 15.2.3 Applications of DSC and DTA Table 15.1 Glass Transition Temperatures (Tg) and Crystalline Melting Points of Selected Polymers 15.3 THERMOGRAVIMETRIC ANALYSIS 15.3.1 TGA Instrumentation 15.3.2 Applications of TGA 15.4 THERMOMECHANICAL ANALYSIS 15.4.1 TMA Instrumentation 15.4.2 Applications of TMA 15.5 DYNAMIC MECHANICAL ANALYSIS 15.6 ENTHALPIMETRIC ANALYSIS 15.6.1 Thermometric Enthalpimetric Titrations Figure 15.1 Typical Thermometric Titration Curves for an Exothermic Process 15.6.2 Direct-Injection Enthalpimetry 15.7 THERMOMETRY Table 15.2 Fixed Points in the ITS-90 15.8 THERMOCOUPLES Table 15.3 Thermoelectric Values in Millivolts at Fixed Points for Various Thermocouples Table 15.4 Type B Thermocouples: Platinum–30% Rhodium Alloy vs. Platinum–6% Rhodium Alloy Table 15.5 Type E Thermocouples: Nickel–Chromium Alloy vs. Copper–Nickel Alloy Table 15.6 Type J Thermocouples: Iron vs. Copper–Nickel Alloy Table 15.7 Type K Thermocouples: Nickel–Chromium Alloy vs. Nickel–Aluminum Alloy Table 15.8 Type N Thermocouples: Nickel–14.2% Chromium–1.4% Silicon Alloy vs. Nickel–4.4% Silicon–0.1% Magnesium Alloy Table 15.9 Type R Thermocouples: Platinum–13% Rhodium Alloy vs. Platinum Table 15.10 Type S Thermocouples: Platinum–10% Rhodium Alloy vs. Platinum Table 15.11 Type T Thermocouples: Copper vs. Copper–Nickel Alloy Bibliography

15.1

15.1 15.2 15.2 15.3 15.3 15.4 15.5 15.6 15.6 15.7 15.7 15.7 15.7 15.8 15.8 15.9 15.9 15.10 15.10 15.11 15.12 15.13 15.13 15.14 15.14 15.15 15.15 15.16 15.16 15.17

INTRODUCTION Thermal analysis includes a group of techniques in which specific physical properties of a material are measured as a function of temperature. The techniques include the measurement of temperatures at which changes may occur, the measurement of the energy absorbed (endothermic transition) or evolved (exothermic transition) during a phase transition or a chemical reaction, and the assessment of physical changes resulting from changes in temperature. 15.1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

THERMAL ANALYSIS

15.2

SECTION FIFTEEN

Various environments (vacuum, inert, or controlled gas composition) and heating rates from 0.1 to 500°C ⋅ min−1 are available for temperatures ranging from −190 to 1400°C. The analysis of gas(es) released by the specimen as a function of temperature is possible when thermal analysis equipment is coupled with Fourier-transform infrared detection or with a mass spectrometer. The applications of thermal analysis are many and varied. For environmental measurements, these parameters can be measured: vapor pressure, thermal stability, flammability, softening temperatures, and boiling points. Compositional analysis offers phase diagrams, free versus bound water, solvent retention, additive analysis, mineral characterization, and polymer system analysis. In the important area of product reliability, thermal methods provide heat-capacity data, liquid-crystal transitions, solid fat index, purity, polymer cures, polymer quality control, glass transitions, Curie point, and fiber properties. Information on stability can be obtained from modulus changes, creep studies, expansion coefficients, and antioxidant evaluation. Dynamic properties of materials are found from viscoelastic measurements, impact resistance, cure characteristics, elastic modulus, loss modulus, and shear modulus. Lastly, chemical reactions can be followed through heats of transition, reaction kinetics, catalyst evaluation, metal–gas reactions, and crystallization phenomena.

15.2 DIFFERENTIAL SCANNING CALORIMETRY AND DIFFERENTIAL THERMAL ANALYSIS Differential scanning calorimetry (DSC) and quantitative differential thermal analysis (DTA) measure the rate and degree of heat change as a function of time or temperature. In addition to these direct energy measurements, the precise temperature of the sample material at any point during the experiment is also monitored. Since DSC can measure both temperatures and heats of transitions or reactions, it has replaced DTA as the primary thermal analysis technique, except in certain high-temperature applications. These methods are used to investigate the thermal properties of inorganic and organic materials. The procedure involves recording a temperature (DTA) or power (DSC) difference between a sample and a reference container as both are carried through a temperature program. DTA detects the physical and chemical changes that are accompanied by a gain or loss of heat in a substance as the temperature is altered. DSC provides quantitative information about any heat changes, including the rate of heat transfer. When a thermal transition occurs in the sample, thermal energy is added either to the sample or the reference holders in order to maintain both the sample and reference at the sample temperature. Because the energy transferred is exactly equivalent in magnitude to the energy absorbed or evolved in the transition, the balancing energy yields a direct calorimetric measurement of the transition energy at the temperature of the transition. Elevated pressure or reduced pressure is often beneficial to DSC experiments for such applications as the study of pressure-sensitive reactions, accelerated oxidation of lubricants or polymers, evaluation of catalysts, and resolution of overlapping transitions.

15.2.1 Instrumentation for DSC In a cell designed for quantitative DSC measurements, two separate sealed pans, one containing the material of interest and the other containing an appropriate reference, are heated (or cooled) uniformly. The enthalpic difference between the two is monitored either at any one temperature (isothermal) or the temperature can be raised (or lowered) linearly. If maximum calorimetric accuracy is desired, the sample and reference thermocouples must be removed from direct contact with the materials. The gradient temperature experiments can be run slowly (0.1°C ⋅ min−1) or rapidly (up to 300°C ⋅ min–1). The electronic circuitry detects any change in heat flow in the sample versus the reference cell. This event in turn is sent as an analog signal to an output device such as a strip-chart recorder, digital integrator, or a computer. Information may be obtained with samples as small as 0.1 mg.

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THERMAL ANALYSIS

THERMAL ANALYSIS

15.3

However, quantitative studies usually require at least 1 mg of sample. Expanding the upper temperature limit to 1500°C allows for DSC analyses to be done on materials such as ceramics, metal alloy systems, silicates, and high-temperature composites. The DSC cell uses a constantan disk as the primary means of transferring heat to the sample and reference holders and also as one element of the temperature-sensing thermoelectric junction. Samples in powder, sheet, film, fiber, crystal, or liquid form are placed in disposable aluminum sample pans of high thermal conductivity and weighed on a microbalance. The sample is placed in one sample holder and an empty sample holder serves as reference. Sample sizes range from 0.1 to 100 mg. The differential heat flow to the sample and reference through the disk is monitored by the chromel–constantan thermocouples formed by the junction of the constantan disk and the chromel wafer covering the underside of each platform. Chromel and alumel wires connected to the underside of the wafers form a chromel–alumel thermocouple, which is used to monitor the sample temperature. Thin-layer, large-area sample distribution minimizes thermal gradients and maximizes temperature accuracy and resolution. Generally, holders are covered with domed, aluminum sample-holder covers. The covers are essentially radiation shields that enhance baseline linearity and reproducibility from run to run. An airtight sample-holder enclosure isolates the holders from external thermal disturbances and allows vacuum or controlled atmosphere operation. A see-through window permits observation of physical changes in unencapsulated samples during a scan. Side ports allow addition of catalysts and seeding of supercooled melts. Three different DSC cells are available: standard (as described), dual sample, and pressure capability. The dual-sample cell provides an immediate twofold increase in sample throughput, while the pressure DSC cell provides capability from vacuum to 7 MPa (1000 lb ⋅ in–2). 15.2.2 Instrumentation for DTA In DTA a thermocouple is inserted into the center of the material in each sample holder. The sample is in one holder and reference material is placed in the other sample holder. The sample blocks are then heated. The difference in temperature between sample and reference thermocouples is continuously measured. Furnace temperature is measured by an independent thermocouple. Any transition that the sample undergoes results in liberation or absorption of energy by the sample with a corresponding deviation of its temperature from that of the reference. A plot of the differential temperature versus the programmed temperature indicates the transition temperature(s) and whether the transition is exothermic or endothermic. For high-temperature studies (1200 to 1600°C), the sample holders are fabricated from platinumiridium.

15.2.3 Applications of DSC and DTA Applications include melting-point and glass transition measurements, curing studies, oxidative stability testing, and phase-transition studies. 15.2.3.1 Glass Transition Temperature. The DSC trace of a polymeric material will show a step in the recorded signal by the polymer undergoing a second-order heat-capacity change at the glass transition temperature (Tg). As the temperature continues to increase, viscosity drops, and the polymer becomes mobile enough to realign into a crystalline structure. The heat released from the so-called “cold crystallization” produces an exothermic peak on the DSC curve. The area under the peak is proportional to the heat of crystallization. At higher temperatures the crystalline material melts, and an endothermic peak is recorded. The glass transition determines the useful lower temperature range of an elastomer, which is normally used well above this temperature to ensure the desired softness. In thermoset materials the glass transition temperature is related to the degree of cure, with the glass transition temperature steadily increasing as the degree of cure increases. Additionally, the glass transition temperature is often used to determine the storage temperature of uncured or partially cured material,

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THERMAL ANALYSIS

15.4

SECTION FIFTEEN

TABLE 15.1 Glass Transition Temperatures (Tg) and Crystalline Melting Points of Selected Polymers Values in parentheses are approximate or uncertain. Polymer Polybutadiene Polyisobutylene Poly(ethylene vinyl acetate) copolymer Polyethylene, low density Polybutene Polyethylene, high density Poly(oxymethylene) copolymer Polypropylene Poly(vinyl chloride) soft Poly(vinylidene chloride) Poly(oxymethylene) homopolymer Poly(cinylidene fluoride) Polyamide 11 Poly(vinyl acetate) Poly(vinyl chloride) Poly(butylenes terephthalate) Polyamide 6 Polyamide 6,10 Poly(vinyl alcohol) Polystyrene Poly(methyl methacrylate) Epoxy resin Poly(phenylene oxide) Polycarbonate Polyamide 6,6 Poly(ethylene terephthlate) Poly(ethylene tetrafluoroethylene) copolymer Poly(fluoroethylene propylene) Poly(phenylene sulfide) Polyacrylonitrile Polytetrafluoroethylene

Tg, °C −86 −73 −20 to 20* (−100) (−70) (−30) −40 to 10† −17

mp, °C (−20) (44) 40 to 100* 120 130 135 164 to 168 165

175 to 180 178 186 30 85 65 (40) (46) 85 90 to 100 105 50 to 150‡ 155 (50) (69)

80 100 (−20)

(190) 220 220 226

230 (235) 255 256 270 280 280 (320) 327

* Depending on ethylene content. † Depending on plasticizer content. ‡ Depending on formula.

since curing does not begin until the material is heated above its glass transition temperature. Table 15.1 contains glass transition temperatures and crystalline melting points of selected polymers. 15.2.3.2 Crystallization and Fusion. The exothermal crystallization enthalpy as well as the endothermal heat of fusion (at −25°C) can be evaluated by integrating curve areas. In a semicrystalline polymer the crystallinity can be calculated from the measured heat of fusion, which is a characteristic property and widely used for quality control. 15.2.3.3 Curing Reactions. During a curing reaction, energy is released from the sample as cross-linking occurs and a large exothermic peak follows the glass transition. 15.2.3.4 Curie Point. An interesting application of DSC is the determination of the Curie point temperatures of ferromagnetic materials. The Curie point is discussed in more detail in Sec. 15.3. Selected Curie point standards are alumel, 163°C; nickel, 354°C; perkalloy, 596°C, and iron, 780°C.

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THERMAL ANALYSIS

THERMAL ANALYSIS

15.5

15.2.3.5 Oxidative Stability. DSC is a useful tool for generating data on oxidative stability of fats and oils. In either the isothermal or the temperature-programmed mode, the onset of a deviation from the baseline can be related to the oxidative induction period. In practice, the system would be brought to the test temperature and held isothermally in a nitrogen atmosphere. Oxygen is then introduced and the time necessary to give the first indication of oxidation (exotherm) is noted. DSC can differentiate quickly between stable and unstable systems and determine which antioxidant system would best preserve the specimen. 15.2.3.6 Chemical Reactions. Depending upon the polymer and its intended use, various types of chemical reactions may be studied. Exotherms produced by polymerization, curing, oxidative degradation, and other reactions can be studied. Such information is useful in processing, in shelflife estimates, and in hazards evaluation for energetic polymers. In many cases it is possible to derive kinetics information from the DSC curves. One of the simplest and most broadly applicable methods is based upon the change in the DSC peak maximum temperature with heating rate. Samples are temperature-programmed at several rates (b) and the corresponding exothermic peak temperatures (T) are plotted as d log b ≈ 1.912 E/R d (1/ T )

(15.1)

where E is the Arrhenius activation energy in joules per mole. The reaction frequency factor (Z ) is calculated from the same curves as Z=

b E e E / RT RT 2

(15.2)

The polymer reaction rate (k) at any temperature is available from the Arrhenius equation k = Z e − E/RT

(15.3)

The equation for Z is a first-order form; however, the error incurred for other reaction orders is small and can usually be neglected.

15.3

THERMOGRAVIMETRIC ANALYSIS1 Thermogravimetric analysis (TGA) monitors the change in mass of a substance as a function of temperature or time as the sample is subjected to a controlled temperature program. TGA has often been used to rank polymeric materials in order of their thermal stability by comparing losses of weight versus temperature. Many important TGA procedures involve isothermal monitoring of the mass loss from a material. Thus, when one or more isothermal holds are involved in the total heating program, the mass loss is often monitored versus time. In addition, plotting heating rate versus the temperature for a specified degree of conversion yields kinetic data for the decomposition process. The weight-loss method for determining kinetic values avoids difficulties that occur for DSC when exothermic and endothermic events coincide and interfere with the analysis. A second use of TGA is the determination of the loss rate of moisture, diluent, and unreacted monomer that must be removed from the polymer. Polymers can also be pyrolyzed in the TGA equipment to enable determination of carbon black fillers or residual inorganic material. Another important use of TGA is in helping in the interpretation of DSC and DTA thermograms. For example, early endothermic activity in a programmed DSC curve might represent low-melting-point 1

C. M. Earnest, “Modern Thermogravimetry,” Anal. Chem. 56:1471A (1984).

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THERMAL ANALYSIS

15.6

SECTION FIFTEEN

polymer, or it may be due to volatilization of low-molecular-weight material. A TGA run would resolve the question.

15.3.1 TGA Instrumentation To perform TGA, the equipment must be capable of both simultaneous heating and weighing. Instrumentation includes the following: 1. A sensitive recording microbalance capable of detecting weight changes as small as 0.1 mg. 2. A furnace and an appropriate enclosure for heating the sample specimen. For most applications the temperature range will be from ambient to 1000°C. A small low-mass designed furnace permits rapid linear heating and cooling rates as fast as 200°C per minute. 3. A temperature programmer, heat-control circuitry, and associated electronics. Linear heating rates from 5 to 10°C ⋅ min–1 are typical, although much higher rates are available. Many important TGA procedures involve isothermal monitoring of the mass loss from a material. 4. A pneumatic system for dynamic purging of the furnace and sample chamber. 5. A data acquisition system. Additional features might be a purge-gas switching capability for those applications in which the purge gas is changed during the experiment, and software to generate a first-derivative differential thermogravimetric curve from the TG data after storage. Thermal curves obtained by TGA offer what is probably the most accurate ordinate scale of weight or weight percent of all thermal analysis techniques. The temperature axis is often less well defined. First, the thermocouple is generally close but not in contact with the sample; also the dynamic gaseous atmosphere surrounding the TG sample may influence the sample temperature.

15.3.2 Applications of TGA Thermogravimetry provides the laboratory chemist with a number of important testing applications. The most important applications include compositional analysis and decomposition profiles of multicomponent systems studied at varying temperatures and atmospheric conditions, parameters that can be tailored and switched at any point during the experiment. Other important applications include the rapid proximate analysis of coal, quantitative separation of the main sample components in multicomponent mixtures, the determination of the volatile and moisture components in a sample material, kinetic studies, accelerated aging tests, and oxidation–reduction reactions. Curie point measurements by TGA provide an accurate method for the calibration of the temperature axis of the thermogram since the Curie point temperature of many materials is well known and characterized. The ferromagnetic material is suspended in a magnetic field that is oriented such that a vertical component of magnetic force acts on the sample. This magnetic force acts as an equivalent magnetic mass on the TGA microbalance beam to indicate an apparent sample weight. When the sample is heated through its Curie point, the magnetic mass will be lost and the microbalance will indicate an apparent weight loss. Moisture determination is an important application of TGA. In many industries even small amounts of moisture have serious consequences. When the sample is rapidly heated to 105°C and held at this temperature, any moisture present in the sample is lost. Moisture levels at 0.5% and often below can be determined. Determination of the temperature of oxidation of a sample is another TGA application. For example, if magnesium powder is heated from 300 to 900°C in an oxidizing (air) atmosphere, at approximately 682°C a sharp increase in sample weight is noted that corresponds to the rapid oxidation of the material.

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THERMAL ANALYSIS

THERMAL ANALYSIS

15.4

15.7

THERMOMECHANICAL ANALYSIS Thermomechanical analysis (TMA) measures either the dimension or deformation of a substance under nonoscillatory load as a function of temperature or time. It is used to measure changes in the physical properties of sample materials, such as compression, coefficients of expansion, softening temperatures, viscosity/flow, stress relaxation/viscoelasticity, composite delamination, glass transition temperature, heat deflection temperature measurements, and creep, fiber, or film shrinkage. Useful information is provided with regard to behavior of the material at either elevated or reduced temperatures while under an external load, which can be varied. Very little chemical information can be determined by TMA.

15.4.1 TMA Instrumentation The analyzer measures the change in dimension as a function of temperature and stress. One of several selectable probes makes contact with a sample. The probe touches the upper surface of the sample and applies a well-defined stress. As the sample expands, contracts, or softens, the position of the probe will change. This position is monitored by a linear variable differential transformer (LVDT) that provides a signal proportional to the probe displacement. Several probe configurations are available for TMA. In the simplest arrangement, with a sharp probe placed under load on the sample, glass transition temperatures as well as softening and flow temperatures can be extrapolated. The Tg determination is generally more sensitive than that by DSC. A flat probe on the sample with little or no load allows determination of a linear coefficient of expansion. In the expansion, penetration, and hemispherical configurations, the outer member is the quartz platform and is fixed. The inner members are movable and connected to the LVDT core. Another probe, in which the sample expands in a glass bulb filled with microbeads, serves as a dilatometer to determine cubical expansion versus temperature. The tension and fiber probes put the sample in tension by pulling instead of pushing. The dilatometer probe is designed to measure volume changes. 15.4.2 Applications of TMA In the penetration and expansion modes, the sample rests on a quartz stage surrounded by the furnace. Under no load, expansion with temperature is observed. At the glass transition temperature, there is a discontinuous change in expansion coefficient, as evidenced by the elbow in the TMA scan. The thermal coefficient of linear expansion is calculated directly from the slope of the resulting curve. In the penetration mode the penetration of the probe tip into the sample is observed under a fixed weight placed in the weight tray. Sample sizes may range from a 0.1-mil coating to a 0.5-in thick solid; sensitivities down to a few microinches are observable. In the penetration mode, TMA is a sensitive tool for the characterization and quality control of thin films and coatings. For the measurement of samples in tension, the sample holder consists of stationary and movable hooks constructed of fused silica. This permits extension measurements on films and fibers; these measurements are related to the tensile modulus of a sample. For a swelling measurement, the sample is placed on the bottom of a small cup and is covered by a small disk of aluminum oxide. In an isothermal experiment, the cup is filled with solvent at time zero. The swelling of the sample increases its thickness, which is measured by the probe.

15.5

DYNAMIC MECHANICAL ANALYSIS Dynamic mechanical analysis (DMA) is the measurement of the mechanical properties of materials as they are deformed under periodic stress. Of special importance is the modulus of materials as well

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THERMAL ANALYSIS

15.8

SECTION FIFTEEN

as their damping characteristics under oscillatory load as a function of temperature. It involves measuring the resonant frequency and mechanical damping of a material forced to flex at a selected amplitude. These parameters define the inherent stiffness of a material and its tendency to convert mechanical energy into heat when stressed. Two parallel, balanced sample-support arms are free to oscillate around flexure pivots. A specimen material of known dimensions is clamped between the two arms. The force modulator is an electromagnet used in place of the weights required in the static TMA mode. The magnet receives its current impulses through the circuit with an integrated function generator and power amplifier; this oscillates the sample–arm–pivot system. The frequency and amplitude of this oscillation are detected by a linear variable differential transformer positioned at the opposite end of the active arm. The frequency of oscillation is directly related to the modulus of the sample, while the energy needed to maintain constant amplitude oscillation is a measure of damping within the sample. DMA allows the user to observe elastic and inelastic deformations of materials, providing information relative to changes in moduli of shear and elasticity, as well as changes in loss modulus. Microprocessor data-reduction techniques provide graphical and tabular outputs of these properties as functions of time or temperature. It also makes possible the detection of glass transitions generally before such are detectable by other means.

15.6

ENTHALPIMETRIC ANALYSIS 2,3 Enthalpimetric analysis, which includes the titrimetric and calorimetric modes, utilizes the temperature change in a system while a titrant is gradually added or measures the thermal energy released during a controlled reaction of the specimen. The main types of enthalpimetric analysis are thermometric enthalpy titrations and direct injection enthalpimetry. In practical terms they can be differentiated by the way the reactant is introduced to the adiabatic cell.

15.6.1 Thermometric Enthalpimetric Titrations Thermometric enthalpimetric titrations (TET) are characterized by the continuous addition of the titrant to the sample under effectively adiabatic conditions. The total amount of heat evolved (if the reaction is exothermic) or absorbed (if the reaction is endothermic) is monitored using the unbalance potential of a Wheatstone bridge circuit, incorporating a temperature-sensitive semiconductor (thermistor) as one arm of the bridge. Simple styrofoam-insulated reaction cells will maintain pseudoadiabatic conditions for the short period of a titration. The heat capacity of the system will remain essentially constant if the change in volume of the solution is minimized and if the titrant and titrate are initially at the same temperature (usually room temperature). The TET ethalpogram (a thermometric titration curve), shown in Fig. 15.1, illustrates an exothermic titration reaction. The base line AB represents the temperature–time blank, recorded prior to the start of the actual titration. B corresponds to the beginning of the addition of the titrant, C is the end point, and CD is the excess reagent line. In order to minimize variations in heat capacity during titrations, it is customary to use a titrant that is 50 to 100 times more concentrated than the specimen being titrated. Thus the volume of the titrate solutions is maintained virtually constant, but the titrant is diluted appreciably. Correction for the latter is conveniently made by linear back-extrapolation CB′. Under these conditions, the extrapolated ordinate height, BB′, represents a measure of the change of temperature due to the titration reaction. 2 A. J. Hogarth and J. D. Stutts, “Thermometric Titrimetry: Principles and Instrumentation,” Am. Lab. 13 (January):18 (1981). 3 J. Jordan et al., “Enthalpimetric Analysis,” Anal. Chem. 48:427A (1976).

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THERMAL ANALYSIS

THERMAL ANALYSIS

15.9

FIGURE 15.1 Typical thermometric titration curves for an exothermic process: (a) idealized curve, and (b) actual curve, illustrating extrapolation correction for curvature due to incompleteness of reaction. AB—temperature–time blank, slope due to heat leakage into or out of titration cell; B—start of titration; BC—titration branch; C′—end point; CD—excess reagent line, slope due to heat leakage and temperature difference between reagent and solution being titrated; ∆T— corrected temperature change; ∆V—end-point volume. (From L. Meites, ed., Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963.)

In contrast to most analytical procedures that depend on a property related solely to equilibrium constants (i.e., free energy methods), TET depend on the heat of the reaction as a whole, viz., ∆H = ∆F + T ∆ S

(15.4)

Consequently, thermometric titration curves may yield a well-defined end point when all free energy methods fail if the entropy term in Eq. (15.4) is favorable. This is indeed the case in many alkalimetric titrations of weak acids. The corresponding enthalpograms for the titration of hydrochloric acid and boric acid are strikingly similar, because the heats of neutralization are comparable: –56.5 and –42.7 J/mol, respectively, for HCl and boric acid. For boric acid (pKa = 9.24) the direct free energy (potentiometric titration) method for boric acid fails to provide a sharp end point. 15.6.1.1 Applications. Since heat of reaction is the most general property of chemical processes, thermometric titrations have a very wide range of applicability in quantitative analysis. Nonaqueous systems are well suited for this method. Thermometric titrations are very useful in titrating acetic anhydride in acetic acid–sulfuric acid acetylating baths, water in concentrated acids by titration with fuming acids, and free anhydrides in fuming acids. Precipitation and ion-combination reactions such as the halides with silver and cations with ethylene–diaminetetraacetate are other possibilities. Even halide titrations in fused salts have been done. 15.6.2

Direct-Injection Enthalpimetry Direct-injection enthalpimetry (DIE) involves the (virtually) instantaneous injection of a single “shot” of reactant into the solution under investigation, the reagent being in stoichiometric excess. The corresponding heat evolved (or absorbed) is directly proportional to the number of moles of analyte reacted. Unstandardized reagents can be used, provided they are added in sufficient excess to make the reaction at least 99% complete. Circuitry is identical to that employed with TET. The amount of heat evolved or absorbed is calculated from Joule heating calibration experiments. Precise volume measurements are not a prerequisite of DIE. The calorimeter is calibrated by passing a constant current i through a calibration heater resistor (which is immersed in the solution) and a standard resistor in the external circuit. The heat dissipated within the calorimeter may then be calculated from the expression: Q(J) = i 2 RH t =

VS VH t RS

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(15.5)

THERMAL ANALYSIS

15.10

SECTION FIFTEEN

where VS and VH are potential drops measured across the standard resistor RS and the calibration heater RH. DIE is a method of virtually universal applicability. Any rapid process that involves a heat of reaction that is equal or greater than 4.18 kJ per mole is amenable for use, whether exothermic or endothermic. Continuous-flow enthalpimetry is utilized for the on-line analysis of industrial process streams. The technique consists of passing two reactant solutions at a constant rate through a mixing chamber and continuously monitoring the heat output of the product stream. The reagent must be in stoichiometric excess. The direct thermodynamic characterization of biological molecular interactions has been summarized.4

15.7

THERMOMETRY The international temperature scale, known as ITS-90, was adopted in September 1989. Neither the definition of thermodynamic temperature nor the definition of the kelvin or the Celsius temperature scales has changed; it is the way in which we are to realize these definitions that has changed. The changes concern the recommended thermometers to be used in different regions of the temperature scale and the list of secondary standard fixed points. The changes in temperature determined using ITS-90 from the previous IPTS-68 are always less than 0.4 K, and almost always less than 0.2 K, over the range 0 to 1300 K. The ultimate definition of thermodynamic temperature is in terms of pV (pressure × volume) in a gas thermometer extrapolated to low pressure. The kelvin (K), the unit thermodynamic 4

E. Treire, O. L. Mayorga, and M. Straume, “Isothermal Titration Calorimetry,” Anal. Chem. 62:950A (1990).

TABLE 15.2 Fixed Points in the ITS-90 T, K

t, °C

Triple point of hydrogen Boiling point of hydrogen at 33 321.3 Pa Boiling point of hydrogen at 101 292 Pa Triple point of neon Triple point of oxygen Triple point of argon Triple point of mercury Triple point of water Melting point of gallium Freezing point of indium Freezing point of tin Freezing point of zinc Freezing point of aluminum Freezing point of silver Freezing point of gold Freezing point of copper

13.8033 17.035 20.27 24.5561 54.3584 83.8058 234.3156 273.16 302.9146 429.7458 505.078 692.677 933.473 1234.93 1337.33 1357.77

−259.3467 −256.115 −252.88 −248.5939 −218.7916 −189.3442 −38.8344 0.01 29.7646 156.5985 231.928 419.527 660.323 961.78 1064.18 1084.62

Secondary reference points to extend the scale (IPTS-68) Freezing point of platinum Freezing point of rhodium Freezing point of iridium Melting point of tungsten

2042 2236 2720 3660

1769 1963 2447 3387

Fixed points

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

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THERMAL ANALYSIS

THERMAL ANALYSIS

15.11

temperature, is defined by specifying the temperature of one fixed point on the scale—the triple point of water which is defined to be 273.16 K. The Celsius temperature scale (°C) is defined by the equation °C = K − 273.15

(15.6)

where the freezing point of water at 1 atm is 273.15 K. The fixed points in the ITS-90 are given in Table 15.2. Platinum resistance thermometers are recommended for use between 14 and 1235 K (the freezing point of silver), calibrated against the fixed points. Below 14 K either the vapor pressure of helium or a constant-volume gas thermometer is to be used. Above 1235 K radiometry is to be used in conjunction with the Planck radiation law, Ll = c1l−5 (e 2 / lT − 1) −1 c

(15.7)

where Lλ is the spectral radiance at wavelength l. The first radiation constant c1 is 3.741 83 × 10–16 W ⋅ m2 and the second radiation constant c2 has a value of 0.014 388 m ⋅ K.

15.8

THERMOCOUPLES The thermocouple reference data in Tables 15.3 to 15.11 give the thermoelectric voltage in millivolts with the reference junction at 0°C. Note that the temperature for a given entry is obtained by adding the corresponding temperature in the top row to that in the left-hand column, regardless of whether the latter is positive or negative. The noble metal thermocouples, types B, R, and S, are all platinum or platinum–rhodium thermocouples and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures can readily change the platinum wire calibration; hence platinum wires should only be used inside a nonmetallic sheath such as high-purity alumina. Type B thermocouples (Table 15.4) offers distinct advantages of improved stability, increased mechanical strength, and higher possible operating temperatures. They have the unique advantage that the reference junction potential is almost immaterial, as long as it is between 0 and 40°C. Type B is virtually useless below 50°C because it exhibits a double-value ambiguity from 0 to 42°C. Type E thermoelements (Table 15.5) are very useful down to about liquid-hydrogen temperatures and may even be used down to liquid-helium temperatures. They are the most useful of the commercially standardized thermocouple combinations for subzero temperature measurements because of their high Seebeck coefficient (58 mV/°C), low thermal conductivity, and corrosion resistance. They also have the largest Seebeck coefficient (voltage response per degree Celsius) above 0°C of any of the standardized thermocouples, which makes them useful for detecting small temperature changes. They are recommended for use in the temperature range from –250 to 871°C in oxidizing or inert atmospheres. They should not be used in sulfurous, reducing, or alternately reducing and oxidizing atmospheres unless suitably protected with tubes. They should not be used in vacuum at high temperatures for extended periods of time. Type J thermocouples (Table 15.6) are one of the most common types of industrial thermocouples because of the relatively high Seebeck coefficient and low cost. They are recommended for use in the temperature range from 0 to 760°C (but never above 760°C due to an abrupt magnetic transformation that can cause decalibration even when returned to lower temperatures). Use is permitted in vacuum and in oxidizing, reducing, or inert atmospheres, with the exception of sulfurous atmospheres above 500°C. For extended use above 500°C, heavy-gauge wires are recommended. They are not recommended for subzero temperatures. These thermocouples are subject to poor conformance characteristics because of impurities in the iron.

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15.12

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. −0.000 −0.0024 0.0332 0.0561 0.1019 0.2474 0.3477 0.4971 0.5182 0.6197 0.8678 1.4951 1.9784 2.1668 4.4908 5.4336 5.6263 9.5766 10.025 10.721 13.262

−268.934 −259.347* −252.88* −248.594* −246.048 −218.792* −210.001 −195.802 −182.962 −78.474 −38.834* 0.000 26.87 100.00 122.37 156.598* 231.928* 271.442 321.108 327.502 356.66 419.527* 548.23 630.74 660.37 961.93* 1064.43* 1084.5 1455 1494 1554 1772

Helium nbp Hydrogen tp Hydrogen nbp Neon tp Neon nbp Oxygen tp Nitrogen tp Nitrogen nbp Oxygen nbp Carbon dioxide sp Mercury tp Ice point Diphenyl ether tp Water bp Benzoic acid tp Indium fp Tin fp Bismuth fp Cadmium fp Lead fp Mercury bp Zinc fp Cu–Al eutectic fp Antimony fp Aluminum fp Silver fp Gold fp Copper fp Nickel fp Cobalt fp Palladium fp Platinum fp

−9.8331 −9.7927 −9.7447 −9.7046 −9.6776 −9.2499 −9.0629 −8.7168 −8.3608 −4.2275 −2.1930 0.000 1.6091 6.3171 7.8468 10.260 15.809 18.821 22.684 23.186 25.489 30.513 40.901 47.561 49.941 73.495

Type E

−8.0957 −7.7963 −7.4807 −3.7187 −1.4849 0.000 1.3739 5.2677 6.4886 8.3743 12.552 14.743 17.493 17.846 19.456 22.926 30.109 34.911 36.693 55.669 61.716 62.880

Type J

0.000 1.076 4.0953 5.0160 6.0404 9.4201 11.029 13.085 13.351 14.571 17.223 22.696 26.207 27.461 39.779 43.755 44.520

−6.4569 −6.4393 −6.4167 −6.3966 −6.3827 −6.1446 −6.0346 −5.8257 −5.6051 −2.8696

Type K

* Defining fixed points of the International Temperature Scale of 1990 (ITS-90). Except for the triple points, the assigned values of temperature are for equilibrium states at a pressure of one standard atmosphere (101 325 Pa). Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

Type B

°C

Fixed point

Abbreviations used in the table fp, freezing point bp, boiling point nbp, normal boiling point tp, triple point

TABLE 15.3 Thermoelectric Values in Millivolts at Fixed Points for Various Thermocouples

−4.345 −4.334 −4.321 −4.271 −4.300 −4.153 −4.083 −3.947 −3.802 −1.939 −0.985 0.000 0.698 2.774 3.446 4.508 6.980 8.336 10.092 10.322

Type N

−0.1830 0.000 0.1517 0.6472 0.8186 1.0956 1.7561 2.1250 2.6072 2.6706 2.9630 3.6113 5.0009 5.9331 6.2759 10.003 11.364 11.635 16.811 17.360 18.212 21.103

Type R

−0.1895 0.000 0.1537 0.6453 0.8129 1.0818 1.7146 2.0640 2.5167 2.5759 2.8483 3.4479 4.7140 5.5521 5.8591 9.1482 10.334 10.570 15.034 15.504 16.224 18.694

Type S

−6.2563 −6.2292 −6.1977 −6.1714 −6.1536 −5.8730 −5.7533 −5.5356 −5.3147 −2.7407 −1.4349 0.000 1.0679 4.2773 5.3414 7.0364 11.013 13.219 16.095 16.473 18.218

Type T

THERMAL ANALYSIS

THERMAL ANALYSIS

THERMAL ANALYSIS

15.13

TABLE 15.4 Type B Thermocouples: Platinum–30% Rhodium Alloy vs. Platinum –6% Rhodium Alloy Thermoelectric voltage in millivolts; reference junction at 0∞C. °C

0

0 0.00 100 0.0332 200 0.1782 300 0.4305 400 0.7864 500 1.2415 600 1.7912 700 2.4305 800 3.1540 900 3.9565 1000 4.8326 1100 5.7769 1200 6.7833 1300 7.8446 1400 8.9519 1500 10.0940 1600 11.2574 1700 12.4263 1800 13.5845

10

20

−0.0019 −0.0026 0.0427 0.0534 0.1987 0.2202 0.4615 0.4935 0.8275 0.8696 1.2923 1.3440 1.8512 1.9120 2.4991 2.5686 3.2308 3.3084 4.0409 4.1260 4.9241 5.0162 5.8749 5.9734 6.8871 6.9914 7.9634 8.0627 9.0648 9.1780 10.2097 10.3255 11.3743 11.4913 12.5429 12.6594 13.6991 13.8135

30 −0.0021 0.0652 0.2428 0.5266 0.9127 1.3967 1.9738 2.6390 3.3867 4.2119 5.1090 6.0726 7.0963 8.1724 9.2915 10.4415 11.6082 12.7757

40

50

60

−0.0005 0.0023 0.0062 0.0780 0.0920 0.1071 0.2665 0.2912 0.3170 0.5607 0.5958 0.6319 0.9567 1.0018 1.0478 1.4503 1.5048 1.5603 2.0365 2.1000 2.1644 2.7101 2.7821 2.8548 3.4658 3.5457 3.6264 4.2984 4.3857 4.4737 5.2025 5.2966 5.3914 6.1724 6.2728 6.3737 7.2017 7.3076 7.4140 8.2826 8.3932 8.5041 9.4053 9.5194 9.6338 10.5577 10.6740 10.7905 11.7252 11.8422 11.9591 12.8918 13.0078 13.1236

70

80

90

0.0112 0.1232 0.3438 0.6690 1.0948 1.6166 2.2296 2.9284 3.7078 4.5624 5.4868 6.4753 7.5210 8.6155 9.7485 10.9071 12.0761 13.2391

0.0174 0.1405 0.3717 0.7071 1.1427 1.6739 2.2957 3.0028 3.7899 4.6518 5.5829 6.5774 7.6284 8.7273 9.8634 11.0237 12.1929 13.3545

0.0248 0.1588 0.4006 0.7462 1.1916 1.7321 2.3627 3.0780 3.8729 4.7419 5.6796 6.6801 7.7363 8.8394 9.9786 11.1405 12.3100 13.4696

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

The type K thermocouple (Table 15.7) is more resistant to oxidation at elevated temperatures than the type E, J, or T thermocouple and consequently finds wide application at temperatures above 500°C. It is recommended for continuous use at temperatures within the range −250 to 1260°C in inert or oxidizing atmospheres. It should not be used in sulfurous or reducing atmospheres, or in vacuum at high temperatures for extended times. The type N thermocouple (Table 15.8) is similar to type K but it has been designed to minimize some of the instabilities in the conventional chromel–alumel combination. Changes in the alloy TABLE 15.5 Type E Thermocouples: Nickel–Chromium Alloy vs. Copper–Nickel Alloy Thermoelectric voltage in millivolts; reference junction at 0∞C. °C

0

10

20

30

40

50

60

70

80

90

−200 −100 −0 0 100 200 300 400 500 600 700 800 900 1000

−8.824 −5.237 0.000 0.000 6.317 13.419 21.033 28.943 36.999 45.085 53.110 61.022 68.783 76.358

−9.063 −5.680 −0.581 0.591 6.996 14.161 21.814 29.744 37.808 45.891 53.907 61.806 69.549

−9.274 −6.107 −1.151 1.192 7.683 14.909 22.597 30.546 38.617 46.697 54.703 62.588 70.313

−9.455 −6.516 −1.709 1.801 8.377 15.661 23.383 31.350 39.426 47.502 55.498 63.368 71.075

−9.604 −6.907 −2.254 2.419 9.078 16.417 24.171 32.155 40.236 48.306 56.291 64.147 71.835

−9.719 −7.279 −2.787 3.047 9.787 17.178 24.961 32.960 41.045 49.109 57.083 64.924 72.593

−9.797 −7.631 −3.306 3.683 10.501 17.942 25.754 33.767 41.853 49.911 57.873 65.700 73.350

−9.835 −7.963 −3.811 4.394 11.222 18.710 26.549 34.574 42.662 50.713 58.663 66.473 74.104

−8.273 −4.301 4.983 11.949 19.481 27.345 35.382 43.470 51.513 59.451 67.245 74.857

−8.561 −4.777 5.646 12.681 20.256 28.143 36.190 44.278 52.312 60.237 68.015 75.608

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

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THERMAL ANALYSIS

15.14

SECTION FIFTEEN

TABLE 15.6 Type J Thermocouples: Iron vs. Copper–Nickel Alloy Thermoelectric voltage in millivolts; reference junction at 0∞C. °C −200 −100 −0 0 100 200 300 400 500 600 700

0 −7.890 −4.632 0.000 0.000 5.268 10.777 16.325 21.846 27.388 33.096 39.130

10

20

30

40

50

60

−8.096 −5.036 −0.501 0.507 5.812 11.332 16.879 22.397 27.949 33.683 39.754

−5.426 −0.995 1.019 6.359 11.887 17.432 22.949 28.511 34.273 40.482

−5.801 −1.481 1.536 6.907 12.442 17.984 23.501 29.075 34.867 41.013

−6.159 −1.960 2.058 7.457 12.998 18.537 24.054 29.642 35.464 41.647

−6.499 −2.431 2.585 8.008 13.553 19.089 24.607 30.210 36.066 42.283

−6.821 −2.892 3.115 8.560 14.108 19.640 25.161 30.782 36.671 42.922

70 −7.122 −3.344 3.649 9.113 14.663 20.192 25.716 31.356 37.280

80

90

−7.402 −3.785 4.186 9.667 15.217 20.743 26.272 31.933 37.893

−7.659 −4.215 4.725 10.222 15.771 21.295 26.829 32.513 38.510

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

content have improved the order–disorder transformations occurring at 500°C, and a higher silicon content of the positive element improves the oxidation resistance at elevated temperatures. The type R thermocouple (Table 15.9) was developed primarily to match a previous platinum– 10% rhodium British wire, which was later found to have 0.34% iron impurity in the rhodium. Comments on type S also apply to type R thermocouples. The type S thermocouple (Table 15.10) is so stable that it remains the standard for determining temperatures between the antimony point (630.74°C) and the gold point (1064.43°C). The other fixed point used is that of silver. The type S thermocouple can be used from −50°C continuously up

TABLE 15.7 Type K Thermocouples: Nickel–Chromium Alloy vs. Nickel–Aluminum Alloy Thermoelectric voltage in millivolts; reference junction at 0∞C. °C

0

10

20

30

40

50

−200 −100 −0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

−5.891 −3.553 0.000 0.000 4.095 8.137 12.207 16.395 20.640 24.902 29.128 33.277 37.325 41.269 45.108 48.828 52.398

−6.035 −3.852 −0.392 0.397 4.508 8.537 12.623 16.818 21.066 25.327 29.547 33.686 37.724 41.657 45.486 49.192 52.747

−6.158 −4.138 −0.777 0.798 4.919 8.938 13.039 17.241 21.493 25.751 29.965 34.095 38.122 42.045 45.863 49.555 53.093

−6.262 −4.410 −1.156 1.203 5.327 9.341 13.456 17.664 21.919 26.176 30.383 34.502 38.519 42.432 46.238 49.916 53.439

−6.344 −4.669 −1.517 1.611 5.733 9.745 13.874 18.088 22.346 26.599 30.799 34.909 38.915 42.817 46.612 50.276 53.782

−6.404 −4.912 −1.889 2.022 6.137 10.151 14.292 18.513 22.772 27.022 31.214 35.314 39.310 43.202 46.985 50.633 54.125

60 −6.441 −5.141 −2.243 2.436 6.539 10.560 14.712 18.839 23.198 27.445 31.629 35.718 39.703 43.585 47.356 50.990 54.466

70

80

90

−6.458 −5.354 −2.586 2.850 6.939 10.969 15.132 19.363 23.624 27.867 32.042 36.121 40.096 43.968 47.726 51.344 54.807

−5.550 −2.920 3.266 7.338 11.381 15.552 19.788 24.050 28.288 32.455 36.524 40.488 44.349 48.095 51.697

−5.730 −3.242 3.681 7.737 11.793 15.974 20.214 24.476 28.709 32.866 36.925 40.879 44.729 48.462 52.049

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

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THERMAL ANALYSIS

15.15

THERMAL ANALYSIS

TABLE 15.8 Type N Thermocouples: Nickel–14.2% Chromium–1.4% Silicon Alloy vs. Nickel–4.4% Silicon–0.1% Magnesium Alloy Thermoelectric voltage in millivolts; reference junction at 0∞C. °C

0

10

20

30

40

50

60

70

80

90

−200 −100 −0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

−3.990 −2.407 0.000 0.000 2.774 5.912 9.340 12.972 16.744 20.609 24.526 28.456 32.370 36.248 40.076 43.836 47.502

−4.083 −2.612 −0.260 0.261 3.072 6.243 9.695 13.344 17.127 20.999 24.919 28.849 32.760 36.633 40.456 44.207

−4.162 −2.807 −0.518 0.525 3.374 6.577 10.053 13.717 17.511 21.390 25.312 29.241 33.149 37.018 40.835 44.577

−4.227 −2.994 −0.772 0.793 3.679 6.914 10.412 14.091 17.896 21.781 25.705 29.633 33.538 37.402 41.213 44.947

−4.277 −3.170 −1.023 1.064 3.988 7.254 10.772 14.467 18.282 22.172 26.098 30.025 33.926 37.786 41.590 45.315

−4.313 −3.336 −1.268 1.339 4.301 7.596 11.135 14.844 18.668 22.564 26.491 30.417 34.315 38.169 41.966 45.682

−4.336 −3.491 −1.509 1.619 4.617 7.940 11.499 15.222 19.055 22.956 26.885 30.808 34.702 38.552 42.342 46.048

−4.345 −3.634 −1.744 1.902 4.936 8.287 11.865 15.601 19.443 23.348 27.278 31.199 35.089 38.934 42.717 46.413

−3.766 −1.972 2.188 5.258 8.636 12.233 15.981 19.831 23.740 27.671 31.590 35.476 39.315 43.091 46.777

−3.884 −2.193 2.479 5.584 8.987 12.602 16.362 20.220 24.133 28.063 31.980 35.862 39.696 43.464 47.140

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

TABLE 15.9 Type R Thermocouples: Platinum–13% Rhodium Alloy vs. Platinum Thermoelectric voltage in millivolts; reference junction at 0∞C. °C (Below zero) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

0

10

20

30

40

50

60

70

80

90

0.0000 0.6472 1.4681 2.4000 3.4072 4.4706 5.5823 6.7412 7.9488 9.2034 10.5032 11.8463 13.2239 14.6239 16.0347 17.4447 18.8424 20.2151

−0.0515 0.0543 0.7228 1.5571 2.4978 3.5112 4.5796 5.6960 6.8598 8.0722 9.3313 10.6356 11.9827 13.3631 14.7647 16.1759 17.5852 18.9810 20.3497

−0.100 0.1112 0.8000 1.6471 2.5963 3.6157 4.6892 5.8101 6.9789 8.1960 9.4597 10.7684 12.1194 13.5025 14.9056 16.3172 17.7256 19.1194 20.4834

−0.1455 0.1706 0.8788 1.7381 2.6954 3.7208 4.7992 5.9246 7.0984 8.3203 9.5886 10.9017 12.2565 13.6421 15.0465 16.4583 17.8659 19.2575 20.6161

−0.1877 0.2324 0.9591 1.8300 2.7953 3.8264 4.9097 6.0398 7.2185 8.4451 9.7179 11.0354 12.3939 13.7818 15.1876 16.5995 18.0059 19.3953 20.7475

−0.2264 0.2965 1.0407 1.9229 2.8957 3.9325 5.0206 6.1554 7.3390 8.5703 9.8477 11.1695 12.5315 13.9218 15.3287 16.7405 18.1458 19.5329 20.8777

0.3627 1.1237 2.0167 2.9968 4.0391 5.1320 6.2716 7.4600 8.6960 9.9779 11.3041 12.6695 14.0619 15.4699 16.8816 18.2855 19.6702 21.0064

0.4310 1.2080 2.1113 3.0985 4.1463 5.2439 6.3883 7.5815 8.8222 10.1086 11.4391 12.8077 14.2022 15.6110 17.0225 18.4251 19.8071

0.5012 1.2936 2.2068 3.2009 4.2539 5.3562 6.5054 7.7035 8.9488 10.2397 11.5745 12.9462 14.3426 15.7522 17.1634 18.5644 19.9437

0.5733 1.3803 2.3030 3.3037 4.3620 5.4690 6.6230 7.8259 9.0758 10.3712 11.7102 13.0849 14.4832 15.8935 17.3041 18.7035 20.0797

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

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THERMAL ANALYSIS

15.16

SECTION FIFTEEN

TABLE 15.10 Type S Thermocouples: Platinum–10% Rhodium Alloy vs. Platinum Thermoelectric voltage in millivolts; reference junction at 0∞C. °C

0

10

20

30

40

50

60

70

80

90

(Below zero) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

0.0000 0.6453 1.4400 2.3227 3.2597 4.2336 5.2373 6.2743 7.3449 8.4483 9.5847 10.7536 11.9471 13.1550 14.3680 15.5765 16.7712 17.9417

−0.0527 0.0552 0.7194 1.5250 2.4143 3.3557 4.3327 5.3394 6.3799 7.4537 8.5605 9.7002 10.8720 12.0674 13.2762 14.4892 15.6967 16.8895 18.0562

−0.1028 0.1128 0.7948 1.6109 2.5065 3.4519 4.4320 5.4417 6.4858 7.5629 8.6730 9.8159 10.9907 12.1878 13.3975 14.6103 15.8168 17.0076 18.1698

−0.1501 0.1727 0.8714 1.6975 2.5991 3.5485 4.5316 5.5445 6.5920 7.6724 8.7858 9.9320 11.1095 12.3084 13.5188 14.7314 15.9368 17.1255 18.2823

−0.1944 0.2347 0.9495 1.7849 2.6922 3.6455 4.6316 5.6477 6.6986 7.7823 8.8989 10.0485 11.2286 12.4290 13.6401 14.8524 16.0566 17.2431 18.3937

−0.2357 0.2986 1.0287 1.8729 2.7858 3.7427 4.7318 5.7513 6.8055 7.8925 9.0124 10.1652 11.3479 12.5498 13.7614 14.9734 16.1762 17.3604 18.5038

0.3646 1.1089 1.9617 2.8798 3.8403 4.8323 5.8553 6.9127 8.0030 9.1262 10.2823 11.4674 12.6707 13.8828 15.9042 16.2956 17.4474 18.6124

0.4323 1.1902 2.0510 2.9742 3.9382 4.9331 5.9595 7.0202 8.1138 9.2403 10.3997 11.5871 12.7917 14.0041 15.2150 16.4148 17.5942

0.5017 1.2726 2.1410 3.0690 4.0364 5.0342 6.0641 7.1281 8.2250 9.3548 10.5174 11.7069 12.9127 14.1254 15.3356 16.5338 17.7105

0.5728 1.3558 2.2316 3.1642 4.1348 5.1356 6.1690 7.2363 8.3365 9.4696 10.6354 11.8269 13.0338 14.2467 15.4561 16.6526 17.8264

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

to about 1400°C, and intermittently at temperatures up to the freezing point of platinum (1769°C). The thermocouple is most reliable when used in a clean oxidizing atmosphere, but may also be used in inert gaseous atmospheres or in a vacuum for short periods of time. It should not be used in reducing atmospheres nor in those containing metallic vapor (such as lead or zinc), nonmetallic vapors (such as arsenic, phosphorus, or sulfur), or easily reduced oxides, unless suitably protected with nonmetallic protecting tubes. The type T thermocouple (Table 15.11) is popular for the temperature region below 0°C (but see under type E). It can be used in vacuum or in oxidizing, reducing, or inert atmospheres.

TABLE 15.11 Type T Thermocouples: Copper vs. Copper–Nickel Alloy Thermoelectric voltage in millivolts; reference junction at 0∞C. °C

0

10

20

30

40

50

60

70

80

90

−200 −100 −0 0 100 200 300 400

−5.603 −3.378 0.000 0.000 4.277 9.286 14.860 20.869

−5.753 −3.656 −0.383 0.391 4.749 9.820 15.443

−5.889 −3.923 −0.757 0.789 5.227 10.360 16.030

−6.007 −4.177 −1.121 1.196 5.712 10.905 16.621

−6.105 −4.419 −1.475 1.611 6.204 11.456 17.217

−6.181 −4.648 −1.819 2.035 6.702 12.011 17.816

−6.232 −4.865 −2.152 2.467 7.207 12.572 18.420

−6.258 −5.069 −2.475 2.908 7.718 13.137 19.027

−5.261 −2.788 3.357 8.235 13.707 19.638

−5.439 −3.089 3.813 8.757 14.281 20.252

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

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THERMAL ANALYSIS

THERMAL ANALYSIS

15.17

Bibliography Brennen, W. P., R. B. Cassel, and M. P. DiVito, “Materials and Process Characterization by Thermal Analysis,” Am. Lab., 20 (January):32 (1988). Burros, B. C., “Thermal Analysis and Food Chemistry,” Am. Lab. 18 (January):20 (1986). Daly, K. F., “Applications of Thermal Analysis to Pharmaceutical Compounds and Related Materials,” Am. Lab. 7 (January):57 (1975). DiVito, M. P., W. P. Brennan, and R. L. Fyans, “Thermal Analysis: Trends in Industrial Applications,” Am. Lab. 18 (January):82 (1986). Gibbons, J. J., “Applications of Thermal Analysis Methods to Polymers and Rubber Additives,” Am. Lab. 19 (January):33 (1987). Gill, P. S., “Thermal Analysis Developments in Instrumental and Applications,” Am. Lab. 16 (January):39 (1984). Kolthoff, I. M., P. J. Elving, and C. Murphy, eds., Treatise on Analytical Chemistry, 2d ed., Part I, Vol. 12, Thermal Methods, Wiley, New York, 1983. Marcozzi, C., and K. Reed, “Simultaneous TGA-DTA, Theory and Applications,” Am. Lab. 25 (January):33 (1993). Staub, F., “Application of TA to Elastomers,” Am. Lab. 18 (January):56 (1986). Svehla, G., ed., Wilson and Wilson’s Comprehensive Analytical Chemistry, Vol. XII, Thermal Analysis, Elsevier, New York, 1984. Welton, R. E., et al., “Third Generation DMTA Instrumentation,” Am. Lab. 25 (January):15 (1993). Wendlandt, W., Thermal Methods of Analysis, 3d ed., Wiley, New York, 1986. Widmann, G., “Thermal Analysis of Plastics,” Am. Lab. 19 (January):98 (1987). Wunderlich, B., Thermal Analysis, Academic, New York, 1990.

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THERMAL ANALYSIS

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 16

OPTICAL ACTIVITY AND ROTATORY DISPERSION 16.1 INTRODUCTION 16.2 CHIRALITY AND OPTICAL ACTIVITY 16.2.1 Chiral Center 16.2.2 Enantiomers 16.2.3 Optically Inactive Chiral Compounds 16.3 SPECIFIC ROTATION 16.4 OPTICAL ROTATORY DISPERSION 16.5 CIRCULAR DICHROISM Table 16.1 Optical Rotation of Pure Organic Liquids Table 16.2 Optical Rotation of Carbohydrates and Related Compounds Table 16.3 Optical Rotations of Natural Amino Acids and Related Compounds Table 16.4 Optical Rotations of Selected Organic Compounds 16.6 RELATION OF ORD AND CD 16.7 MEASUREMENT OF OPTICAL ROTATION 16.7.1 Polarimeters 16.7.2 Spectropolarimeters Bibliography

16.1

16.1 16.1 16.1 16.2 16.2 16.2 16.3 16.3 16.4 16.5 16.7 16.9 16.18 16.18 16.18 16.19 16.19

INTRODUCTION Optical activity (the property of rotating the plane of polarization of light) is characteristic of many organic compounds and of a few inorganic complexes. The determination of the angle of optical rotation is a classical technique of quantitative analysis. More recently, the technique has been extended to structural and stereochemical compounds, and in these the rotatory dispersion (the variation of specific rotation of plane-polarized light with the wavelength of the light) has been found most revealing.

16.2

CHIRALITY AND OPTICAL ACTIVITY A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral elements, namely, a chiral center, a chiral axis, or a chiral plane.

16.2.1 Chiral Center The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four

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OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.2

SECTION SIXTEEN

different atoms or groups. One “ligand” may be a lone pair of electrons; another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon.

16.2.2 Enantiomers Two nonsuperimposable structures that are mirror images of each other are known as enantiomers. Enantiomers are related to each other in the same way that a right hand is related to a left hand. Except for the direction in which they rotate the plane of polarized light, enantiomers are identical in all physical properties. Enantiomers have identical chemical properties except in their reactivity toward optically active reagents. Enantiomers rotate the plane of polarized light in opposite directions but with equal magnitude. If the light is rotated in a clockwise direction, the sample is said to be dextrorotatory (to the right) and is designated as (+). When a sample rotates the plane of polarized light in a counterclockwise direction, it is said to levorotatory (to the left) and is designated as (−). Use of the former designations d and l (or D and L) is discouraged.

16.2.3 Optically Inactive Chiral Compounds Although chirality is a necessary prerequisite for optical activity, chiral compounds are not necessarily optically active. With an equal mixture of two enantiomers, no net optical rotation is observed. Such a mixture of enantiomers is said to be racemic and is designated as (±) and not as dl or DL. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image; it has an internal mirror plane. This is an example of a diastereromer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers.

16.3

SPECIFIC ROTATION Optical rotation is caused by individual molecules of the optically active compound. The amount of rotation depends upon how many molecules the light beam encounters in passing through the sample tube. When allowances are made for the length of the tube that contains the sample and the sample concentration, it is found that the amount of rotation, as well as its direction, is a characteristic of each individual optically active compound. Specific rotation is the number of degrees of rotation observed if a 1-dm tube is used and the compound being examined is present to the extent of 1 g per 100 mL. For a pure liquid, its density replaces the solution concentration. Specific rotation = [a ] =

observed rotation (degrees) length (dm) × (g/100 mL)

(16.1)

The temperature of the measurement is indicated by a superscript and the wavelength of the light employed by a subscript written after the bracket; for example, [a]20 589 implies that the measurement was made at 20°C using 589-nm radiation. Values are always understood to apply to a wavelength of 589 nm (sodium D line) except as otherwise indicated by the appearance of a different wavelength as a superscript.

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OPTICAL ACTIVITY AND ROTATORY DISPERSION

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.3

Rotatory power is also expressed in terms of molar rotation [f] (formerly [M]): [f ] =

[a ] × molecular weight 100

(16.2)

The effect of solvent is often small, but it may become extremely large if it affects the state of ionization or conformation. The effect of temperature on rotation is usually small, but it, too, may become larger whenever it affects the structure of the compound. This is particularly true in those cases in which two forms are in an equilibrium that shifts with the temperature. In carbohydrates, such equilibria give rise to mutarotation. Whenever a sample of the a or b form is dissolved, the rotation changes until the equilibrium is established. For carbohydrates (see Table 16.2), the initial values of rotation are listed with arrows pointing to the final values. Where no temperature is given, the data cited were obtained at room temperature. The optical rotation of pure organic liquids is given in Table 16.1. Table 16.2 contains the optical rotation of carbohydrates and related compounds; Table 16.3 has similar information for natural amino acids and related compounds. Optical rotations of selected organic compounds are in Table 16.4.

16.4

OPTICAL ROTATORY DISPERSION An optical rotatory dispersion curve (ORD) is related to the change in refractive index of a compound as a function of wavelength. The observed rotation is a=

p (h − h r ) l l

(16.3)

where l = wavelength hl = refractive index for left-circularly polarized light hr = refractive index for right-circularly polarized light An optically inactive substance will retard the speeds of the two circularly polarized components to the same extent, and no net rotation can be observed. However, the speed of the two circularly polarized components are retarded by an optically active substance to a different extent, and this results in the rotation of the plane of polarization. Since changes in refractive index are greatest in the vicinity of absorption bands, ORD spectra are most pronounced in these wavelength regions. For each ORD spectrum there is an anomalous curve within the spectral region of the optically active absorption band that is called the Cotton effect. The tailings outside the absorption band are called the plain curve. Each Cotton effect consists of two extremes, a geometric maximum called a peak and a geometric minimum called a trough. A Cotton effect is further characterized by its amplitude, defined as the vertical distance between peak and trough, its breadth, which is the horizontal distance between the peak and trough, and its sign. A positive Cotton effect curve has its peak at the longer wavelength region, while a negative Cotton effect curve is defined as having its trough appear at the longer wavelength.

16.5

CIRCULAR DICHROISM Circular dichroism (CD) is concerned with the intensities of the right- and left-circularly polarized components of the linearly polarized light beam. CD is related directly to the molecular absorption

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OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.4

SECTION SIXTEEN

TABLE 16.1 Optical Rotation of Pure Organic Liquids Compound

Molecular weight

(+)-2-Bromobutane (−)-2-Bromobutane (+)-2-Bromooctane (−)-2-Bromooctane 2-Bromopropanoic acid

137.03 137.03 193.13 193.13 152.98

(+)-2-Butanol (−)-2-Butanol sec-(+)-Butyl acetate (+)-Carvone (−)-Carvone (−)-Diethyl hydroxysuccinate (+)-Diethyl tartrate (−)-Ethyl lactate (+)-Fenchone (−)-Fenchone

74.12 74.12 116.16 150.21 150.21 190.19 206.19 118.13 152.23 152.23

(−)-2-Heptanol (+)-2-Hydroxy-2-butanone

116.20 88.10

(+)-12-Hydroxyoleic acid (+)-Limonene

298.45 136.23

(−)-Limonene (+)-Linalool (−)-Linalool (+)-N-Methylbenzylamine (+)-3-Methylbutanal (+)-3-Methylbutanoic acid 1-Methyl-2-propylpiperidine Nicotine Nicotine hydrochloride 10% in H2O (+)-2-Octanol

(+)-a-Pinene hydrochloride (−)-a-Pinene hydrochloride (+)-b-Pinene (−)-b-Pinene (+)-Propylenediamine (−)-Propylenediamine 2-Propylpiperidine (coniine) Thiolactic acid Thujol (−)-a-Thujone (+)-b-Thujone

136.23 154.24 154.24 121.18 86.13 102.13 141.25 162.23 198.71 130.22

172.71 172.71 136.23 136.23 74.13 74.13 127.22 106.14 154.2 152.23 152.23

[a] +10.8 −12.2 +27.5 −27.5 −27 +70 +13.52 −13.51 +25.4 +61.2 −62.46 −10.2 +7.5 −10 +66.9 −51.2 −77.2 −154 −10.48 +4.5 +8.0 +17.6 +57.3 +6.67 +85 +123.8 +136 +233 −103.1 +19.3 −20.1 +40.3 +23.6 +18.0 +81 −169 −203 +104 +7.5 +9.89 +26.2 +51.14 −51.28 +28.59 −22.4 +29.7 −28 −14.2 −45.5 +68 −19.2 +72.5

Temperature, °C

Wavelength, nm

25 25 17 17 20 20 27 25 20 20 20 20 20 14 20 16 18 18 17 20 20 20 20 22 22 19.3 22 22 19.3 20 20 22 20 17 24 20 20 20 20 20 20 20 20

D* 400 D D D D

D D D

546 436 D

671 546 436 340 D

671 D

546 436 D D D D

D D

546 D

671 D

382 D D D

25 25 23 15 20 20 15

* Sodium D line, 589 nm.

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D

D D

180.16 180.16 150.13 152.15 342.30 355.31 164.16 164.16 134.13 180.16 260.14 164.16 164.16 180.16 180.16 194.14 194.14 342.30 342.30 196.16 179.17 180.16 180.16 260.14

(+)-b-Allopyranose (b-allose)

(+)-Altropyranose (b-altrose) (−)-b-Arabinose (+)-Arabitol Cellobiose Chondrosine 2-Deoxy-(+)-glucose 6-Deoxy-(+)-glucose (isorhamnose) 2-Deoxy-(+)-ribose Fructose Fructose-6-dihydrogen phosphate

(+)-Fucose

(−)-Fucose

(+)-a-Galactose (+)-b-Galactose (+)-a-Galacturonic acid (+)-b-Galacturonic acid a-Gentiobiose b-Gentiobiose Gluconic acid Glucosamine a-(+)-Glucose b-(+)-Glucose a-Glucose-l-phosphate a-Glucose-1-phosphate, potassium salt Glucose-6-phosphate, potassium salt (+)-Glucuronic acid (+)-Glucuronic acid g-lactone (+)-Glyceraldehyde (−)-Glyceraldehyde Glycogen (high-mol.-wt. polymer) 336.33 194.14 176.12 90.08 90.08

411.21

Molecular weight

a-Acetobromoglucose

Compound

The value of [a] was measured at 589 nm (sodium D line).

+199.3 +230 +0.58 (4 min) → +3.26 (10 min) → +14.41 (20 h, max) +32.6 +173 (6 min) → +105.1 (22.5 h) +7.7 +14.2 → +34.6 (15 h) +39 +38.5 → +45.5 (35 min) +73 (6 min) → +30 (3 h, final) −56.2 (final value) −132 → −92 (rapid) +2.5 +1.2 +127.0 (7 min) → +89.4 (31 min) → +76.0 (146 min, final value) −124.1 (10 min) → −108.0 (20 min) → −75.6 (24 h) +150.7 → +80.2 +52.8 → +80.2 +98.0 → +50.9 +27 → +55.6 +16 (3 min) → +8.3 (3.5 h) −5.9 (6 min) → +9.6 (6 h) −6.7 +100 → +47.5 (30 min) +112.2 → +52.7 +18.7 → +52.7 +120 +78 +21.2 +11.7 → +36.3 (2 h) +19.8 +8.7 −8.7 +196 to +197

[a]

TABLE 16.2 Optical Rotation of Carbohydrates and Related Compounds

Water Water Water Water Water Water Water Water Water Water Water Water Water

Water Water Water Water

Water

Water Water Satd. aq. Na2B4O7 Water Water Water Water Water Water Water Water Water

CHCl3 Benzene Water

Solvent

4 3 1 1 10 10 1 4 1.3 6 5.19 2 2

5 4 10

9

0.52 8.3 1 2 3 0.9 10

7.6 3 9.26 8

3 9 6

Concentration, g/100 mL

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(Continued)

20 20 20 20 22 22 20 20 20 20 25 20 24 24 25 25 25 25

20

20 12 20 20 20 17.5 20 22 20 21 21 19

19 15 20

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.5

180.16 180.16 180.16 180.16 360.32 342.30 150.13 360.33 180.16 180.16 270.3 378.33 228.68 678.61 180.16 594.54 164.16 164.16 150.13 210.14 182.17 180.16 342.30

180.16 120.10 120.10 378.34 342.30 150.13

b-Lactose

a-(+)-Lyxose Maltose monohydrate a-(+)-Mannose b-(+)-Mannose b-(+)-Mannose phenylhydrazone Melibiose dihydrate N-Methyl-a-(−)-glucosamine HCl Octaacetyl-b-cellobiose (+)-Psicose Raffinose pentahydrate a-(−)-Rhamnose b-(−)-Rhamnose (+)-Ribose (+)-Tetrahydroxyhexanedioic acid Sorbitol (−)-Sorbose Sucrose

(+)-Tagatose (+)-Threose (−)-Threose Trehalose dihydrate Turanose Xylose

Molecular weight

(+)-Gulose (−)-Gulose (+)-Idose (−)-Idose a-Lactose monohydrate

Compound Water Water Water Water Water

−20.4 +21.3 +15.8 −17.4 +92.6 → +83.5 (10 min) → +69 (50 min) → +52.3 (22 h) +34 (2 min) → +39 (6 min) → +46 (1 h) → +52.3 (22 h) +5.5 → −14.0 +111.7 → +130.4 +29.3 → +14.2 −17.0 → +14.2 +26.3 → +33.8 +111.7 → +129.5 −103 → −88 −14.7 +4.7 +105.2 −7.7 → +8.9 +31.5 (1 min) → +8.9 −25 +6.86 → +20.60 −2.0 −42.7 +66.47 to +66.49 +78.3 (546), +128.4 (436), +309.5 (302), +541.2 (284) −2.3 −12.3 (20 min, final) +13.10 (final) +178.3 +27.3 → +75.8 +92 → +18.6 (16 h) Water Water Water Water Water Water

Water Water Water Water Pyridine Water Water CHCl3 Water Water Water Water Water Water Water Water Water Water

Water

Solvent

[a]

TABLE 16.2 Optical Rotation of Carbohydrates and Related Compounds (Continued)

2.19 4 4.5 7 4 10

5 26 26

4 0.6 5 4.3 4 4 4 4

0.82 4 4 4

4

4.58 2.3 3.6 4.5

Concentration, g/100 mL

20 20 20 20 20 20

20 20 20 20 20 20 25 20 25 20 20 20 24 19 20 30 20 20

25

13 20 20

20

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.6

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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 210.68 168.60 133.10 133.10 121.16 140.30 140.30 140.59 140.59 432.97 197.19 197.19 147.13 147.13 146.15 307.33 155.16 119.12 163.14 131.13

Arginine ⋅ HCl (−)-Asparagine ⋅ HCl

(+)-Aspartic acid (−)-Aspartic acid Cysteine (+)-Cystine (−)-Cystine (−)-2,3-Diaminopropionic acid ⋅ HCl (+)-2,3-Diaminopropionic acid ⋅ HCl (−)-3,5-Diiodotyrosine

(+)-3-(3,4-Dihydroxyphenyl)alanine (DOPA) (−)-3-(3,4-Dihydroxyphenyl)alanine (−)-Glutamic acid (+)-Glutamic acid Glutamine Glutathione Histidine

(−)-Homoserine

Hydroxyglutamic acid

4-Hydroxy-(−)-proline cis form (−)-Isoleucine

131.17

103.12 139.60 103.12 140.26 174.20

89.09

Molecular weight

a-(−)-Aminobutyric acid a-(−)-Aminobutyric acid ⋅ HCl b-(+)-Aminobutyric acid Anserine (−)-Arginine

(−)-Alanine

Compound

−13.1 +31.4 −30.5 +6.1 −21 −39.74 +47.6 −8.8 +18.3 +17.6 +1.2 −76.5 −58.1 +11.29 +40.61

+2.8 +10.3 (660), +14.3 (589), +23.3 (500), +39.3 (440) +8.40 +12.90 +35.20 +12.3 +12.5 +26.9 +12.0 −5.42 +20.0 −23.0 +25.0 +9.8 +223 −223.4 +25.3 −25.0 +2.89 +2.27 +13.0

[a]

TABLE 16.3 Optical Rotations of Natural Amino Acids and Related Compounds

1N HCl 6N HCl 6N HCl Water Water Water Hydrochloride in H2O Water 2N HCl 6N HCl Water Water Water Water 6.1N HCl

Water Water Water Water Water 6.0N HCl Water Water 1M HCl 6N HCl 6N HCl Water 1.0N HCl 1.0N HCl 1.0M HCl 1.0M HCl 5 g of 4% HCl 5 g of 25% NH3 1N HCl

Water 3M HCl

Solvent

5.12 1 1 3.6 2.74 1.13 2 5 5 2 2 1 5.2 3 4.6

5 3.5 1.65 4 1.3 (1 mol) 2.3 1.97 1.3 1 1 5 5 0.246 0.227 5.27

4 3.64

6 4.4

Concentration, g/100 mL

(Continued)

18 20 20

13 22.4 20 23 27 20 20 26 26 20 20

16 19 20 30 20 20 20 20 20 27 20 30 20 20 20 20 20 20 11

25 22

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.7

117.15 117.15 131.17 146.19 149.21 218.25 131.17 117.15 117.15 132.16 165.19 165.19 115.13 105.09 119.12 776.93 776.93 204.22 181.19 117.15

(−)-Methionine

N-Methyl-(−)-tryptophan (Abrine) (−)-Norleucine

(−)-Norvaline (+)-Norvaline Ornithine Ornithine ⋅ 2HCl (−)-Phenylalanine (+)-Phenylalanine

(−)-Proline

(−)-Serine

(−)-Threonine (−)-Thyroxine (+)-Thyroxine (−)-Tryptophan

(−)-Tyrosine

(−)-Valine

Molecular weight

(−)-Isovaline (+)-Isovaline (−)-Leucine (−)-Lysine

Compound

Solvent Water Water Water Water 6.0N HCl Water 1M HCl 0.5M HCl 5N HCl Water 20% HCI 20% HCl Water Water Water Water 18% HCl Water 0.50N HCl Water 5.6N HCl Water 0.13N NaOH in 70% EtOH 6 g 0.5N NaOH and 14 g EtOH Water 0.5N HCl 1.0N HCl 3N NaOH Water 20 HCl

[a] +11.13 −11.28 −10.8 +14.6 +25.9 −8.2 +19.3 +44 +23.1 +6.26 +23.0 −24.2 +11.5 +16.6 −35.1 +35.0 +7.1 −85.0 −52.6 −6.83 +14.45 −28.3 −4.4 +2.97 at 546 nm −31.5 +2.4 −10.6 −13.2 +13.9 +22.9

TABLE 16.3 Optical Rotations of Natural Amino Acids and Related Compounds (Continued)

5 5 2.2 6.5 2 1 3.4 2.8 4.25 0.7 10 10 6.5 5.3 1.94 2.04 3.8 1 0.58 10 0.5 1.1 3% 0.74 1 1 4 4 0.9 0.8

Concentration, g/100 mL

25 25 25 20 23 25 28 21 20 20 20 20 25 23 20 20 20 23.4 20 20 25 26 20 21 23 20 22 18 26 23

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.8

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2% NaOH Water CHCl3 1N NaOH 6N HCl Acetone CHCl3 CHCl3 CHCl3 CHCl3 1,4-Dioxane Water Methanol Methanol 0.1N HCl Water Methanol Methanol (neat) Ethanol 6N HCl 0.1N HCl Water

−46.6 −26.7 +300 −9.8 −35.9 −7 +150 +97 +113.4 +95 +94 +141 −88.9 −155.3 +16.7 +18.3 +98.9 −101.9 +79 −16 −30 +276.3 +246

264.31 188.18 203.24 191.26 191.25 645.72 708.8 267.24 365.24 365.24 507.21 300.40 416.56 160.17 885.20 360.45 390.52 332.49 346.45 364.47 348.39 283.40 311.39 266.25 119.12 296.40 296.40 130.19 103.17 212.21 216.26 365.42

(−)-cis,trans-Abscisic acid N 2-Acetyl-(−)-glutamine (−)-Acetylcarnitine (+)-N-Acetylmethionine (+)-N-Acetylpenicillamine Aconitine Aconitine nitrate Adenosine (−)-Adenosine 3′-monophosphate hydrate (3′-Adenylic acid) (−)-Adenosine 5′-monophosphate hydrate (5′-Adenylic acid, AMP) Adenosine triphosphate Adrenosterone Agaricic acid (−)-Alanyl-(−)-alanine Alborixin Aldosterone Alfadolone acetate Alfaxalone Algestone Allotetrahydrocortisone Alstonine N-Allyl-3-hydroxymorphinan N-Allylnormorphine (+)-N-(4-Aminobenzoyl)glutamic acid (+)-4-Amino-3-hydroxybutyric acid (+)-Aminopentamide (−)-Aminopentamide (R)-(+)-1-Amino-2-(methoxymethyl) pyrrolidine (R)-(−)-2-Amino-3-methyl-butanol (+)-threo-2-Amino-1-(4-nitrophenyl)-1,3-propanediol (+)-6-Aminopenicillanic acid (6-APA) Amoxicillin

Absolute alcohol EtOH 0.005N H2SO4 in MeOH 0.005N H2SO4 Water Water Water 50% EtOH Chloroform Water Water Water

Solvent

−106 +411.10 +426.5 −426.2 −12.5 −19.52 +20.3 +18 +17.3 −35 −58.2 −41.6

[a]

302.44 264.31

Molecular weight

Abietic acid (+)-cis,trans-Abscisic acid

Compound

TABLE 16.4 Optical Rotations of Selected Organic Compounds

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10 1

3.095 1 1 2 4 0.1 1.02 1.2 0.81 1.45 0.4 3 3 2 2 1 1

(Continued)

22 20

20 25

22 20 25 22 20 25 26 26 23 25 25 20 25 20 20 23 23 18

22

20 9 22

2 0.658 1 2

24 20 20 20 20 20 25 25

Temperature, °C

1 1 1 1 2.9 6 4 1

Concentration, g/100 mL

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.9

1290.46 924.11

349.42 457.42 426.70 426.70 162.24 330.49 160.21 290.43 265.30 315.32 798.72 282.35 176.12 173.13 212.22 212.22 244.31 154.24 231.14 394.47 444.55 361.54 416.65 136.23 136.23 152.23 200.23 232.31 842.00 826.00 136.24 161.20 150.22 150.22 290.28

Ampicillin Amygdalin a-Amyrin b-Amyrin Anabasine Anagestone (as acetate) (−)-Anatabine Androstenediol Anisomycin Anthramycin Aplasmomycin Artimisinin (−)-Ascorbic acid Azaserine (−)-Benzoin (+)-Benzoin Biotin (+)-Borneol 3-Bromo-(+)-camphor Brucine Bufotalin (+)-Butaclamol

Calcitriol (+)-Camphene (−)-Camphene Camphor Camphoric acid Camphor-b-sulfonic acid Carbomycin A Carbomycin B 3-Carene (−)-Carnitine (R)-(−)-Carvone (S)-(+)-Carvone (+)-Catechin hydrate

Molecular weight

Amphomycin Amphotericin B.

Compound

−33.6 +281 −42 +91.6 +99.8 −83.1 +24 −177.8 −55.5 −30 +930 +225 +66.3 +48 −0.5 −118 +120.5 +91 +37.7 +122.7 −127 +5.4 +218.5 (as HCl salt) +48 +103.5 −119.11 +41 to +43 +47 to +48 +21.5 −58.6 −35 +15 −23.9 −58 +58 +16.0

+7.5 +333

[a]

TABLE 16.4 Optical Rotations of Selected Organic Compounds (Continued)

Methanol Diethyl ether Benzene U.S.P. alcohol Ethanol Water Chloroform Chloroform (neat) Water (neat) (neat) Water

Water, pH 6 Acidic dimethylformamide (DMF) 0.1N HCl in MeOH Water Water Benzene Benzene Water CHCl3 Water 2-Propanol Methanol Dimethylformamide Chloroform Chloroform Methanol Water, pH 8.6 Acetone Acetone 0.1N NaOH Ethanol Benzene, 100 g Chloroform Chloroform Methanol

Solvent

16.10

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0.86

1 9.67 2.33 10 2.5 4.3 1 1

1 1 1 1.3 1.3 1 1 1 0.4 1 1 1.24 1.64 1 8.46 1.2 1.2 1 5 14.5 1 0.5 1

1 1

Concentration, g/100 mL

25 17 21 25 20 20 25 25 20 30 20 20 21

20 25

24 21 20 17 19 20 20 17 18 23 25 22 17 23 27.5 12 12 21 20 20

25 24

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

466.60 162.18 353.36 309.54 323.14 90.55 186.68 354.30 893.5 907.5 478.88 386.64 408.56 294.38 294.38 148.11 154.24 156.26 303.35 299.36 315.36 399.43 342.35 360.46 369.44 310.38 162.18 243.22 323.19 323.19 174.11 371.42 392.56 372.49 567.62

Cephaeline (+)-Chalcose

Chelidonine a-Chloralose Chloramphenicol 3-Chloro-1-butene

3-Chloro-d-camphor

Chlorogenic acid Chlorophyll a Chlorophyll b Chlortetracycline

Cholesterol Cholic acid Cinchonidine Cinchonine Citramalic acid

Citronellal b-Citronellol

Cocaine Codeine

Codeine N-oxide Colchicine

Coniferin Cortisone

Corydaline Cuprein Cymarose Cytidine 2′-Cytidylic acid 3′-Cytidylic acid Dehydroascorbic acid Demecolcine Deoxycholic acid Deoxycorticosterone acetate Deoxydihydrostreptomycin

−43.4 +120 (2 min) → +97 (10 min) → +76 (3 h) +117 +19 +18.6 −2.52 D (−) form +5.87 L (+) form +96.1 endo form +35 exo form −35 (hemihydrate) −262 −267 −275.0 −240 (hydrochloride) −31.5 +37 −109.2 +229 +23.6 (+) form −23.4 (−) form +11.50 +5.22 (+) form −4.76 (−) form −16 −136 −112 −97.1 (monohydrate) −121 −429 −68 +209 +269 (546 nm) +311 −176 +54.7 (24 h) +31 +20.7 +49.4 +56 −129.0 +55 +168 to +175 −102.5 Ethanol Chloroform Ethanol Chloroform Water Chloroform Water Water Ethanol Benzene Ethanol Methanol Water Water Water Water Water Chloroform Ethanol 1,4-Dioxane Water

Water Ethanol

Chloroform 98% Ethanol Ethanol (neat) (neat) Ethanol Ethanol Water Acetone Acetone + methanol Methanol Water Diethyl ether Ethanol Ethanol Ethanol

Chloroform Water

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4 2 2 2.1 0.9 1.72 0.5 1.2 0.125 0.8 1.8 3.2 0.7 1 1 1 1

3

0.6

2 0.6

5 5 2.8

3 5 4.86

2 1.5

16.11

(Continued)

20 20 15 15 18 17 17 20 25 25 20 17 20 25 20 20 20 20 20 20 20

22 25

26 20 20 23 23 20 20 20 20

20 22 25 20 25 20

20 24

OPTICAL ACTIVITY AND ROTATORY DISPERSION

246.20 480.63 669.57 165.23 183.20 325.39 581.65 733.92 120.11 340.28 270.36 441.40 458.53 346.37 2218.75 355.42 181.19 178.14 179.17 307.33 217.26 465.61 154.25 352.77

Epinephrine Ergonovine Ergotamine Erythromycin (+)-Erythrose Esculin Estrone Folic acid Fumagillin Gibberellic acid Giractide (+)-Glaucine Glucamine Gluconolactone Glucosamine Glutathione Glutethimide

Glycocholic acid Grandisol Griseofulvin

392.52 369.40 235.25 211.22 178.18 448.62 1229.30 374.50 764.92 301.37 583.67 287.35 158.11

Molecular weight

Doxifluridine Emetine Enterobactin Ephedrine

Dextromoramide Diacetylmorphine Dideoxyadenosine Dideoxycytidine Digitalose Digitogenin Digitonin Digitoxigenin Digitoxin Dihydrocodeine Dihydroergotamine Dihydromorphine 4,5-Dihydroorotic acid

Compound +25.5 −166 −25.2 +81 +109 (15 min; 546 nm) −81 −54 +19 +4.8 −72 to −75 (acid tartrate) −79 (546 nm) −112 (as HCl) +33.23 (−) form −31.54 (+) form +18.4 −50 +7.40 +62 (+) form HCl −33 to −35.5 (−) form HCl −53.5 +90 −160 −78 +1 to −14.5 (3 d) −78.4 (sesquihydrate) +152 +23 −26.6 +86 −51.4 ± 1.9 +115 −7.95 +61.7 +100 → +47.5 (30 min) −18.9 +176 (+) form −181 (−) form +30.8 +18.5 +370

[a]

TABLE 16.4 Optical Rotations of Selected Organic Compounds (Continued)

Benzene Methanol Water Water Water Chloroform Methanol Methanol 1,4-Dioxane Water Pyridine Water 1% NaHCO3 1% NaHCO3 Water Chloroform Ethanol Water Water 0.5M HCl Water Chloroform Ethanol Water 50% 1,4-Dioxane Chloroform 0.1M NaOH Ethanol Water 0.1N Acetic acid Ethanol Water Water Water Water Methanol Methanol Ethanol Hexane Chloroform

Solvent

16.12

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0.8 5 5 1 1 2 1 2.5 1 0.5 1 2.12 0.472 3 10 1 1 4.653

5 1.49 1.01 0.635 1.7 1.4 2.8 1.36 1.2 1 0.5 1.6 1.992 2.01 0.419 2

Concentration, g/100 mL 20 25 25 25 27 19 20 25 20 25 20 25 25.3 25.3 25 20 25 20 25 25 20 20 25 20 18 22 25 25 19 23.5 20 15 20 20 25 20 20 23 21.5 17

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

289.36 268.23 348.22 ca. 5000 777.09 570.93 206.32 206.32 206.32 206.32 296.31 569.61 176.12 666.77 378.45 146.15 273.32 268.34 309.43

Hyoscyamine Inosine 5′-Inosinic acid Inulin Iopamidol Iopanoic acid

(+)-cis-a-Irone (+)-trans-a-Irone (+)-b-Irone (+)-cis-g-Irone Isatropic acid Isepamicin

Isoascorbic acid Isobutol Isoflupredone (−)-Isoglutamine Isoladol Isolysergic acid Isomethadone 286.32 311.37 371.39 213.23 1188.44 484.51 328.46 208.21 90.08

610.55 382.44 383.39 104.10 402.67 163.14 330.45 220.22

Guaran Hesperidin Heterophylline (Aricine) Hydrastine 3-Hydroxybutanoic acid 25-Hydroxycholesterol b-Hydroxyglutamic acid 17a-Hydroxyprogesterone 5-Hydroxytryptophan

cis-Isopilosine Isothebaine Isradipine Kainic acid Kallidin Kanamycin A 11-Ketoprogesterone (−)-Kynurenine (+)-Lactic acid

283.24 363.23

Guanosine 3′-Guanylic acid

−16.6 +3 +108 +20.5 −150 (−) form +281 (dihydrate) +20.8 (+) form −20 (−) form +83.9 +285 +6.7 S(+) form −14.8 −57 +146 +270 −29 (hydrate) −2.6 (546 nm)

−60.5 −65 −8 +53 −76 −91 −50 +24.3 −39.0 +17.6 +105.6 −32.5 (−) form +16.0 (−) form −21.0 −49.2 −18.5 −40 −2.01 −5.2 (−) form 5.1 (+) form +109 +420 +59 +2 +9.94 +110.9 0.1N NaOH 5% NaOH Water 1N NaOH Pyridine Chloroform Ethanol Water Chloroform 6N HCl Chloroform Water 4N HCl Ethanol Water 2.5% HCl Water Water Ethanol Ethanol Dichloromethane Dichloromethane Dichloromethane Dichloromethane Ethanol Water (disulfate hydrate) Water Water Ethanol Water Ethanol Pyridine (neat) Ethanol Ethanol Ethanol Ethanol Water 1N Acetic acid 0.1N H2SO4 Chloroform Water Water

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1.5 1 1 1.5 1.01 1 1

1 5% 0.611 6.1 0.952 1

6.7 1

2 1.4 0.3 2.226 1.05 2 1.042 1 1 1 0.9 3 g Ba salt 2 10 2 2

3 2 2

16.13

(Continued)

16.5 20 23 21 20 20 25 25 20 18 20 24 21 24 25 20 21.5

20 20 20 25 20 20 20 10 25 20 17 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20

OPTICAL ACTIVITY AND ROTATORY DISPERSION

406.56 651.01 338.46 376.56 337.47 404.55 426.70 267.32 268.32 134.09 133.14 340.51 2846.54 156.26 154.24 345.90 328.46 211.21 339.42 195.22 193.20 194.18 374.46 302.44 514.45 285.33 321.81

Methotrimeprazine (−)-Methyldopa Methylergonovine N-Methylglucamine N-Methyl-a-(−)-glucosamine a-Methylglucoside Methylprednisolone 17-Methyltestosterone Mildiomycin Morphine Morphine hydrochloride

154.24

Linalool

Lincomycin Liothyronine Lisuride Lithocholic acid Lobeline Lovastatin Lupeol Lysergamide Lysergic acid Malic acid Mandelonitrile Medrogestone Melittin Menthol (−)-Menthone Methadone hydrochloride

90.08 358.30 342.30 426.70 208.24 590.80 1209.42 197.19 136.23

Molecular weight

(−)-Lactic acid Lactobionic acid Lactulose Lanosterol Lanthionine Lasalocid A. Leuprolide Levodopa Limonene

Compound +2.6 (546 nm) +53.0 → +22.6 (6 h) −51.4 (24 h) +62.0 +9.4 (−) form −7.55 −31.7 −13.1 +123.8 (+) form −101.3 (−) form −20.1 (−) form +19.3 (+) form +137 (HCl, hemihydrate) +21.5 +31.3 +33.7 −43 (−) form +323 +27.2 15 (546 nm) +40 −2.3 (−) form 43.75 (+) form +79 −89.52 −50 −24.8 −145 (−) form −169 (−) form −17 −4.0 (sesquihydrate) −45 −23 −62 +158.9 +83 +69 to +75 +100 −132 (monohydrate) −113.5 (trihydrate)

[a]

TABLE 16.4 Optical Rotations of Selected Organic Compounds (Continued)

Water 33% 1N HCl + 67% ethanol Pyridine Ethanol Ethanol Acetonitrile Chloroform Pyridine Pyridine Water Benzene Chloroform Water 10% Ethanolic solution Ethanol Water Ethanol Chloroform 0.1N HCl Pyridine Water Methanol Water 1,4-Dioxane 1,4-Dioxane Water Methanol Water

Water Water Water, pH 4.8 Chloroform 2.4N NaOH Methanol 1% Acetic acid 1N HCl (neat) (neat)

Solvent

1 4.75 0.60 1.5 1 0.5 4.8 0.5 0.5 8.5 5 1 0.409 5.5 5.5 2.5 2.1 5 1 0.4 1 1 10 1 1 0.5 1 2.2 (anhyd.)

2.5 8 4 1 1.4 1 1 5.12

Concentration, g/100 mL

20 25 25 25 13 19.5 19.5 20 20 25 29.5 20 20 15 25 20 20 20 19 25 23 21 18 20 20 20 20 20 20 20 25 20 20 25 20 20 25

21−22 20

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.14

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238.40 274.39 399.39 322.4 299.36 475.60 162.23 385.38 418.45 169.18 298.41 312.44 148.20 151.20 612.65 293.37 315.36 460.44 130.14 219.23 112.12 149.21 909.11 517.63 1265.79 372.47 359.40 350.38 334.37 268.27 150.21 136.23 136.23 206.24 436.40 275.34 602.57

Muscone Nandrolone Narcotoline Neomycin Neopine Netilmicin Nicotine Nilvadipine

Nimodipine

Norepinephrine Norethindrone Norgestrel Nornicotine Norpseudoephedrine Novobiocin Nystatin Ondansetron

Oxycondone Oxytetracycline Pantolactone Pantothenic acid Parasorbic acid Penicillamine Penicillin G benzathine Penicillin G benzhydrylamine Penicillin G hydrabamine Penicillin G potassium Penicillin N Penicillin V 2-Pentenylpenicillin sodium Pentostatin Perillaldehyde a-Phellandrene

b-Phellandrene

Pheneturide Phloridzin Physostigmine Picrotoxin

−13(−) form +55 −189 +112.8 −28 (hydrobromide) 164 +104 (hydrochloride) +222.42 (+) form −219.62 (−) form +7.9 (+) form −7.93 (−) form −37.3 (−) form −31.7 −42.5 (−) form −89 +37.9 (+) form −63.0 −10 −14 3S form +16 3R form −125 (hydrochloride) −196.6 (dihydrate) −50.7 (−) form +37.5 +210 −63 (hydrochloride) +206 (hydrated) 206 +115.3 +285 to +310 +187 (barium salt) +223 (potassium salt) +316 +76.4 +127 −217 (−) form +86.4 (+) form +65.2 (+) form −51.9 (−) form +54.0 (+) form −52 (dihydrate) −76 −29.3 Chloroform Chloroform (20-cm tube) Water Chloroform Water Water Methanol Methanol 1,4-Dioxane 1,4-Dioxane Water +1 equiv. HCl Chloroform Chloroform Water Methanol Ethanol Glacial acetic acid Methanol Methanol Water 0.1N HCl Water Water Ethanol 1N NaOH Formamide Water Chloroform Water Water Water Water Water Carbon tetrachloride (neat) (neat) (neat) (neat) Ethanol Ethanol Chloroform Ethanol 1 3.2 1.3 2.31

2 1 0.105 1 10 0.7 0.6 0.2 0.88 1 13.1

0.93 0.4 1 7.5 3 10 1 1 0.439 0.374 5 1 1 100 3 1 1 0.19 0.34 2.5 1 2.05

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(Continued)

17 23 20 20 23 26 20 20 20 20 20 25 20 25 22 20 24 25 25 24 20 25 25 25 19 25 25 20 25 22 20 25 15 25 20 20 16 20 20 17 25 17 16

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.15

208.25 136.23 463.55 360.44 358.44 339.48 794.0 810.0 303.35 152.23 328.4 372.4 129.11 486.67 416.42 324.41 324.41 746.93 164.16 271.41 634.71 608.70 376.36 298.45 610.51 326.30 286.27 751.02 246.29 305.20 303.35 348.39 174.15 414.69 234.37 581.58 743.75 363.4

b-Pinene Pivampicillin Prednisolone

Prednisone a-(+)-Propoxyphene Protoveratrine A Protoveratrine B Pseudococaine Pulegone Pyrethrin I Pyrethrin II (−)-Pyroglutamic acid Quillaic acid Quinacillin Quinidine Quinine Quinine sulfate Quinovose Racemethorphan Rescinnamine Reserpine Riboflavine Ricinoleic acid Rutin Rutinose Salicin Salinomycin a-Santonin

(−)-Sarcolysine Scopolamine Serpentine Shikimic acid b-Sitosterol Sparteine Streptomycin Streptomycin B Streptothricin F

Molecular weight

Pilocarpine

Compound +106 +91 (hydrochloride) +28.59 +196 +102 +116 (21-acetate) +186 (21-acetate) +59.8 (hydrochloride) −40.5 −37 +42 −22.5 −14 +14.7 −11.9 +56.1 +183.5 +230 −169 −220 (dihydrate) +75 (5 min) → +30 (3 h, final) +27.6 (hydrobromide) −27 −118 −112 to −122 +7.15 +13.82 +3.2 → +0.8 −45.6 −37 (sodium salt) −170 to −175 (−) form +165.9 (+) form −31.5 −28 +292 −183.8 −37 −16.4 −84 (trihydrochloride) −47 (trihydrochloride monohydrate) −51.3 (hydrochloride)

[a]

TABLE 16.4 Optical Rotations of Selected Organic Compounds (Continued)

1,4-Dioxane Water Pyridine Pyridine Chloroform (neat) Isooctane Isooctane–diethyl ether Water Pyridine Water Chloroform Ethanol 0.5N HCl Water Water Chloroform Chloroform 0.02M NaOH in ethanol Acetone Ethanol Water Abs. ethanol Ethanol Ethanol Ethanol Methanol Water Methanol Water Chloroform Abs. ethanol Water Water Water

Water 1,4-Dioxane

Water Water

Solvent

1 1 2 2.9 1 1.8 2 5 8.3 1.5 1 1 0.4 5 1 4 0.6 1 2 1.92 0.67 2.7 0.27 4.03 2 10 1 1.35 1.4

1 0.6 1 1 5

1 1

2 2

Concentration, g/100 mL

25 25 25 25 20 23 20 19 20 20 23 15 15 15 20 20 24 23 25 26 23 20 20 25 25 20 22 22 25 18 25 23 25 25 25

20 25

18 18

Temperature, °C

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.16

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404.49 334.40 397.64 678.58 150.09 853.92 172.19 154.24 288.41 311.37 206.32 196.22 299.31 242.23 776.93

416.66 416.66 416.66 681.66 144.20

Strophanthidin Strychnine Sucralose Sucrose octaacetate Tartaric acid Taxol Terpenylic acid a-Terpineol Testosterone Thebaine Thioctic acid 5-Thio-(+)-glucose Thioguanosine Thymidine Thyroxine

b-Tocopherol g-Tocopherol d-Tocopherol Tubocurarine chloride Uridine

−4.4 (−) form +6.37 −2.4 +3.4 (546 nm) +190 +4

+43.1 −139.3 +68.2 +58.5 −12 (−) form, +12 (+) form −49 +56.3 (+) form, −56.5 (−) form +92.45 (+) form, −100 (−) form +100 −219 +104 (+) form, −113 (−) form +188 −64 +30.6 +2.97 (+) form

Ethanol Ethanol Water Water

20 4 2 0.88 1.56 1.3 1.029 0.74

Ethanol Ethanol Ethanol Benzene Water 0.1M NaOH Water 6 g 0.5M NaOH and 14 g ethanol 0.13M NaOH in 70% EtOH

16 0.5 2

3%

2.8 1 1 2.56 20 1

Methanol Chloromethane Ethanol Abs. ethanol Water Methanol

20 20 20 25 22 20

25.4 20 20 25 20 24 15 23 20 22 25 21

25 18

OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.17

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OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.18

SECTION SIXTEEN

coefficients of a compound. The difference between ⑀l and ⑀r , the absorption coefficients for left- and right-circularly polarized light, respectively, is a measure of the intensity of circular dichroism. If the medium is optically inactive, the decrease of their intensities is equal. However, an optically active medium absorbs the two components to a different extent, resulting in an absorption difference. The CD band can be positive or negative. The shape of the CD band is the same as the absorption band and is observable only within the spectral region in which absorption occurs. The maximum or minimum of the CD band corresponds to the maximum of the absorption band.

16.6

RELATION OF ORD AND CD ORD and CD are both manifestations of the same underlying phenomenon arising from the interaction of polarized light with asymmetric structural elements of a molecule. CD is a measure of the differential absorption of left- and right-handed circularly polarized light. ORD appears as a rotation of the direction of vibration of linearly polarized light. The point of inflection of the Cotton effect is located at the same wavelength as the maximum of the CD band, and the sign of the Cotton effect is the same as that of the corresponding CD. On a purely theoretical basis both methods will provide the same amount of structural information. However, practical instrumentation does impose some different limitations, so that each has its advantages and each may complement the other. Sometimes resolution of the ORD spectra is difficult because of overlapping of the tailings of each of the Cotton effects. In contrast, relatively weak transitions can be identified by CD. The ready resolution of CD bands make it superior to ORD, particularly in the structural studies of complex molecules and biomolecules that may possess many asymmetric elements. On the other hand, some optically active absorption bands may be located in a spectral region inaccessible to the instruments. Under such circumstances, while no CD measurements can be made, the continuing tailing of the Cotton effect of an ORD spectrum that usually extends into the instrumentally accessible spectral region can be turned to advantage in providing structural information originating from the inaccessible Cotton effect.

16.7

MEASUREMENT OF OPTICAL ROTATION

16.7.1 Polarimeters When ordinary white light, which is vibrating in all possible planes, is passed through a Nicol prism (the polarizer), two polarized beams of light are generated. One of these beams passes through the prism, while the other beam is reflected and does not interfere with the plane-polarized beam, the one that is coincident with the propagation axis of light. If the beam of plane-polarized light is also passed through a second Nicol prism (the analyzer), it can be transmitted only if the second Nicol prism has its axis oriented so that it is parallel to the plane-polarized light. If its axis is perpendicular to that of the plane-polarized light, the light will not pass through. The sample cell is placed between the two Nicol prisms. If an optically active substance is in the sample tube, the light is deflected. The analyzer prism is rotated to permit maximum passage of light and is then said to be aligned. The angle of rotation (in degrees) is measured. The standard wavelength is that of the green mercury line at 546.1 nm, although the sodium doublet has been widely employed, especially in the older measurements. Additional wavelengths can be obtained from the mercury lamp and suitable filters: 365, 405, 436, and 633 nm. The standard temperature is 20°C. Although the standard length of sample tubes is 100 ± 0.03 mm, lengths of 50 and 200 mm are available. Sample volumes will range from 0.1 up to 12.0 mL, depending both on length and internal diameter of the sample tubes.

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OPTICAL ACTIVITY AND ROTATORY DISPERSION

16.19

16.7.2 Spectropolarimeters The basic requirements for ORD measurements are (1) a means of producing an intense linearly polarized beam of monochromatic light at various wavelengths, and (2) a detection system with its associated electronic circuitry. 16.7.2.1 Light Source. A high-pressure, 450-W, xenon arc lamp provides high-intensity, continuous emission throughout the range 185 to 600 nm. UV-transmitting quartz is used to construct the lamp envelope. The entire optical train must be purged with dry nitrogen (about 1 L ⋅ min−1) to remove corrosive ozone from the system and to minimize the absorption due to atmospheric oxygen, water vapor, and carbon dioxide when working at wavelengths less than 200 nm. 16.7.2.2 Monochromator. A double monochromator is used in order to reduce stray light, which will affect the apparent rotation detected by the analyzer since some of this stray light may be polarized. The type of linear polarizer most suitable for ORD and CD measurements in the ultraviolet and visible region is the birefringent polarizer. It produces light within a highly defined plane of polarization and possesses high transmittance at a wide range of wavelengths. This type of polarizer is made from double refracting uniaxial crystals such as calcite or quartz. Most spectropolarimeters employ Rochon-type polarizers, which are made from two prisms cut from a uniaxial crystal with their optic axes perpendicular to one another and one of them parallel to the direction of the incident light. Specially cut crystal quartz prisms permit the prism to be used simultaneously as a monochromator (Jasco J710–J720 series). When a polarized light beam passes through an optically active sample, its plane of vibration is rotated by an angle that can be measured by the use of an analyzer. In visual polarimeters, constant light intensity reaching the eye is used to determine the closeness to crossed position of the polarizer and the analyzer. In recording spectropolarimeters the polarizer can be oscillated through a few degrees about a mean angle, which results in extinction when the mean angle is equal to zero. Another technique uses a Faraday cell to rotate the plane of the polarized beam so as to compensate for rotation due to an optically active sample. 16.7.2.3 Detector. The detector is a photomultiplier tube with a working range of 185 to 800 nm (an S-20 surface, end-on configuration). 16.7.2.4 Calibration. ORD instruments can be calibrated or checked with standard sucrose or dextrose solution [NIST, standard reference materials (SRMs) 17d and 41c] at three wavelengths: 546, 589, and 633 nm.

Bibliography Charney, E., The Molecular Basis of Optical Activity, Wiley, New York, 1979. Crabbe, P., ORD and CD in Chemistry and Biochemistry: An Introduction, Academic, New York, 1972. Djerassi, C., Optical Rotatory Dispersion, McGraw-Hill, New York, 1960. Wong, K-P., “Optical Rotatory Dispersion and Circular Dichroism,” J. Chem. Educ. 51:A573 (1974); 52:A9 (1975); 52:A83 (1975).

SECTION 17

REFRACTOMETRY 17.1 INTRODUCTION 17.1.1 Refractive Index Table 17.1 Atomic and Group Refractions 17.1.2 Refractometers Table 17.2 Refractometer Sensitivity for Solutions and Organic Mixtures

17.1

17.1 17.1 17.2 17.3 17.3

INTRODUCTION When light passes from one medium into another, its velocity is changed. The ratio of the velocity of light in a vacuum to that in a substance is known as the index of refraction or refractive index of that substance. The index of refraction varies with the wavelength of light employed, with temperature, and with pressure (for gases). A variety of instruments, called refractometers, permits the measurement of indices of refraction of gases, liquids, and solids. Refractometry is the term applied to the group of optical methods for the analysis of either relatively pure substances or complex mixtures, based on refractive-index measurements. It, usually, is applied to identify pure substances. Some substances, called isotropic materials, transmit light with equal velocity in all directions and have only one index of refraction. Gases, liquids, glasses, and most solids of the isometric system belong to the isotropic group of materials. Other solids, which do not transmit light with equal velocity in all directions, are called anisotropic materials.

17.1.1

Refractive Index The refractive index of a liquid is the ratio of the velocity of light in a vacuum to the velocity of light in the liquid. The angle of refraction varies with the wavelength of the light used. Usually the yellow sodium doublet lines are used; they have a weighted mean of 589.26 nm and are symbolized by D. A typical refractive index (h) would be expressed as h 5D = 1.4567

where the superscript indicates the temperature and the subscript indicates the wavelength of the light source. The refractive indices for many thousands of compounds will be found in Ref. 1. When only a single refractive index is available, approximate values over a small temperature range may be calculated using a mean value of 0.000 45 per degree for dh/dt, remembering that h decreases with an increase in temperature. If a transition point lies within the temperature range, extrapolation is not reliable. The refractive index of moist air can be calculated from the expression (h − 1) × 10 6 =

1

103.49 177.4 86.26  5748 p1 + p2 + 1+ p T T T  T  3

(17.1)

J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

17.1

17.2

SECTION SEVENTEEN

where p1 is the partial pressure of dry air (in mmHg), p2 is the partial pressure of carbon dioxide (in mmHg), p3 is the partial pressure of water vapor (in mmHg), and T is the temperature (in kelvins). 17.1.1.1 Specific Refraction. The specific refraction rD is independent of the temperature and pressure and may be calculated by the Lorentz and Lorenz equation: rD =

h2D − 1 r (h2D + 2)

(17.2)

where r is the density at the same temperature as the refractive index. The empirical Eykman equation  h2D − 1  1  h + 0.4 r = const  D 

(17.3)

offers a more accurate means for checking the accuracy of experimental densities and refractive indices, and for calculating one from the other. The molar refraction is equal to the specific refraction multiplied by the molecular weight. It is a more or less additive property of the groups or elements comprising the compound. A set of atomic refractions is given in Table 17.1; an extensive discussion is given in Ref. 2. 2 N. Bauer, K. Fajans, and S. Lewin, in A. Weissberger, ed., Physical Methods of Organic Chemistry, 3d ed., Wiley-Interscience, New York, 1960, Vol. 1, Pt. II, Chap. 28.

TABLE 17.1 Atomic and Group Refractions Group

MrD

Group

MrD

H C Double bond (C == C) Triple bond (C ≡≡ C) Phenyl (C6H5) Naphthyl (C10H7) O (carbonyl)(C == O) O (hydroxyl)(O −− H) O (ether, ester)(C−− O −− ) F (one fluoride) F (polyfluorides) Cl Br I S (thiocarbonyl)(C−− O) S (thio)(S−− H) S (dithia)(−− S−− S−− ) Se (alkyl selenides) Three-membered ring Four-membered ring

1.100 2.418 1.733 2.398 25.463 43.00 2.211 1.525 1.643 0.95 1.1 5.967 8.865 13.900 7.97 7.69 8.11 11.17 0.71 0.48

N (primary aliphatic amine) N (sec-aliphatic amine) N (tert-aliphatic amine) N (primary aromatic amine) N (sec-aromatic amine) N (tert-aromatic amine) N (primary amide) N (sec-amide) N (tert-amide) N (imidine) N (oximido) N (carbimido) N (hydrazone) N (hydroxylamine) N (hydrazine) N (aliphatic cyanide)(C≡≡ N) N (aromatic cyanide) N (aliphatic oxime) NO (nitroso) NO (nitrosoamine) NO3 (alkyl nitrate) NO2 (alkyl nitrite) NO2 (aliphatic nitro) NO2 (aromatic nitro) NO2 (nitramine)

2.322 2.499 2.840 3.21 3.59 4.36 2.65 2.27 2.71 3.776 3.901 4.10 3.46 2.48 2.47 3.05 3.79 3.93 5.91 4.37 7.59 7.44 6.72 7.30 7.51

REFRACTOMETRY

17.1.2

17.3

Refractometers Two types of refractometers are available, the differential and the critical angle. In the differential refractometer, a light beam is transmitted through a partitioned cell that refracts the beam at an angle that depends on the difference in refractive index between the sample liquid in one part and a standard liquid in the other. In the critical-angle refractometer the light incident on the surface of the solution changes sharply from reflected to transmitted light at a critical angle. Refractometers differ in the ranges they cover, the accuracy obtainable, the type of light source employed, and the presence or absence of color-compensating prisms. In general, when white light is used for illumination, the color-compensating prisms are provided. 17.1.2.1 Abbé Refractometer. The Abbé refractometer, an example of a critical-angle refractometer, compares the angles at which light from a point source passes through the test liquid and into a prism the refractive index of which is known. A drop of the sample is placed between the upper and lower prisms and, following the directions supplied by the manufacturer, the refractive index of the sample is read from the dial. The critical angle is the angle from the perpendicular at which the beam changes from light transmitted into the liquid to light totally reflected at the liquid surface. At angles smaller than the critical angle the light is transmitted into the liquid. The critical angle depends not only on the solution composition but also on the prism material. The significant feature of a critical angle refractometer is that it measures the refractive index at the surface of a solution. Since surface reflection requires no penetration of the light beam into the solution, this type of refractometer may be used for highly opaque samples and various murky solutions and suspensions, as well as transparent samples. The range of the Abbé refractometer is normally 1.3000 to 1.7000, the maximum precision attainable being 0.0001. This refractometer reads the refractive index directly and requires only a drop of sample. Process versions of the critical-angle refractometer make it uniquely suitable for many types of binary mixtures. Applications, with an indication of sensitivities attainable, are shown in Table 17.2. TABLE 17.2 Refractometer Sensitivity for Solutions and Organic Mixtures System

Minimum full-scale span, wt. %

Water In acetic acid In ethanol In methanol

0–0.40 0–1.07 0–0.79

Ethanol In water In benzene

0–0.59 0–0.26

Methanol in water Ethylene glycol in water Propylene glycol in water Glycerol in water Acetone in water

0–1.78 0–0.42 0–0.37 0–0.33 0–0.58

Benzene In ethanol In cyclohexane

0–0.31 0–0.75

Cyclohexane in benzene Trichlorfluoromethane in dichlorodifluoromethane Sodium chloride in water Ammonium sulfate in water

0–0.40 0–0.36 0–0.41 0–0.28

17.4

SECTION SEVENTEEN

17.1.2.2 Differential Refractometers. Differential refractometers are intended primarily for the analysis of liquid mixtures. They are applicable to any mixture whose refractive index is a singlevalued function of the composition; as such they are uniquely applicable as detectors in high-performance liquid chromatography in which they monitor the difference in refractive index between the mobile phase (reference) and the column effluent. The sensitivity of differential refractometers is 0.000 001 refractive-index units. Liquid samples must be clear and clean. Different refractometers operate on one of two principles, as discussed in the following sections. 17.1.2.2.1 Deflection Type. The deflection type measures the deflection of a beam of monochromatic light by a double prism. The sample (or column eluent) is placed in (or flows through) half of the prism; the reference liquid (or pure mobile phase) fills the other half. The reference and sample compartments are separated by a diagonal glass divider. If the refractive index of the sample differs from that of the reference, or the eluent from a chromatographic column differs from the pure mobile phase, the beam from the sample compartment is slightly deflected. The cell volume is 15 to 25 mL. Deflection refractive-index detectors have the advantage of a wide range of linearity. One cell covers the entire refractive-index range. 17.1.2.2.2 Reflection Type. In the optical path of the reflection-type refractometer, two collimated beams from the light source (with masks and lens) illuminate the reference (mobile phase only) and sample (eluent) cells. The cells’ volume (3 mL) is a depression formed with a Teflon gasket that is clamped between the prism and a reflecting backplate (finely ground to diffuse the light). The diffuse reflected light passes through the flowing liquid film and is imaged onto dual photodetectors. Since the percentage of reflected light at the glass–liquid interface changes as the refractive index of the liquid changes, a signal arises when a solute emerges. The detector is adjusted to zero with mobile phase in both cells. This type of refractometer has a relatively limited range. Two different prisms must be used to cover the usual refractive-index range.

SECTION 18

ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS 18.1

MICRODETERMINATION OF CARBON, HYDROGEN, AND NITROGEN 18.1.1 Removal of Interfering Substances 18.1.2 Measurement of Combustion Gases 18.2 TOTAL CARBON AND TOTAL ORGANIC CARBON ANALYZERS 18.3 KJELDAHL DETERMINATION OF NITROGEN 18.3.1 Special Situations 18.3.2 Automated Kjeldahl Method 18.4 DETERMINATION OF SULFUR 18.4.1 Measuring the Sulfur in the Combustion Products 18.4.2 Tube Combustion (Manual) 18.4.3 Schöninger Combustion 18.5 DETERMINATION OF HALOGENS 18.5.1 Decomposition of the Organic Material 18.5.2 Measurement of the Halides by Amperometric Titration 18.5.3 Separation of the Halides by Ion Exchange 18.6 OXYGEN DETERMINATION 18.7 DETERMINATION OF OTHER NONMETALS 18.7.1 Antimony 18.7.2 Arsenic 18.7.3 Bismuth 18.7.4 Boron 18.7.5 Fluoride 18.7.6 Phosphorus 18.7.7 Silicon 18.7.8 Selenium and Tellurium 18.8 DETERMINATION OF TRACE METALS IN ORGANIC MATERIALS 18.9 METHODS FOR MULTIELEMENT TRACE ANALYSES Bibliography

18.1 18.2 18.2 18.3 18.3 18.4 18.4 18.4 18.4 18.5 18.5 18.5 18.5 18.5 18.5 18.6 18.6 18.6 18.6 18.7 18.7 18.7 18.7 18.8 18.8 18.8 18.8 18.9

18.1 MICRODETERMINATION OF CARBON, HYDROGEN, AND NITROGEN Since all organic compounds contain carbon and hydrogen, and a large number of them also additionally contain nitrogen, it can be seen that the ability to measure these elements accurately is of extreme importance for characterization and identification of such organic compounds. The microcombustion technique is the principal means for determining carbon, hydrogen, and nitrogen. Automated elemental analyzers that are commercially available offer multisample and unattended operation. The combustion operation is completely automated and is followed by an on-line measurement of the components in the combustion gases. Computerization permits 18.1

18.2

SECTION EIGHTEEN

extensive data reduction, calculation, reporting, and storage capabilities. The technique involves several steps. 1. In the purge mode, the weighed sample is dropped into the loading head, which is then sealed and all the interfering gases are purged from the combustion path. 2. In the burn mode, the sample is moved onto a ceramic crucible and into the furnace for combustion in a flowing stream of pure oxygen at 900°C. The sample boat can subsequently be removed for weighing any residue. Alternatively, the sample can be mixed with cobalt(III) oxide [or a mixture of manganese dioxide and tungsten(VI) oxide] to provide the oxygen and heated to the same combustion temperature. Carbon dioxide, water vapor, nitrogen and some oxides of nitrogen, and oxides of sulfur are possible products of combustion of an organic compound. The burn time is 10 to 12 min. 3. The removal of interfering elements is effected. 4. The measurement of the carbon dioxide, nitrogen, and water vapor formed is performed. 18.1.1 Removal of Interfering Substances The interfering substances encountered in the determination of carbon and hydrogen are sulfur, the halogens, and nitrogen. Hot copper at 550 to 670°C reduces the nitrogen oxides to nitrogen and removes residual oxygen. In carbon–hydrogen analyzers, oxides of nitrogen are removed with manganese dioxide. Copper oxide converts any carbon monoxide to carbon dioxide. A magnesium oxide layer in the middle of the furnace removes fluorine. A silver-wool plug at the exit removes chlorine, iodine, and bromine, and also any sulfur or phosphorus compounds that result from the combustion of the sample. Calcium oxide removes oxides of sulfur in a secondary combustion zone so that water vapor cannot combine to form sulfurous or sulfuric acid. 18.1.2 Measurement of Combustion Gases A variety of techniques have been used to separate and measure the components in the combustion gases. These include gas chromatography (Sec. 5.2), thermal conductivity (Sec. 5.2.5.1), infrared spectrometry (Sec. 7), and coulometry (Sec. 14.8). 18.1.2.1 Gas-Chromatographic Method. In one approach excess oxygen is removed from the combustion gases and the nitrogen oxides are reduced to nitrogen with copper. Helium, used as carrier gas, sweeps the carbon dioxide, water, and nitrogen onto a chromatographic column for separation. Signals from the three chromatographic peaks are integrated to ascertain the quantities present in the sample. In this method relatively small samples must be used so that the combustion products represent a “slug” injection on the chromatographic column. In another approach the gases pass through a charge of calcium carbide in which water vapor is converted to acetylene. A nitrogen cold trap freezes the sample gases and isolates them in a loop of tubing. A valve seals off the combustion train, which is then ready for another sample. The chromatographic separation is begun by removing the cold trap and heating the loop. Another stream of dry helium gas carries the gases from the trap into the chromatographic column where the three gases (nitrogen, carbon dioxide, and acetylene) are completely separated. The chromatographic separation requires about 10 min. 18.1.2.2 Thermal Conductivity Detection. Three pairs of thermal conductivity detectors are used in a differential manner. Specific absorbents are placed between the detectors. The helium carrier gas

ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

18.3

fills a mixing volume to a specified pressure. The mixed gas is passed through the sample side of detector 1. Water is then removed by a magnesium perchlorate trap from the gases, which are then passed through the reference side of the same detector. Similarly carbon dioxide is determined by passing the effluent from the first detector into the sample side of detector 2, removing carbon dioxide from the gas with a soda–asbestos trap, and passing the stripped gas through the reference side of detector 2. Nitrogen is determined by detector 3, which compares the effluent gas from the second detector after removal of carbon dioxide (and water in the earlier measurement) with pure helium. 18.1.2.3 Infrared Detection Methods. Dispersive (Sec. 7.2.3.1) and nondispersive infrared detectors are available for water and carbon dioxide. One type of nondispersive spectrometer uses filters to isolate the wavelength desired. A simple filter infrared analyzer is designed around a multisegment circular interference filter drive system. 18.1.2.4 Coulometric Detection. Coulometric detectors for carbon dioxide provide 100% efficiency and an absolute digital readout in terms of micrograms of carbon. Hydrogen can be determined by trapping the water from the combustion step on calcium chloride. The water is desorbed by heating and passed through a proprietary material that quantitatively converts water to carbon dioxide for measurement by coulometry.

18.2

TOTAL CARBON AND TOTAL ORGANIC CARBON ANALYZERS The total carbon (TC) and total organic carbon (TOC) analyzers are designed for natural and wastewater samples and seawater. Periodically (often every 2.5 min for TC and 5 min for TOC) an aspirated sample is injected into a high-temperature (900°C) reaction chamber through which flows a nitrogen carrier gas with a constant level of oxygen. All oxidizable components are combusted to their stable oxides and all inorganic and organic carbon in the aqueous sample are converted to carbon dioxide. The carbon dioxide generated is transferred by the carrier gas through a scrubber to remove corrosive impurities and interferences. Then the carrier gas flows through a nondispersive infrared analyzer. To differentiate between total carbon and total organic carbon, a separate sample is drawn and mixed with acid at a constant sample-to-acid ratio. Inorganic carbon in the sample is converted to carbon dioxide and is removed by nitrogen sparging. Then the sample no longer containing any inorganic carbon is treated as described above for total carbon to determine the total organic carbon.

18.3

KJELDAHL DETERMINATION OF NITROGEN In the Kjeldahl method the sample is digested with sulfuric acid and a catalyst. Organic material is destroyed and the nitrogen is converted to ammonium hydrogen sulfate. The heating is continued until the solution becomes colorless or light yellow. Selenium, copper, and mercury, and salts of each, have been used as the catalyst. Potassium sulfate added to the catalyst raises the temperature and thereby speeds the decomposition. A blank should be carried through all the steps of the analysis. After the digestion is complete, allow the flask to cool. Cautiously dilute with distilled water and cool to room temperature. Arrange a distillation apparatus with a water-jacketed condenser the adapter tip of which extends just below the surface of the solution in the receiver. Carefully pour a concentrated solution of NaOH down the side of the digestion flask so that little mixing occurs with the solution in the flask. Add several pieces of granulated zinc and a piece of pH test paper. Immediately connect the flask to a spray trap and the condenser. Swirl the solution until mixed; the test paper should indicate an alkaline value. Bring the solution to a boil and distill at a steady rate until only one-third of the original solution remains. When the reaction mixture is made alkaline, ammonia is liberated and removed by steam distillation. (1) In the classical method, the distillate is collected in a known excess of standard HCl

18.4

SECTION EIGHTEEN

solution. The unused HCl is titrated with a standard solution of NaOH, using as indicator methyl red or bromocresol green. These indicators change color at the pH that corresponds to a solution of ammonium ions. (2) In the boric acid method, the distillate is collected in an excess of boric acid (crystals). The borate ion formed is titrated with a standard solution of HCl, using methyl red or bromocresol green as the indicator.

18.3.1 Special Situations Compounds containing N — O or N — N linkages must be pretreated or subjected to reducing conditions prior to the Kjeldahl digestion. The N— O linkages are reduced with zinc or iron in acid. There is no general technique for the N — N linkages. Samples containing very high concentrations of halide can in some instances cause trouble because of the formation of oxyacids known to oxidize ammonia to nitrogen. For nitrate-containing compounds, salicylic acid is added to form nitrosalicylic acid which is reduced with thiosulfate. Then the digestion can proceed as described.

18.3.2 Automated Kjeldahl Method The Technicon AutoAnalyzer utilizes a procedure based on the Kjeldahl digestion. The sample is digested using a mixture containing selenium dioxide, sulfuric acid, and perchloric acid. The digest is then made up to a given volume and placed in the autoanalyzer. The ammonium hydrogen sulfate in the digest is automatically sampled and treated with sodium hydroxide. The ammonia liberated is mixed with a phenol–hypochlorite reagent to produce a blue color, which is then measured with a filter photometer.

18.4

DETERMINATION OF SULFUR The determination of sulfur follows the same basic steps outlined earlier for the determination of carbon and hydrogen. A solid sample up to 1 g is weighed in a combustion boat on the integral electronic balance. Coal or coke samples are then covered with a layer of vanadium pentaoxide powder. The vanadium pentaoxide acts as a flux to moderate sample combustion. The boat with sample is inserted through the open port combustion tube of the resistance furnace where it is correctly positioned under the oxygen inlet by a mechanical stop. Oxidative combustion converts the organic material into carbon dioxide, water, sulfur dioxide, and sulfur trioxide. Raising the temperature to 1350°C ensures the production of sulfur dioxide with no sulfur trioxide. Moisture and dust are removed by appropriate traps. Liquids, such as petroleum samples, are loaded dropwise onto a bed of vanadium pentoxide contained in a crucible with cover. The use of a crucible cover serves to retard the combustion of volatile samples.

18.4.1 Measuring the Sulfur in the Combustion Products Various methods exist for determining sulfur after oxidation. A simple, straightforward method measures the sulfur dioxide gas by a selective, solid-state, infrared detector. In the amperometric titration method, the sulfur dioxide is pumped from the furnace to a reaction vessel in the analyzer via a heated manifold. The sulfur dioxide is bubbled through a specially formulated diluent and determined directly through an iodometric titration. An electrical current is preset for a platinum electrode in the diluent. As the diluent absorbs sulfur dioxide, the current decreases. The current decrease triggers an integral burette, which automatically releases a precisely monitored

ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

18.5

quantity of titrant to restore the current in the diluent to the preset level. A sensitivity of 0.005% sulfur can be achieved for a nominal 100-mg sample. The specificity of the method eliminates any interference from chlorine, organic nitrogen, phosphorus, and lead antiknock compounds.

18.4.2 Tube Combustion (Manual) In the manual method the sample is burned with the aid of a vanadium(V) oxide [or tungsten(VI) oxide] catalyst and pure oxygen in an alundum tube maintained at 1000°C. The combustion products pass successively through magnesium perchlorate, 8-hydroxyquinoline, and free copper (heated at 840°C), which remove water, the halogens, and oxygen. The residual gases are absorbed in neutral hydrogen peroxide, which converts all the sulfur oxides to sulfuric acid, which is determined by titration with standard base.

18.4.3 Schöninger Combustion In the Schöninger combustion technique, the sulfur is converted by oxidation to sulfur dioxide and sulfur trioxide and subsequently oxidized to sulfuric acid with hydrogen peroxide. The methodology is described in Sec. 1.7.3.1. This method is useful for nonvolatile compounds only.

18.5

DETERMINATION OF HALOGENS Here the term halogen refers only to chlorine, bromine, and iodine.

18.5.1 Decomposition of the Organic Material The sample is decomposed in an oxygen atmosphere at 700°C in the presence of a platinum catalyst. The evolved gases are absorbed in a sodium carbonate solution with hydrazine present.

18.5.2 Measurement of the Halides by Amperometric Titration Iodide, bromide, and chloride can be successively titrated in mixtures with silver nitrate, using a rotating microelectrode (see Sec. 14.5.6, Amperometric Titrations). In a 0.1M to 0.3M solution of ammonia only silver iodide precipitates. The indicator electrode is held at –0.2 V versus SCE. During the titration of iodide, the current remains constant at zero, or nearly so, until the iodide ions are consumed, and then it rises. After three or four points have been recorded past the end point, the solution is acidified to make it 0.8M in nitric acid. Immediately the silver ions added in excess and now released from the silver ammine complex combine with the bromide ions and precipitate as silver bromide, and the current drops to zero. The indicator electrode is held at +0.2 V versus SCE. A second rise in the current indicates the end point of the bromide titration. A chloride end point can be obtained by adding sufficient gelatin to make the solution 0.1% in gelatin. Gelatin suppresses the current due to silver chloride, and the titration is continued until the current again rises after the chloride end point.

18.5.3 Separation of the Halides by Ion Exchange The anion-exchange separation of the halides is carried out on a column of Dowex 1-X10 in the nitrate form by elution with sodium nitrate solutions. For example, a Dowex 1-X10 column, 3.7 cm2 × 7.4 cm, is eluted at 1.0 mL ⋅ min–1 with 0.5M sodium nitrate for chloride ion, which elutes within 50 mL.

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ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

18.6

SECTION EIGHTEEN

The eluant is then changed to 2.0M sodium nitrate. Bromide elutes in the next 50 mL. Finally, iodide elutes as a peak extending over the volume from 75 to 275 mL of the stronger nitrate solution. The individually separated halides may be determined by potentiometric titration with silver nitrate.

18.6

OXYGEN DETERMINATION For oxygen determination a quartz pyrolysis tube that contains platinized carbon is employed. This is followed by a tube that contains copper(II) oxide. The operating temperature is 900°C and a helium atmosphere is used. Any oxygen in the sample forms carbon monoxide, which is converted in the copper(II) oxide tube to carbon dioxide. The carbon dioxide is measured by GC, IR, or coulometric techniques as previously described for the carbon determination.

18.7 18.7.1

DETERMINATION OF OTHER NONMETALS Antimony For organic materials that contain antimony, wet digestion in a Kjeldahl flask with H2SO4 and H2O2 is often employed. If the compound contains chlorine, the digestion is started with (1 : 2) HNO3 and then 18M H2SO4 and H2O2 are added until the solution becomes clear. The excess H2O2 is decomposed by heating; then antimony(V) is reduced to antimony(III) with Na2SO3 or N2H4 ⋅ 2HCl. After decomposition of the excess sulfite ions, antimony(III) is titrated with 0.01N I2 containing Na2CO3. When using the Rhodamine B colorimetric procedure, destruction of organic matter and conversion of antimony to the quinquevalent state is best effected with nitric, sulfuric, and perchloric acids. If iron is present, SbCl5 is extracted with diisopropyl ether from 1.5M HCl solution. The organic phase is shaken with a 1M HCl solution of Rhodamine B and the red-violet color is measured in the organic layer at 545 to 555 nm.1

18.7.2

Arsenic Dry-ashing is not suitable for decomposing organic arsenic (or antimony) compounds or organic substances that contain these metals because of the danger of losses of volatile arsenic or antimony compounds. An exception is dry-ashing at 600°C for materials high in lipids after addition of MgO and Mg(NO3)2. A strong oxidizing attack with a mixture of nitric and sulfuric acids or of nitric, sulfuric, and perchloric acids is usually employed for most organic samples. Oils, fats, and tobacco are hard to decompose completely by wet oxidation. In these instances, the oxygen flask method is used in which the sample is combusted in quartz wool. The arsenic oxides formed are absorbed in alkaline solution. The most rapid method for the determination of arsenic in the acid digest consists of the distillation of arsenic(V) after the addition of bromide, followed by the direct application of the molybdenum blue method. Other methods for the isolation of arsenic in the digest consist of the distillation as AsCl3, evolution as AsH3, or extraction with diethylammonium diethyldithiocarbamate. Colorimetric methods complete the determination.2 In the heteropoly molybdenum blue procedure, isolated arsenic is oxidized to the pentavalent state, ammonium molybdate is added to the acid solution, and then hydrazine is added to reduce the heteropoly acid to molybdenum blue.3 The silver 1

R. W. Ramette and E. B. Sandell, Anal. Chim. Acta 13:455 (1955). A. D. Wilson and D. T. Lewis, Analyst 100:54 (1975); Z. Stefanac, Mikrochim. Acta 1962:1108,1115; B. Griepink and W. Krijgsman, ibid. 1973:574. 3 R. E. Stauffer, Anal. Chem. 55:1205 (1983). 2

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ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

18.7

diethyldithiocarbamate method involves evolution of AsH3 and its absorption in a pyridine solution of the reagent (0.5% w/v). The red reaction product is colloidal silver; the absorbance is measured at 540 nm.4 Evolved AsH3 can be collected and the gas released into an atomic absorption spectrometer. The complexometric titration method follows the precipitation of arsenate ions with AgNO3. The precipitate is dissolved in a reagent solution containing tetracyanonickelate(II) complex, the nickel ions being liberated, and titrated with EDTA in the presence of murexide indicator. 18.7.3

Bismuth The destruction of organic matter is effected by digestion with nitric and sulfuric acid, often together with perchloric acid. Organobismuth compounds can also be decomposed by closed-flask combustion using a silica spiral, with HCl as absorbent. After combustion by the latter method, H2SO4 is added and the mixture is heated to incipient dryness. Treat the residue with H2SO4, ascorbic acid, and KI; Bi is determined by measuring the absorbance at 465 nm. This method is suitable when the bismuth content of the sample is greater than 5 to 10 mg ⋅ mL–1. For bismuth content less than 10 mg, the dithizone method is preferable.5

18.7.4 Boron Oxygen flask combustion is suitable for the mineralization of most organic boron compounds.6 Volatile compounds are mixed with Na2CO3 and glucose and placed in a methylcellulose capsule. Low results for boron are reported for fluorine-containing organic boron compounds, but the Wickbold combustion is satisfactory.7 Wet digestion in a Kjeldahl flask equipped with a reflux condenser is practical if the boron content is low, and thus larger amounts have to be used for analysis. For very reactive compounds, oxidation is done in a Carius bomb with fuming nitric acid or trifluoroperoxoacetic acid to give boric acid. Nonvolatile boron compounds can be fused with Na2CO3 in a platinum crucible. After the dissolution step, the boric acid formed is titrated with 0.01N to 0.1N NaOH in the presence of excess mannitol to an end point of pH 8.6 with a glass–calomel electrode pair; for small amounts of boric acid, use an ultramicro burette. For spectrophotometric methods, see Table 6.16. 18.7.5

Fluoride For the determination of fluoride after closed flask combustion, the absorbent solution is titrated with 0.01M Th(NO3)4 at pH 3.0 with sodium alizarinsulfonate as indicator.

18.7.6 Phosphorus For the decomposition of organophosphorus compounds, the principally used methods are oxygen flask combustion, with water as absorbent, or digestion with HNO3. In the oxygen flask method, the paper containing the compound is placed in a silica holder since platinum is attacked. The absorbing solution is 0.4N H2SO4 containing peroxodisulfate as the oxidant or a mixture of NaOH with Br2. Phosphate ions are determined spectrophotometrically using the molybdophosphate method or, better, the molybdovanadophosphoric acid method measured at 430 nm.8 Phosphorus can also be determined by induction coupled plasma–optical emission spectroscopy (ICP-OES). 4

V. Vasak and V. Sedivec, Collect. Czech. Chem. Commun. 18:64 (1953). D. M. Hubbard, Anal. Chem. 20:363 (1948); J. C. Gage, Analyst 83:672 (1958). S. K. Yasuda and N. R. Rogers, Microchem. J. 4:155 (1960). 7 B. Schreiber and R. W. Frei, Mikrochim. Acta 1:219 (1975). 8 J. P. Dixon, Modern Methods in Organic Microanalysis, Van Nostrand, London, 1968, pp. 160–162. 5 6

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ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

18.8

18.7.7

SECTION EIGHTEEN

Silicon Organosilicon compounds can be digested with HNO3–H2SO4 and programmed heating: dissolution at 200°C, wet-ashing at 300°C, dry-ashing at 600°C, and calcination at 950°C. The resulting SiO2 is determined gravimetrically. Another method involves digesting nonvolatile silicon compounds in a platinum dish by heating with a mixture of H2SO4 and peroxodisulfate. The dehydrated silica is filtered, ignited, and weighed. Phosphorus and titanium, if present, can be determined in the filtrate. Two methods are available for the colorimetric determination of silica. In strongly acidic HCl solutions, a yellow complex is formed with ammonium molybdate, a reliable but not very sensitive method. Much more sensitive is the formation of molybdenum blue in a less acidic solution. The first method suffers no interference from fluoride, which does affect the formation of the molybdenum blue color. A review details problems in the analysis of silanes and siloxanes.9

18.7.8 Selenium and Tellurium Organic selenium compounds can be digested with an oxidizing acid or a mixture of acids. After (Sec. 2.5.2). In the oxygen flask combustion, bromine water is used for absorption. The finish method can be the iodometric determination of selenate ions or spectrophotometrically with 3,3′diaminobenzidine (see Table 6.16). The results of four different methods suggested for the determination of selenium have been compared.10 Tellurium can be determined by using methods similar to those for selenium.

18.8 DETERMINATION OF TRACE METALS IN ORGANIC MATERIALS Metal ions of some organic compounds may react directly in solution with certain reagents, but most of them must be digested before the metals are determined by inorganic microanalytical methods. The pure metal is obtained after pyrolytic decomposition of gold, platinum, and silver organic compounds. The list extends to nickel and cobalt if, after pyrolytic decomposition, the residue is heated in a stream of hydrogen to reduce their oxides. Heating Al, Cr, Cu, Fe, Mg, Sn, and Zn compounds in air gives metal oxides with stoichiometric compositions. Organic metal compounds can be digested with oxidizing acids or a mixture of acids, fused with oxidizing reagents, or combusted in the oxygen flask method. In addition to closed-flask combustion methods already described, microwave digestion is gaining acceptance because of operational ease and the saving of time.

18.9

METHODS FOR MULTIELEMENT TRACE ANALYSES Anodic stripping voltammetry is applicable to traces of Cd, Co, Cu, Ni, Pb, and Zn in biological and plant materials after prior sample decomposition or digestion. The major advantage of stripping voltammetry in the preconcentration by factors of 100 or more of the analyte(s) on or in the small volume of a microelectrode before a voltammetric analysis. Combined with differential pulse voltammetry in the stripping step, solutions as dilute as 10–11M can be analyzed. Atomic absorption spectrometry is frequently used as the method of finish for metals; see Table 8.8. Elements analyzed in biological, feedstuff, food, marine, petroleum, pharmaceutical, 9 10

J. C. Smith, Analyst 85:465 (1960). Z. Stefanac, M. Tomaskovic, and I. Bregovec. Microchem. J. 16:226 (1971).

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ELEMENTAL ANALYSIS OF ORGANIC COMPOUNDS

18.9

plant, and polymer samples are listed below. Atomic emission (ICP-OES) extends the elements applicable to include those in parentheses. 1. 2. 3. 4. 5. 6. 7.

Biological: Ca, Cu, Fe, K, Mg, Na, Zn (Cd, Co, Mn, P) Fodder: Ca, Cu, Fe, K, Mg, Mn, Na, Zn (Al, Co, Cd, P, Pb, Sr) Food: Al, Ca, Cd, Cr, Cu, Fe, K, Na, Ni, Sn, Zn (As, Mg, Mo, P, Se, Sr) Marine: Ag, Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pb, Zn Pharmaceutical: Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn Plant: Al, Co, Cu, Fe, Mn, Ni, Sb, V, Zn (As, Cd, Ce, La, Mo, Pb, Sn, Tl) Wine: As, Cd, Cr, Hg, Pb, Se (Ag, Ba, Ca, Cu, K, Li, Mg, Na, Ru, Sb, Sr)

Ion-selective electrodes find use for these elements when biological materials are being analyzed: Ca, Cl, Fe, K, Li, Mg, Na; see Table 14.12. ISEs are adaptable to monitoring specific metals during processing industrial liquids. Neutron activation is one of the preferred methods if a large number of elements are required to be determined in a sample of organic material, particularly for biological and marine samples, and if the applicable elements are desired. Spectrophotometric methods in the visible and ultraviolet portions of the spectrum are often used when analyzing for trace levels of many elements; see Table 6.15. X-ray fluorescence has been used on biological materials for the elements Br, Ca, Cr, Cu, Fe, K, Mn, Ni, Pb, Ti, Zn; and on plant materials for Br, Ca, Cl, Cu, Fe, K, Ni, Pb, Rb, Sr, Ti, Zn.

Bibliography R. Boch, A Handbook of Decomposition Methods in Analytical Chemistry, International Textbook Company, London, 1979. E. C. Dunlap and C. R. Grinnard, Chap. 2, “Decomposition and Dissolution of Samples: Organic,” in I. M. Kolthoff and P. J. Elving, eds., Treatise on Analytical Chemistry, Part I, Vol. 5, 2d ed., Wiley-Interscience, New York, 1978. L. Mázor, Methods of Organic Analysis, Vol. XV in G. Svehla, ed., Wilson and Wilson’s Comprehensive Analytical Chemistry, Elsevier, Oxford, 1983.

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 19

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS 19.1

UNSATURATION Table 19.1 Characteristic Infrared and Raman Bands for Measurement of Unsaturation Table 19.2 Measurement of Unsaturation by Spectral Methods: Ultraviolet Absorption Bands 19.1.1 Characteristic Infrared and Raman Wavelengths 19.1.2 Ultraviolet Absorption Bands 19.1.3 Detection and Determination of Unsaturation Table 19.3 Recommended Conditions for Visual Nonaqueous Titrations of Carboxylic Acids 19.2 CARBOXYL GROUPS 19.2.1 Spectral Methods Table 19.4 Recommended Conditions for Potentiometric Nonaqueous Titrations of Carboxylic Acids 19.2.2 Separation of Mixtures of Acids (or Their Esters) 19.2.3 Determination of Salts of Carboxylic Acids 19.3 METHODS FOR THE DETERMINATION OF ALCOHOLS AND PHENOLS (−− OH GROUP) Table 19.5 Scope and Limitations of Procedures Employed for the Determination of Hydroxyl Groups Table 19.6 Conditions Frequently Employed in Volumetric Methods for Determination of Hydroxyl Groups Table 19.7 Methods for the Determination of Hydroxyl Groups 19.4 METHODS FOR THE DETERMINATION OF ALDEHYDES AND KETONES Table 19.8 Methods for the Determination of Aldehydes and Ketones 19.5 METHODS FOR THE DETERMINATION OF ESTERS 19.6 METHODS FOR THE DETERMINATION OF OTHER OXYGEN-BASED FUNCTIONAL GROUPS 19.6.1 Ethers Table 19.9 Methods for the Determination of Aldehydes Table 19.10 Methods for the Determination of Ketones 19.6.2 The Epoxy Group (Oxiranes) 19.6.3 Peroxides Table 19.11 Methods for the Determination of Epoxides (Oxiranes) Table 19.12 Chemical Methods for the Determination of Peroxides 19.6.4 Quinones 19.7 METHODS FOR THE DETERMINATION OF FUNCTIONAL GROUPS CONTAINING NITROGEN AND OXYGEN 19.7.1 Determination of Nitrates, Nitro, and Nitroso Compounds 19.8 DETERMINATION OF AMINES AND AMINE SALTS 19.8.1 Determination of Primary Amines

19.2 19.3 19.6 19.8 19.8 19.8 19.10 19.12 19.12 19.13 19.14 19.14 19.14 19.15 19.16 19.17 19.19 19.20 19.21 19.21 19.21 19.22 19.24 19.26 19.27 19.28 19.29 19.30 19.30 19.30 19.31 19.31

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.2

SECTION NINETEEN

Table 19.13

Typical Amine Mixtures That Can Be Analyzed by Differentiating Titrations in Nonaqueous Solvents 19.8.2 Determination of Secondary Amines 19.8.3 Determination of Tertiary Amines 19.8.4 Determination of Primary Plus Secondary Amines 19.8.5 Determination of Secondary Plus Tertiary Amines 19.8.6 Determination of Primary, Secondary, and Tertiary Amines in Mixtures 19.9 DETERMINATION OF AMINE AND QUATERNARY AMMONIUM SALTS 19.10 DETERMINATION OF AMINO ACIDS 19.11 METHODS FOR THE DETERMINATION OF COMPOUNDS CONTAINING OTHER NITROGEN-BASED FUNCTIONAL GROUPS 19.11.1 Determination of Amides 19.11.2 Methods for the Determination of Azo Compounds 19.11.3 Methods for the Determination of Hydrazines 19.11.4 Determination of Primary Hydrazides 19.11.5 Determination of Oxazolines 19.11.6 Determination of Isocyanates, Isothiocyanates, and Isocyanides 19.11.7 Determination of vic-Dioximes 19.11.8 Determination of Hydroxylamine 19.12 DETERMINATION OF NITRILES 19.13 METHODS FOR THE DETERMINATION OF PHOSPHORUS-BASED FUNCTIONS Table 19.14 Methods for the Determination of Inorganic Phosphorus Groups Table 19.15 Methods for the Determination of Organic Phosphorus Groups 19.14 METHODS FOR THE DETERMINATION OF SULFUR-BASED FUNCTIONAL GROUPS 19.14.1 Determination of Mercaptans (Thiols) and Hydrogen Sulfide Table 19.16 Methods for the Determination of Hydrogen Sulfide Table 19.17 Methods for the Determination of RSH Groups 19.14.2 Determination of Thioethers (R−− S−− R) and Disulfides (R−− S−− S−− R) 19.14.3 Determination of Thioketones 19.14.4 Determination of Sulfoxides and Sulfones Table 19.18 Methods for the Determination of R−− S−− R Groups Table 19.19 Methods for the Determination of Sulfoxides and Sulfones Table 19.20 Determination of Sulfinic and Sulfonic Acids Table 19.21 Methods for the Determination of Specific Substances 19.14.5 Determination of Sulfinic and Sulfonic Acids 19.14.6 Determination of Miscellaneous Sulfur-Based Functional Groups

19.1

19.32 19.32 19.33 19.33 19.33 19.33 19.34 19.34 19.34 19.34 19.35 19.36 19.36 19.36 19.36 19.37 19.37 19.37 19.37 19.38 19.40 19.43 19.43 19.43 19.44 19.46 19.46 19.46 19.47 19.48 19.49 19.50 19.53 19.53

UNSATURATION As a class, unsaturated compounds represent the majority of organic structures, particularly if structures containing other functional groups are included. The analytical methods outlined in Tables 19.1 and 19.2 and elsewhere in this section are generally useful for compounds containing carbon-tocarbon unsaturated hydrocarbon compounds. They may or may not apply to structures containing other functional groups. In some cases, either chemical or instrumental methods will be applicable; the analyst may then choose a method compatible with the circumstances.

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Overtone and combination bands −− C == C== C −− asym stretch −− C ≡≡ C−− stretch == C −− H bend overtone == C −− H bend overtone −− C == C −− stretch −− C == C −− stretch −− C == C −− stretch

−− C == C −− stretch −− C == C −− stretch −− C == C−− stretch

Benzene derivatives Allene and derivatives Acetylene gas RCH== CH2 R1R2C== CH2 R1CH== CHR2, trans R1R2C== CHR3 R1R2C== CR3R4

R1CH== CHR2, cis

R1R2C== CH2 RCH== CH2 Aromatic hydrocarbons Ring vibration

== C−− H stretch == C−− H stretch == C−− H stretch == C−− H stretch −− C ≡≡ C−− stretch −− C ≡≡ C−− stretch −− C ≡≡ C−− stretch −− C ≡≡ C−− stretch −− C ≡≡ C−− stretch

== C−− H stretch == C−− H stretch == C−− H stretch == C−− H stretch == C−− H stretch

≡≡ C−− H sym stretch ≡≡ C−− H stretch ≡≡ C−− H asym stretch == C−− H stretch

Nature of vibration

Aromatic compounds C6H6 gas RCH== CH2 R1R2C== CH2 Mono-, di-, and trisubstituted benzenes C6H6 liquid R1CH== CHR2, cis trans Ethylene gas Dialkyl acetylenes Disubstituted acetylenes Dialkyl acetylenes Cyclic acetylenes Monoalkyl acetylenes

Acetylene gas Monosubstituted acetylenes Acetylene gas Ethylene gas

Type of compound

IR, R IR, R IR, R IR

IR, R

R IR, R IR, R R (not IR) R IR R IR IR R IR IR, R R IR IR IR, R IR, R IR, R

R (not IR) IR, R IR (not R) R (not IR) IR (not R) IR IR IR, R IR, R R

Type of spectrum

Wavelength, mm

6.053–6.068 6.061–6.105 6.211–6.289 6.667–6.757

6.017–6.131

3.266–3.282 3.311–3.339 3.322–3.339 3.312 4.340 4.425–4.651 4.490 4.490–4.541 4.673–4.762 4.630–4.760 5.000–6.061 5.050–5.222 5.066 5.435–5.540 5.525–5.650 5.967–6.006 5.952–6.010 5.952–6.006

2.964 3.012–3.049 3.042 3.218 3.220 3.226–3.333 3.227 3.231–3.252 3.247–3.252 3.257–3.284

TABLE 19.1 Characteristic Infrared and Raman Bands for Measurement of Unsaturation

1662−1648 1650−1638 1610−1590 1500−1480

1662−1631

3062−3047 3020−2995 3010−2995 3019 2304 2260−2150 2227 2227−2202 2140−2100 2125−2118 2000−1650 1980−1915 1974 1840−1805 1810−1770 1676−1665 1680−1664 1680−1665

3374 3320−3280 3287 3108 3106 3100−3000 3099 3095−3075 3090−3075 3070−3045

Wave number, cm–1

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(Continued)

Weak to moderate strength Very strong Weak but characteristic Strong IR and R Very strong Weak Weak Var IR; vs R Weak IR; vs R Very strong; not IR for symmetrically substituted compounds Cyclohexene, 1646 cm−1; cyclopentene, 1611 cm−1; cyclobutene, 1571 cm−1 Very strong R IR moderate; vs R Strong in R Weak or absent in R

Second band at 2227 cm−1 Variable strength Second band at 2304 cm−1

Strong Moderate strength Moderate strength

Moderate strength Moderate IR, weak R Strong

Moderately weak

Strong IR, weak R

Remarks

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.3

Monosubstituted benzene derivatives 1,2,3-Trisubstituted benzene derivatives

Alkylbenzene derivatives Substituted ethylenes RCH== CH2 R1R2C == CH2 R1CH== CHR2 Monosubstituted acetylenes Allene and alkyl derivatives Monosubstituted benzene; also 1,3RCH== CH2 R1CH== CHR2, trans RCH== CH2 R1R2C== CH2 1,2,4-Trisubstituted benzene derivatives Pentasubstituted benzene derivatives 1,2,4,5-Tetrasubstituted benzene derivatives 1,3,5-Trisubstituted benzene derivatives R1R2C== CHR3 1,2,4-Trisubstituted benzene derivatives 1,2,3,4-Tetrasubstituted benzene derivatives 1,3-Disubstituted benzene derivatives Disubstituted ethylene group in six-membered ring 1,2,3-Trisubstituted benzene derivatives 1,2-Disubstituted benzene derivatives

Type of compound

IR IR IR IR IR IR IR IR IR IR IR IR (not R) IR IR (not R) IR (not R) IR (not R) IR (not R)

C−− H bend C−− H bend C−− H bend

−− C == CH out-of-plane C−− H bend C−− H bend C−− H bend C−− H bend C−− H bend C−− H bend C−− H bend C−− H bend C−− H bend

IR IR, R IR, R IR IR IR IR, R R

Type of spectrum

−− C == CH out-of-plane C−− H bend −− C == CH out-of-plane C−− H bend −− C == CH out-of-plane C−− H bend −− C == CH out-of-plane C−− H bend C−− H bend

−− CH3 asym bending −− C == CH in-plane C−− H bend −− C == CH in-plane C−− H bend −− C == CH in-plane C−− H bend −− C == CH in-plane C−− H bend −− C ≡≡ C−− H bend overtone −− C == C== C−− sym stretch

Nature of vibration

19.4

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13.39–13.57

13.16–13.51

12.82–13.16

12.50–15.38

12.50–12.99

12.35–12.50

11.76–12.66 12.12–12.42

11.56–12.35

11.76–11.90

10.04–11.63

10.05–10.44 10.20–10.36 10.99–11.05 11.24 11.11–11.49

6.803–7.042 6.897–8.333 7.042–7.062 7.042–7.082 7.092–7.143 7.937 9.346 9.90–10.10

Wavelength, mm

TABLE 19.1 Characteristic Infrared and Raman Bands for Measurement of Unsaturation (Continued)

745–705

747–737

760−740

780−760

800−650

800−770

810−800

850−790 825−805

865−810

850−840

980−860

995−985 980−965 910−905 890 900−870

1470−1420 1450−1200 1420−1416 1420−1412 1420−1400 1260 1070 1010−990

Wave number, cm–1

Also at 780–760 cm–1

Also at 701−671 cm−1

Also at 745−705 cm−1

Also at 710−690 cm−1

Moderate strength Also at 900−870 cm−1

Strong

Moderate strength; also at 780−760 cm−1 Moderately strong

Very strong; also at 990 cm−1

Very strong; also at 910 cm−1

Strong IR and R Very strong

Moderately weak

Strong R, weak IR

Remarks

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

1,2,4-Trisubstituted benzene derivatives 1,2-Disubstituted benzene derivatives R1CH == CHR2, cis 1,3-Disubstituted benzene derivatives Monosubstituted benzene derivatives 1,3,5-Trisubstituted benzene derivatives Benzene 1,2,3-Trisubstituted benzene derivatives Monosubstituted acetylenes Monosubstituted benzene derivatives R1CH== CHR2, cis 1,3,5-Trisubstituted benzene derivatives RCH== CH2 R1CH== CHR2, trans RCH== CH2 R1R2C == CH2 17.24 17.54–18.05

IR, R R (not IR)

C −− H bend IR IR, R R R

15.62–15.92 15.87–16.52

IR, R R (not IR)

−− C≡≡ C −− H bend C −− H bend

18.18 20.41 22.99 23.04

14.90 14.93–20.00

IR (not R) R (not IR)

C −− H bend C −− H bend

13.70–14.71

IR (not R)

13.79–14.81 13.90–14.06 14.08–14.49 14.27–14.91

13.59–14.06

13.48–13.97

C −− H bend

C −− H bend

IR R (not IR) IR (not R) IR (not R)

R (not IR)

C −− H bend

−− C== CH out-of-plane C—H bend C −− H bend

R (not IR)

C −− H bend

550 490 435 434

580 570–554

640–628 630–605

671 670–500

730–680

725–675 719–711 710–690 701–671

736–711

742–716

Also at 394–361 cm–1

Also in R at 210 cm–1

Also in R at 413 and 297 cm–1

The lighter the mass of the substituent, the higher the frequency Strong; overtone, 1260 cm–1 Moderately strong

Also at 850–830 cm–1

Also at 800–770 cm–1 Also at 747–737 cm–1

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.5

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TABLE 19.2 Measurement of Unsaturation by Spectral Methods: Ultraviolet Absorption Bands

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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* Apparent molecular absorptivity (not its logarithm) in liters per mole of olefins per gram per liter of I2 for a 1-cm path length. † Steroid and triterpenoid dienes. Source: L. Meites, ed., Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963.

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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19.8

SECTION NINETEEN

19.1.1 Characteristic Infrared and Raman Wavelengths Infrared and Raman absorption bands are useful for the identification of compounds containing carbon–carbon double or triple bonds. Table 19.1 lists the characteristic wavelengths associated with stretching or deformation vibrations in various unsaturated structures. Before applying these correlations to analytical problems, it will usually be desirable to consult sources of more detailed spectral data. Pertinent information is contained in Sec. 7 on infrared absorption, Raman frequencies, and spectra–structure correlation charts. The bands listed in Table 19.1 are arranged in order of increasing wavelength (in the fourth column) or decreasing frequency (in the fifth column). The first column gives the type of compound responsible for the band, and sometimes also its physical state. The second column gives the nature of the vibration; sym is symmetrical, while asym is asymmetrical. The third column gives the type of spectrum in which the band is observed: R is Raman and IR is infrared. The entry R (not IR) in this column means that the band in question is observed in the Raman spectrum but not in the infrared spectrum. The characteristic infrared bands in the region from 995 to 685 cm–1 can be used for quantitative determinations. As the neighboring groups have less influence on Raman bands than on infrared bands, Raman spectroscopy is useful for the determination of substituted olefins at 1640 and 1680 cm–1. 19.1.2 Ultraviolet Absorption Bands Such chromophoric groups as carbon–carbon double and triple bonds absorb ultraviolet radiation. This absorption is weak for structures containing only one ethylenic linkage, except in the far-ultraviolet region below 200 nm. Structures containing two or more isolated double bonds behave like simple olefins. On the other hand, conjugation leads to high-intensity absorption in the region from 200 to 1000 nm. (See also Sec. 6.2.1.) The first column of Table 19.2 lists the type of compound; the second column shows the typical structure. The third column gives the position (or, for classes of compounds, the range of positions) of the peak of the absorption band. The fourth column gives values of log10 ⑀max, where ⑀ is the molar absorbance (see Sec. 6.1). Data have been included, for the sake of convenience, for the iodine complexes of a number of simple olefins that do not absorb in the readily accessible portion of the spectrum above about 200 nm. For these compounds the values given in the fourth column are of the apparent molecular absorptivity (not its logarithm), in liters per mole of olefins per gram per liter of I2 for a 1-cm path length. These values are denoted by asterisks. 19.1.3 Detection and Determination of Unsaturation (−− C== C−− ; −− C ≡≡ C−− ) For the analysis of unsaturated compounds there are a number of chemical methods described in a review.1 Of these methods, three methods most commonly used will be discussed in some detail. For the investigation of structural problems or the identification of more complicated bond systems, instrumental methods will be needed. These tables in earlier sections should be examined: the infrared absorption frequencies of alkenes (Table 7.5), triple bonds (Table 7.6), and aromatic and heteroaromatic compounds (Table 7.12); the Raman frequencies of alkenes (Table 7.21), alkynes (Table 7.22), and aromatic compounds (Table 7.29); and the ultraviolet absorption wavelengths of dienes (Table 6.10), enones and dienones (Table 6.11), and benzene and its derivatives (Tables 6.13 and 6.14). 19.1.3.1 Analytical Hydrogenation. Analytical hydrogenation is the most general chemical method for the determination of olefinic unsaturation. The method depends upon the measurement of the amount of hydrogen consumed by the sample when the reaction is complete. Errors due to substitution do not occur. The rate at which hydrogen reacts with different olefins varies considerably. In general, monosubstituted olefins react rapidly, followed in sequence by conjugated olefins and di-, tri-, and tetrasubstituted 1

K. Muller, Z. Anal. Chem. 181:126 (1961).

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.9

compounds. Aromatic double bonds are the most difficult to hydrogenate. By the proper choice of catalyst and operating parameters, selective hydrogenation can be achieved, thereby avoiding interference due to aromatic compounds. Platinum dioxide catalyst generally causes hydrogenations to proceed more rapidly than Raney nickel. The time required per analysis is rather high because of the long reaction time generally needed. The hydrogenated sample can be recovered if necessary. In general, the weighed sample in a capsule is isolated from the catalyst and solvent in the constanttemperature hydrogenation cell. After flushing, the apparatus is filled with hydrogen at the desired pressure. When the take-up of hydrogen by the catalyst ceases, the volume or pressure is noted; then the sample capsule is broken. The amount of hydrogen consumed by the sample is measured volumetrically or manometrically when the reaction is complete. For accurate results, the volume of the apparatus is needed to correct hydrogen-volume readings if the temperature at the end of an analysis is different from the temperature at the beginning. The reactivity of hydrogen with many compounds depends on operating conditions, so that the time of analysis may vary from one-half hour to several hours. Compounds containing carbonyl, epoxy, sulfur, nitrogen, or other reducible groups may interfere. Under the conditions employed, platinum dioxide will saturate the double bonds in a benzene ring whereas Raney nickel will not. 19.1.3.2 Bromination. Bromination procedures are usually too slow and not quantitative unless a catalyst [mercury(II) sulfate] is present. A 25-mL aliquot of a water-soluble sample is used (0.002 equivalent in unsaturation). Carbon tetrachloride is the solvent for hydrocarbon-soluble samples. When volatile, the sample is weighed in a sealed glass ampoule, the ampoule placed under the solvent in a volumetric flask, and the ampoule crushed. Although bromination is the most useful halogenation reaction, knowledge of the types of compounds in the sample is required for interpretation of the results. In the analysis procedure2 a 10% to 15% excess of 0.1000N bromate–bromide solution (about 25 mL) is introduced into the reaction flask. The flask is evacuated with a three-way stopcock, and 5 mL of 6N H2SO4 is added via a funnel at the other stopcock position. Allow 2 to 3 min to elapse for the bromine to be liberated. Next, 10 to 20 mL of 0.2N HgSO4 is added, followed by 25 mL of the sample solution and rinsing the funnel with solvent. If the solvent is CCl4, 20 mL of glacial HOAc is also added. The flask is wrapped in a black cloth and shaken for 7 min (or longer for some samples). Then 15 mL of 2N NaCl and 15 mL of 20% KI are added and the flask is shaken for 30 s. The vacuum is broken, and the free iodine is titrated with 0.05N Na2S2O3, starch indicator being used. A blank is run under the same conditions. The accuracy generally varies from 0.5% to 10%, depending on the reactivity of the compound determined and its concentration in the sample. An accurate method for trace unsaturation is the coulometric titration of the sample with generated bromine to an amperometric end point. It is rapid, only 5 min per determination. Bromine number is the number of grams of bromine reacting with 100 g of sample. 19.1.3.3 Iodine Number. There are instances when the bromination procedure cannot be used because the bromine not only adds on the unsaturated linkage but also substitutes some of the hydrogen atoms. This situation can be averted, or at least minimized, if iodine monobromide is used as the brominating agent. This procedure is used mostly on unsaturated hydrocarbons, fatty acids, esters, vinyl esters, and some unsaturated alcohols. In the Wijs method, excess iodine monochloride reagent is added along with Hg(OAc)2 catalyst. The reaction is allowed to stand, then KI is added, and the liberated iodine is titrated with Na2S2O3. It is widely used for fats and oils, but does not determine conjugated unsaturation.3 Iodine number (or value) is the number of milligrams of iodine reacting with 1 g of sample under specified conditions. 19.1.3.4 Miscellaneous Methods. For the gas-chromatographic separation and determination of olefin mixtures silicone oils and squalane can be used as nonpolar stationary phases in capillary columns. Ethylene glycol–silver nitrate, Carbowax 1500, and trimethyl phosphate are suitable as polar stationary phases. 2 H. 3 F.

J. Lucas and D. Pressman, Ind. Eng. Chem., Anal. Ed. 10:140 (1938). A. Norris and R, J. Buswell, Ind. Chem. Eng., Anal. Ed. 15:258 (1943).

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p-Hydroxym-Nitrop-Nitrotrans-Butenedioic Citric Hexanedioic

p-Aminom-Hydroxyo-Hydroxy-

PhenylAcrylic b-PhenylBenzoic

Acid

blue colorless magenta

Acetic

b c m

rv v y Solvent

Acetonitrile Benzene 1,4-Dioxane or EtOH Acetone, acetonitrile, pyridine, or benzene–EtOH Benzene–MeOH Pyridine Acetonitrile or benzene–MeOH Acetone, acetonitrile pyridine, or benzene–EtOH Pyridine Benzene–MeOH Acetone Pyridine Acetonitrile Chloroform

Acrylonitrile Dioxane Acetone, acetonitrile, benzene–EtOH Acetone Acrylonitrile Chloroform Acetone

red-violet violet yellow

(References are presented below Table 19.4) Abbreviations

Thymol blue/MeOH Azo violet/benzene Thymol blue/MeOH Thymol blue/MeOH Azo violet Thymol blue/MeOH Thymolphthalein/MeOH Azo violet/benzene Thymol blue/MeOH Phenolphthalein/EtOH

NaOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH NaOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH (Butyl)4NOH/benzene–MeOH NaOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH (Butyl)4NOH/benzene–MeOH (Butyl)4NOH/benzene–MeOH NaOEt/EtOH

KOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH KOMe/benzene–MeOH Diphenylguanidine/benzene NaOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH

Thymolphthalein/MeOH Bromothymol blue/MeOH Phenolphthalein/EtOH p-Hydroxyazobenzene/MeOH Thymolphthalein/MeOH p-Hydroxyazobenzene/MeOH Bromophthalein magenta E/benzene Thymol blue/1,4-dioxane Thymol blue/2-propanol

(Butyl)4NOH/benzene–MeOH NaOH/MeOH

Indicator Bromothymol blue/MeOH Thymol blue/1,4-dioxane Thymol blue/2-propanol

NaOH/MeOH NaOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH

Titrant

TABLE 19.3 Recommended Conditions for Visual Nonaqueous Titrations of Carboxylic Acids

y-v y-b c-b y-v y-b c-rv

y-b y-v y-b y-b

c-b y-b c-rv c-y c-b c-y y-m y-b y-b

y-b y-b y-b

Color change

1 6 12 1 1 4

6 1 5 1

12 14 5 12 5 2* 15 15 1

14 16 1

Reference

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.10

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Chloroform 1-Butylamine Acetone or acetonitrile Chloroform Chloroform CHCl3 or acetonitrile Ethylene glycol–EtOH Pyridine Toluene–N,Ndimethylformamide Chloroform Acetone, acetonitrile, pyridine, or benzene–EtOH Chloroform Benzene–MeOH Pyridine Chloroform–EtOH

* Includes information on titrations of 40 substituted aromatic acids.

Succinic Tartaric

Stearic

Propanoic 3-Pyridine carboxylic

Lactic Malonic Mandelic Oleic Palmitic trans-3-Phenyl-propenoic m-Phthalic o-Phthalic p-Phthalic

NaOEt/EtOH NaOMe/benzene–MeOH (Butyl)4NOH/benzene–MeOH NaOEt/EtOH

NaOEt/EtOH (Butyl)4NOH/benzene–MeOH

NaOEt/EtOH NaOMe/benzene–MeOH KOMe/benzene–MeOH NaOEt/EtOH NaOEt/EtOH NaOEt/EtOH KOH/MeOH (Butyl)4NOH/benzene–MeOH

Phenolphthalein/EtOH Thymol blue/MeOH Azo violet/benzene Phenolphthalein/EtOH

Phenolphthalein/EtOH Thymol blue/2-propanol

Phenolphthalein/EtOH Thymol blue/MeOH p-Hydroxyazobenzene/benzene Phenolphthalein/EtOH Phenolphthalein/EtOH Bromothymol blue/MeOH m-Cresol purple/EtOH Azo violet/benzene Bromothymol blue in solvent

c-rv y-b y-v c-rv

c-rv y-b

c-rv y-b c-y c-rv c-rv y-b y-v y-v y-b

4 6, 17 1 4

4 1

4 6 5 4 4 4 3 1 10

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.11

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.12

SECTION NINETEEN

Chromatography columns prepared from sulfonic acid resins in which sulfonic acid protons have been partially replaced with silver ions have been found to be effective in the separation of unsaturated fatty acids and glycerides.4 A reversible olefin trap consisting of a silver-containing microporous copolymer of divinylbenzene and styrene will separate olefins from saturated hydrocarbons. Subsequent analysis is done by gas chromatography.5 A procedure for the separation and double-bond position determination of unsaturated alcohols, aldehydes, and carboxylic acid involves derivatization using dimethyl disulfide and iodine followed by gas chromatography and mass spectral detection.6 The problems and conditions required for the carbon-13 NMR analysis of hexane–hexene mixtures have been discussed.7 Gasolines have been run on a mass spectrometer, after which the olefins were removed from the sample with benzenesulfonyl chloride (which reacts quantitatively with olefins to form a highboiling-point addition product) and the spectrum was obtained on the residue.8

19.2

CARBOXYL GROUPS A wide variety of organic compounds show acid properties and might be classed as acids if salt formation and the neutralization of alkalies were the sole criteria. The functional group that most consistently displays all the phenomena of acidity is the carboxyl or −− COOH group, and it is to substances containing this characteristic group that this subsection is confined. The degree of dissociation of a carboxyl compound and the pH of its solution depend largely upon the nature of R in R−− COOH. The dissociation of simple aliphatic acids is minimal even at high dilutions, and decreases with increasing length of R. The acidity is increased by the presence of a triple bond. Salt formation is characteristic of most acids, with the sodium salts being more soluble in water than the corresponding acids. The hydroxyl group of the carboxyl can be replaced by a halogen atom, giving an acid halide, and acid anhydrides can be formed by the loss of a molecule of water from two molecules of acid. pKa values of a very large number of organic materials in water are available in Table 8.8 of Ref. 9. Organic acids may be titrated if their pKa are not greater than 5 to 6, at a concentration of 0.01N with 0.1N NaOH solution in the presence of phenolphthalein (or thymolphthalein when pKa > 5) with a sharp end point. If the pKa is >6, the titration is best performed in a nonaqueous solvent as discussed in Sec. 4.3. Carboxylic acids with aqueous pKa values of 6 to 9 can usually be titrated in the solvent N,N-dimethylformamide using as titrant potassium methoxide in benzene–methanol (10 :1) and thymol blue (or azo violet) as indicator. Very weak acids have been titrated in tert-butanol with potassium tertbutoxide as titrant.10 Recommended conditions for visual nonaqueous titrations of organic carboxylic acids are given in Table 19.3. Table 19.4 summarizes the conditions that have been recommended for the nonaqueous titration of these acids when employing a potentiometric end point. Aliphatic monobasic acids are found in plant and animal products, both free and as glycerides (fats). In this group of straight-chain acids, those having an odd number of carbon atoms have melting points lower than their neighbors with even numbers of carbon atoms, and similar differences are observed in their surface tensions and molecular volumes.

19.2.1 Spectral Methods It is easy to recognize the presence of carboxylic acids in the infrared spectrum. A characteristic group of small bands (due to combination bands) appear between 3000 and 2500 cm–1. For quantitative purposes, the absorption of the monomeric carboxyl group at 1760 cm–1 (not often observed) and the 4

R. O. Adiof and E. A. Emken, J. Am. Oil Chem. Soc. 57:276 (1980). E. G. Boeren et al., J. Chromatogr. 349:377 (1985). B. A. Leonhardt and E. D. DeVilbiss, J. Chromatogr. 322:484 (1985). 7 D. A. Forsyth et al., Anal. Chem. 54:1896 (1982). 8 L. Mikkelsen, R. L. Hopkins, and D. Y. Yee, Anal. Chem. 30:317 (1958). 9 J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992. 10 J. S. Fritz and L. W. Marple, Anal. Chem. 34:921 (1962). 5 6

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.13

TABLE 19.4 Recommended Conditions for Potentiometric Nonaqueous Titrations of Carboxylic Acids SCE denotes saturated (aqueous) calomel electrode and SMCE saturated methanolic calomel electrode. Titrant

Electrode system

Acetic Phenyl-

Acid

Methyl ethyl ketone Acetone

(Butyl)4NOH/benzene–MeOH (Butyl)4NOH/benzene–2-propanol

Glass–SMCE Pt(10% Rh)–graphite

9 16

Acrylic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Benzoic p-Amino-

Acetone or pyridine 1,2-Ethylenediamine

Glass–SMCE H2–SCE or H2–Sb

7 13

Pyridine Acetone

(Butyl)4NOH/benzene–MeOH NaOC2H4NH2/1,2-ethylenediamine (Butyl)4NOH/benzene–MeOH (Butyl)4NOH/benzene–MeOH

Glass–SMCE Glass–SMCE

1 7

cis-Butenedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

trans-Butenedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Citric

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

1

Crotonic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Dodecanoic

EtOH

KOH/water

Glass–SCE

Formic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Heptanedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Hexanedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Hydroxybutanedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

1, 17 17

m-, o-, or p-Hydroxyp-Nitro-

Solvent

Reference

8

Lactic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

Nonanedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Oleic

N,N-dimethylformamide

KOH/water

Pt–SCE

11 17

Oxalic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

Palmitic

EtOH

KOH/water

Glass–SCE

Pentanedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Phthalic (m-, o-, p-) Methyl-

Pyridine N,N-Dimethylformamide

(Butyl)4NOH/benzene–MeOH KOH/water

Glass–SMCE Pt–SCE

17 11 17

8

Propanedioic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

Stearic

EtOH

KOH/water

Glass–SCE

Succinic

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

1, 17

Tartaric

Pyridine

(Butyl)4NOH/benzene–MeOH

Glass–SMCE

17

Tetradecanoic

EtOH

KOH/water

Glass–SCE

References for Tables 19.3 and 19.4 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Cundiff, R. H., and P. C. Markunas, Anal. Chem. 28:792 (1956). Davis and Hetzer, J. Res. Natl. Bur. Stand. (U.S.) 60:569 (1958). Esposito, G. G., and M. H. Swann, Anal. Chem. 32:49 (1960). Folin and Flanders, J. Am. Chem. Soc. 34:774 (1912). Fritz, J. S., and R. T. Keen, Anal. Chem. 25:179 (1953). Fritz, J. S., and N. M. Lisicki, Anal. Chem. 23:589 (1951). Fritz, J. S., and S. S. Yamamura, Anal. Chem. 29:1079 (1957). Grunbaum, B. W., F. I. Schaffer, and P. L. Kirk, Anal. Chem. 25:480 (1953). Harlow, G. A., C. M. Noble, and G. E. A. Wyld, Anal. Chem. 28:787 (1956). Hensley, A. L., Anal. Chem. 32:542 (1960). Kirrmann and Daune-Dubois, Compt. rend. 236:1361 (1953). Malmstadt, H. V., and D. A. Vassallo, Anal. Chem. 31:862 (1959). Moss, M. L., J. H. Elliot, and R. T. Hall, Anal. Chem. 20:784 (1948). Owens, Jr., M. L., and R. L. Maute, Anal. Chem. 17:1177 (1955). Patchernik, A., and S. Erhlich-Rogozinski, Anal. Chem. 31:985 (1959). Radell, J., and E. T. Donahue, Anal. Chem. 26:590 (1954). Streuli, C. A., and R. R. Miron, Anal. Chem. 30:1978 (1958).

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16

8

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.14

SECTION NINETEEN

dimeric carboxyl group between 1725 and 1700 cm–1 (but often at 1710 cm–1) are useful. More detailed infrared frequencies are given in Table 7.9; those in the Raman spectrum are given in Table 7.26. The colorimetric determination of carboxylic acids using a carbodiimide, a hydroxylammonium salt, and iron(III) ions can be carried out in organic solvents containing as much as 20% water at pH 3 to 6.11 19.2.2 Separation of Mixtures of Acids (or Their Esters) Procedures for the chromatographic determination of carboxylic acids involve derivatization to volatile compounds followed by the use of liquid or gas chromatography. Alkanoic, alkanedioic, and alkenedioic acids and tricarboxylic acids can be analyzed as their tert-butyldimethylsilyl derivatives12; aromatic dicarboxylic acids and higher fatty acids as their acetonyl esters13; and alkanoic, alkanedioic, and aromatic di- and polycarboxylic acids and hydroxyl acids as their butyl esters.14 Selected chromatographic procedures for the separation of organic acids should be perused in these tables in Sec. 5: gas chromatography, Table 5.14; liquid–liquid chromatography, Table 5.20; ion chromatography, Tables 5.21 and 5.22; and planar chromatography, Table 5.27. Of the methods commonly used to analyze mixtures of monomeric and oligomeric fatty acids such as GC, HPLC, and TLC, the TLC method was found to be the best one.15 Detection sensitivity is improved by the use of fluorescent derivatives; detection limits have been as low as 20 fmol. Supercritical fluid chromatography is useful for the determination of fatty acids without derivatization in the presence of their esters; CO2 was the mobile phase.16 19.2.3 Determination of Salts of Carboxylic Acids A 1 : 1 mixture by volume of ethylene or propylene glycol with 2-propanol is used as the solvent medium. The titrant is perchloric acid (or hydrochloric acid) dissolved in the same solvent mixture.17 Indicators or a pH meter is used to indicate the end point. The indicator is an alcoholic solution of methyl red (or methyl orange); the end point is a pink color. Glacial acetic acid is another solvent that has been used for the titration of carboxylic acid salts.18 Very good indicator end points are obtained, but potentiometric titration can also be used with glass and calomel electrodes. This solvent system usually gives sharper titration curves than the glycol–2propanol medium, but the latter medium has the advantage of the flexibility of solvent for dissolving many different types of samples.

19.3 METHODS FOR THE DETERMINATION OF ALCOHOLS AND PHENOLS ( −− OH GROUP) The determination of the hydroxyl group is the most complicated task in organic chemical analysis. The behavior of the hydroxyl group varies, depending on the rest of the molecule to which it is attached. Hydroxyl groups on primary and secondary carbon atoms, as well as phenolic and enolic hydroxyl groups, can be determined by esterification in pyridine solution with an acid anhydride, usually acetic anhydride, added in known excess. After completing the reaction, the excess can be hydrolyzed with water and the resulting acid is titrated with alkali. 11 Y.

Kasai, T. Tanimura, and T. Tamura, Z. Anal. Chem. 47:34 (1975); M. Pesez and J. Bartos, Talanta 21:1306 (1974). T. P. Mawwhinney et al., J. Chromatogr. 361:117 (1986). D. V. McCalley et al., Chromatographia 20:664 (1985). 14 I. Molnar-Perl et al., Chromatographia 20:421 (1985). 15 I. Zeman, M. Ranny, and L. Winterova, J. Chromatogr. 354:283 (1986). 16 A. Nomura et al., Anal. Chem. 61:2076 (1988). 17 S. Palit, Ind. Eng. Chem., Anal. Ed. 18:246 (1946). 18 J. S. Fritz, Anal. Chem. 22:1028 (1950). 12 13

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.15

TABLE 19.5 Scope and Limitations of Procedures Employed for the Determination of Hydroxyl Groups Procedure

Scope

Interferences

1. Acetylation (Ac2O)

Alcohols, essential oils, fats, glycols, hydroxy acids, phenols, sugars, waxes

2. Bromination (KBrO3 + KBr)

6. Grignard reaction (CH3HgI)

Phenols (replacement of H by Br at o- and p-positions) Phenolic compounds Aliphatic and alicyclic alcohols, aromatic alcohols with OH in aliphatic side chain, hydroxy acids Citronellol, essential oils, linalool, easily dehydrated terpene alcohols, terpineol Tertiary OH groups

Acetylated compounds not hydrolyzed with water, low-molecularweight aldehydes, primary and secondary amines, sulfhydryl groups Aliphatic hydrazines, unsaturated compounds Amines, active methylene groups Acetals, aldehydes, amines, ketals, ketones

7. Infrared and Raman

General

8. LiAlH4 reduction

Alcohols, phenols

9. Oxidation (HIO4)

OH groups on adjacent C atoms

3. Coupling 4. Esterification (HOAc + BF3)

5. Formylation (HOAc–HCOOH)

10. Phthalation (phthalic anhydride)

Other compounds or groups containing active H atoms Functional groups that absorb in same area of infrared or Raman spectrum Other compounds or groups containing active H atoms OH and CO, OH and NH2, or CO groups on adjacent C atoms

Alcohols, essential oils

A more powerful reagent is acetyl chloride in pyridine solution.19 3,5-Dinitrobenzoyl chloride reacts faster.20 The excess reagent is hydrolyzed with water, and HCL is titrated in benzene–methanol (7 : 1) solution with 0.200N (C4H9)4NOH using the color change of the titrant from yellow to red or potentiometric end-point detection. Phthalic anhydride is the most selective of all the acylating reagents; aldehydes, ketones, and aromatic hydroxyl compounds do not interfere. The reaction with 3-nitrophthalic anhydride in the presence of triethylamine catalyst in N,N-dimethylformamide solution is complete in 10 min at room temperature. Boron trifluoride is a very strong catalyst in acylation reactions with acetic acid; the water formed during the reaction is determined by the Karl Fischer titration. This method determines the total hydroxyl group content of tertiary aliphatic and alicyclic alcohols including polyhydroxy alcohols. Only the phenolic hydroxyl group does not react. Enolic and some aromatic hydroxyl groups are acidic enough to be titrated with KOH, particularly in nonaqueous solvents. The scope and limitations of procedures employed for the determination of hydroxyl groups are set forth in Table 19.5. Table 19.6 gives the conditions frequently employed in volumetric methods for the determination of hydroxyl groups. Finally, Table 19.7 provides a summary of the methods that have been found to be most useful in dealing with various important classes of compounds containing hydroxyl groups. When several methods are available for dealing with any one class of compounds, these are listed in the same order as they appear in Table 19.6. The infrared (Table 7.7) or Raman (Table 7.24) methods are also applicable for the determination of the hydroxyl group. Primary alcohols have intense absorption bands at 3640 to 3636 cm–1, 19 20

G. A. Olah and M. B. Comisarow, J. Am. Chem. Soc. 88:4442 (1966). W. T. Robinson, Anal. Chem. 33:1030 (1961).

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Procedure

10. Phthalation

8. LiAlH4 9. Oxidation a. OH on two adjacent C atoms b. OH on three adjacent C atoms

2. Bromination 4. Esterification

1. Acetylation

Sample

Depends on OH content

8–15 mmol OH Depends on OH content

Depends on OH content and compound analyzed Equivalent of 0.5 g phenol 1–2 mL in 1,4-dioxane

Concentration

H5IO6 + H2O + HOAc, 5.4 : 100 : 1900 (w/v/v) 60 g NaIO4 + 120 mL 0.05M H2SO4 diluted to 1 L with water Solid phthalic anhydride

Acetic anhydride + pyridine 1 : 3 (v/v) KBrO3 (0.017M) + excess KBr 100 g BF3 + 1–2 mL water diluted to 1 L with HOAc 0.25M LiAlH4 in tetrahydrofuran

Amount

Time

2h

30 min

50 mL

2 g (100% molar excess needed)

30 min

15–30 min

15 min 2h

1h

50 mL

20 mL

5 mL; maintain at least 100% excess acetic anhydride 50 mL 20 mL

Reagent

Conditions

TABLE 19.6 Conditions Frequently Employed in Volumetric Methods for Determination of Hydroxyl Groups

100°C

Room

Room

Room

Room 67°C

Steam bath

Temperature

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.16

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.17

TABLE 19.7 Methods for the Determination of Hydroxyl Groups Compound determined OH groups

Reagent CH3COCl + pyridine Acetic anhydride + pyridine

OH groups (in presence of a-epoxide) Alcohols

LiAlH4 HOAc + BF3 LiAlH4 Phthalic anhydride in pyridine

Alcohols (in amine mixtures) Alcohols (primary and secondary)

Alcohols (tertiary) Carbohydrates Cellulose derivatives Cellulose esters (total and primary OH) Essential oils (alcohols)

Acetic anhydride + pyridine (1 : 3, v/v) Acetic anhydride + pyridine (1 : 3, v/v) Acetic anhydride + ethyl acetate + HClO4 HOAc + BF3 Grignard reagent Acetic anhydride + pyridine Acetic anhydride + pyridine (1 : 19, v/v) Phenyl isocyanide Acetic anhydride + NaOAc

Essential oils (primary alcohols)

Phthalic anhydride + benzene

Essential oils (easily dehydrated alcohols) Glycerol

HCOOH + HOAc Acetic anhydride + pyridine Acetic anhydride NaIO4 + H2SO4 Schiff reagent Acetic anhydride + pyridine (1 : 3, v/v) 0.025M H5IO6

Glycols

0.1M NaIO4 Glycols (mixtures)

0.025M NaIO4

Hydroxy acids

Acetic anhydride + pyridine (1 : 3, v/v) HOAc + BF3

Procedure Acetylate, hydrolyze, and titrate excess reagent Micro technique Measure infrared absorption at 5000–3125 cm–1 Gas chromatography Volumetric estimation of H2 formed Esterify and titrate water formed Titrate excess LiAlH4 with 1-propanol Esterify and titrate excess anhydride Acetylate and determine sponification value of esters formed Acetylate, hydrolyze, and titrate excess reagent Acetylate, hydrolyze, and titrate excess reagent Esterify and titrate water formed Measure volume of CH4 evolved Acetylate, hydrolyze, and titrate excess reagent Acetylate and titrate excess reagent Ultraviolet absorption of carbanilate at 280 nm Acetylate and determine saponification value Esterify and titrate excess anhydride Esterify and determine saponification value Acetylate, hydrolyze, and titrate excess HOAc Acetylate and titrate excess reagent Oxidize and titrate HCOOH formed Colorimetric measurement Acetylate, hydrolyze, and titrate excess reagent Oxidize and titrate excess reagent iodometrically Oxidize and tirate acid formed with standard alkali Oxidize and apply combination of acidimetric and iodometric titrations Acetylate, hydrolyze, and titrate HOAc Semimicroprocedure similar to above Esterify and titrate water formed

Reference 28 30 27 9, 29 20 6 15 11 12 6 13 6 14 23 26 25 17 16 18 1 4 1 3 1 7 10 7, 8

1 5 6 (Continued)

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.18

SECTION NINETEEN

TABLE 19.7 Methods for the Determination of Hydroxyl Groups (Continued) Compound determined

Reagent

Monoglycerides

H5IO6 + HOAc (1 : 370, w/v)

Natural fats

Acetic anhydride + pyridine (1 : 9, v/v) Acetic anhydride

Acetic anhydride Acetic anhydride + pyridine (1 : 3, v/v) KBrO3 + KBr

Phenols

Diazo compounds LiAlH4 Phenols (o-substituted) Polyesters

LiAlH4 F3CCOOH

Sterols

Acetic anhydride + pyridine Gas chromatography–liquid chromatography

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

Procedure Oxidize and determine excess H5IO6 iodometrically Acetylate, hydrolyze, and titrate excess reagent Acetylate and determine saponification values of sample before and after acetylation Acetylate in sealed tube and titrate excess reagent Acetylate, hydrolyze, and titrate excess reagent Brominate and titrate excess Br2 iodometrically Coupling Titrate excess LiAlH4 with 1-propanol Measure volume of H2 evolved Measure water formed by Karl Fischer method Acetylate, hydrolyze, and titrate excess reagent (Also for sterol esters and waxes in oils and fats)

Official and Tentative Methods, 2d ed., American Oil Chemist’s Society, Chicago, 1946. K. Helrich and W. Rieman, Ind. Eng. Chem., Anal. Ed. 19:691 (1947). Basset, Ind. Eng. Chem. 2:389 (1910). Mehlenbacker, in J. Mitchell et al., eds., Organic Analysis, Interscience, New York, 1953, Vol. 1, p. 18. C. L. Ogg, W. L. Porter, and C. O. Willets, Ind. Eng. Chem., Anal. Ed. 17:394 (1945). W. M. D. Bryant, J. Mitchell, Jr., and D. M. Smith, J. Am. Chem. Soc. 62:1 (1940). N. Allen, H. Y. Charbonnier, and R. M. Coleman, Ind. Eng. Chem., Anal. Ed. 12:384 (1940). Pohle and Mehlenbacker, J. Am. Oil Chem. Soc. 24:155 (1947). L. Ginsburg, Anal. Chem. 31:1822 (1959). S. Dal Nogare and A. N. Oemler, Anal. Chem. 24:902 (1952). P. J. Elving and B. Warshowsky, Ind. Eng. Chem., Anal. Ed. 19:1006 (1947). S. Siggia and I. R. Kervenski, Anal. Chem. 23:117 (1951). Fritz, J. S., and G. H. Schenk, Anal. Chem. 31:1808 (1959). W. Fuchs, N. H. Ishler, and A. G. Sandhoff, Ind. Eng. Chem., Anal. Ed. 12:507 (1940). C. J. Linter, R. H. Schleif, and T. Higuche, Anal. Chem. 22:534 (1950). Gunter, The Essential Oils, Van Nostrand, Princeton, N.J., 1948, Vol. 1, p. 275. Ibid., p. 272. Ibid., p. 276. Furman, ed., Scott’s Standard Methods of Chemical Analysis, 5th ed., Van Nostrand, Princeton, N.J., 1939, Vol. II, p. 2253. G. A. Stenmark and F. T. Weiss, Anal. Chem. 28:1784 (1956). T. Higuche, in J. Mitchell et al., eds., Organic Analysis, Interscience, New York, 1954, Vol. 2, p. 123. Arndt, in J. Mitchell et al., eds., Organic Analysis, Interscience, New York, 1953, Vol. 1, pp. 197–239. B. E. Christensen and R. A. Clarke, Ind. Eng. Chem., Anal. Ed. 17:265 (1945). M. Freed and A. M. Wynne, Ind. Eng. Chem., Anal. Ed. 8:278 (1936). C. J. Malm et al., Anal. Chem. 26:188 (1954). C. J. Malm, L. B. Ganung, and R. F. Williams, Jr., Ind. Eng. Chem., Anal. Ed. 14:935 (1942). C. L. Hilton, Anal. Chem. 31:1610 (1959). Smith and Bryant, J. Am. Chem. Soc. 57:61 (1935). H. S. Knight, Anal. Chem. 30:2030 (1958). J. W. Petersen, K. W. Hedberg, and B. T. Christensen, Ind. Eng. Chem., Anal. Ed. 15:225 (1943). C. A. Lucchesi, B. Bernstein, and P. Ronald, Anal. Chem. 47:173 (1975). K. Grob, M. Lanfranchi, and C. Mariani, J. Chromatogr. 471:397 (1989).

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Reference 1 1 1

2 5 19 22 21 20 31 24 32

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.19

secondary alcohols at 3636 to 3630 cm–1, and tertiary alcohols at 3623 to 3620 cm–1. The carboxylic group shows intense absorption in these regions. The association of hydroxyl groups in polymers with tetrahydrofuran, which absorbs strongly at 3450 cm–1, forms the basis of a hydroxyl determination in polymers.21 Chemical, infrared, and NMR spectral methods for the determination of the hydroxyl group have been discussed.22 Hydroxyl groups on adjacent carbon atoms (glycolic hydroxyl) can be readily determined by oxidation with periodic acid. A known excess of the reagent is used, and after the oxidation the iodine liberated is titrated with thiosulfate. In an alternative finish, the unreacted periodate is reduced to iodide, which is then titrated with silver ion using an iodide-selective electrode. The time required for the determination of hydroxyl groups in various polymer polyols can be cut to 15 min by the addition of imidazole or N-methylimidazole as catalyst. The time required for the periodate reaction can be followed with a perchlorate-selective ion electrode and related to the concentration of glycol.23 Spectrophotometric methods are available for the determination of small amounts of alcoholic hydroxyl groups. Secondary alcohols, in the presence of primary alcohols, can be determined by oxidizing the secondary alcohols to ketones with K2Cr2O7 and determining the ketones with 2,4-dinitrophenylhydrazine.24 Primary alcohols can be determined with vanadium 8-hydroxyquinoline.25 For tertiary alcohols, the alcohol can be transformed into alkyl iodide with HI, and the absorption of the iodide is measured.26 Phenols react with nitrous acid to form nitrosophenol, which, on treatment with alcoholic ammonia solution, form an intense color owing to the formation of a quinone. Coupling reactions of phenols with diazonium compounds display an absorption band at 270 to 280 nm, which in alkaline solution shifts to 295 to 300 nm, with an increase in absorbance. For the determination of very small amounts of phenols, such as those in surface waters, 4-aminoantipyrine is added in alkaline solution and in the presence of K3[Fe(CN)6] with the formation of colored compounds.27 The hydroxyl groups of poly(ethylene glycols) are silanized with dimethylaminosilanes, and the derivatives determined photometrically with a sensitivity 1000-fold more sensitive but with the same precision as the acetylation method.28 (R)-(+)-2-Phenylselenopropionic acid is a useful reagent for determining the enantiomeric composition of secondary chiral alcohols.29 For gas chromatography, both carboxylic acids and alcohols (or hydroxy acids) can be derivatized simultaneously with the use of heptafluorobutyric anhydride, pyridine, and ethanol.30 Reference gas-chromatographic data have been recorded for the analysis of alcohols and phenols as trimethylsilyl ethers.31 Both the 19F NMR of trifluoroacetates and the 13C NMR of CH2 and CH groups serve to distinguish primary from secondary alcohols and ethylene oxide from propylene oxide units in the chains of various polymer polyols. Adduct formation between alcohols and hexafluoroacetone allows the determination of hydroxyl groups by the use of 19F NMR.32 Mixtures of different alcohols or mixtures of alcohols and water can be determined simultaneously.

19.4 METHODS FOR THE DETERMINATION OF ALDEHYDES AND KETONES Compounds containing the carbonyl group, such as aldehydes and ketones, can be determined selectively because of their reducing and complex-forming properties. Acetals (the condensation products of alcohols and aldehydes) are grouped with the aldehydes because they can be determined with the same type of reactions. Methods that serve for the determination of both aldehydes and ketones are 21

C. S. Y. Kim et al., Anal. Chem. 54:232 (1982). S. Siggia, J. G. Hanna, and T. R. Stengle, in S. Patal, ed., Chemistry of the Hydroxyl Group, Pt. 1, Wiley, London, 1971. C. H. Efstathiou and T. P. Hadjiioannou, Anal. Chem. 47:864 (1975). 24 F. E. Critchfield and J. A. Hutchinson, Anal. Chem. 32:862 (1960). 25 R. Amos, Anal. Chim. Acta 40:401 (1968). 26 M. W. Scoggins and J. W. Miller, Anal. Chem. 38:612 (1966). 27 M. B. Ettinger, R. J. Ruchhoft, and R. J. Liska, Anal. Chem. 23:1783 (1951); F. W. Ochynsky, Analyst 85:278 (1960). 28 D. F. Fritz et al., Anal. Chem. 51:7 (1979). 29 P. Michelsen and G. Odham, J. Chromatogr. 331:295 (1985). 30 J. B. Brooks, C. C. Alley, and J. A. Liddle, Anal. Chem. 46:1930 (1930). 31 M. Mattsson and G. Petersson, J. Chromatogr. Sci. 15:546 (1977). 32 F. F.-L. Ho, Anal. Chem. 45:603 (1973); ibid. 46:496 (1974). 22 23

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Volumetric (1–5 meq)

Spectrophotometric (0–500 mg/mL)

Volumetric (1–5 meq)

Spectrophotometric (0–400 mg/mL)

a ,b-Unsaturated

b -Dicarbonyl

Acetaldehydes, methyl ketones

Spectrophotometric (0–500 mg/mL)

(1–10 meq)

(1–10 meq)

Potentiometric (0–0.25 meq)

HPLC–mass spectrometry

Gasometric (Grignard reagent) (1–10 meq) Gravimetric (1–5 meq)

Chromatographic

Chromatographic

Technique

Aliphatic

General

Type of compound determined Procedure and reference

Other complexing agents, excess base

EtOH, peroxides

After reaction with alkaline I2 (NaOI), measure absorbance of CHI3 at 347 nm [1].

HCHO, glyoxal, isobutyraldehyde

Aromatic aldehydes and ketones, other reducing agents

Acids, other colored compounds

Peroxides

Acids, bases, peroxides

Compounds containing active H, acid halides, alkyl halides, esters, isonitriles, nitriles Hydrocarbons

Interferences

After reaction with standard Cu(OAc)2, remove Cu(II) complex by filtration and extraction with CHCl3, and determine excess Cu(II) iodometrically [1].

Reaction with m-phenylenediamine, then measure absorbance of colored Schiff’s base [1].

After reaction with excess standard peroxotrifluoroacetic acid in 1,2-dichloroethane, add excess KI and titrate liberated I2 with standard Na2S2O3 [9].

After derivatization with 2,4-dinitrophenylhydrazine, use HPLC. Convert volatile carbonyl compounds to their o-benzyloximes, then separate by GC. Reaction with CH3MgI and decomposition of excess reagent with water. Measure volume CH4 evolved [1,7]. 2,4-Dinitrophenylhydrazine forms precipitate, filter and weigh [1]. A moving belt interface introduces 2,4-dinitrophenylhydrazones of aldehydes and ketones into a mass spectrometer after separation by HPLC. CH4 chemical ionization mode; aldehydes were detected by negative ion mass spectrometry. Reaction with excess H2NON, titration of excess reagent with standard HCl [1]. Reaction with excess H2NOH ⋅ HCl, titration of liberated HCl with standard NaOH solution [1]. After reaction with excess H2NOH ⋅ HCl, titrate liberated water with Karl Fischer reagent. After reaction with excess 2,4-dinitrophenylhydrazine, make alkaline, measure absorption at 480 nm [1].

TABLE 19.8 Methods for the Determination of Aldehydes and Ketones

For references, see list below Table 19.10.

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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19.21

listed in Table 19.8; methods for the determination of aldehydes are listed in Table 19.9; and methods for the determination of ketones are listed in Table 19.10.

19.5

METHODS FOR THE DETERMINATION OF ESTERS The esters of carboxylic acids are relatively stable compounds, and therefore no direct reactions are available for their determination. Esters are most commonly determined by saponification if the corresponding carboxylic acid and alcohol are formed. If a known excess of alkali is added, the excess can be titrated with standard acid to the phenolphthalein end point or performed potentiometrically with the glass–calomel electrode pair. For easily saponifiable esters, the solvent can be C1 to C4 alcohols and water or hydrocarbon– alcohol mixtures. The system is refluxed for 30 min using 0.05M to 1.0M KOH, then cooled, and the unused base is titrated with 0.05M to 1.0M H+.33–36 For difficultly saponifiable esters, the solvents are alcohols higher than C5, glycols, glycol ethers, polyglycols, polyglycol ethers, and other high-boiling-point solvents. Small amounts of water are needed. Reaction conditions are the same as described for easily saponifiable esters. When the alkaline hydrolysis is done in aqueous dimethylsulfoxide solution, many esters react even at room temperature while others require heating for 5 min on a water bath.37 Using the alkaline hydrolysis method, the “ester number” or “saponification number” can be determined. These data are used in industry to characterize fats and waxes. The ester number is the amount of potassium hydroxide (in mg) that is necessary for the saponification of the esters found in 1 g of the sample. Gas chromatography is an excellent method for the separation and determination of esters. Methyl esters of the C12 to C18 fatty acids have been separated on Chromosorb R and Celite 545 as the stationary phase, which are treated with dimethylchlorosilane and then moistened with poly(vinyl acetate).38 A colorimetric method involves reaction of the ester with hydroxylamine to form the corresponding oxime, after which Fe(III) ion is added to form a red chelate.39,40 In the infrared spectrum most esters will show a C −− O stretching vibration at 1100 to 1250 cm–1. Normal saturated esters have a C == O band at 1750 to 1735 cm–1 whereas in unsaturated and aryl ester the same band appears at 1730 to 1717 cm–1. The details of the procedure that is best suited to any particular analytical problem depend so much on the nature of the sample and on the identity of the ester in question that the original literature must be consulted before making a selection.

19.6 METHODS FOR THE DETERMINATION OF OTHER OXYGEN-BASED FUNCTIONAL GROUPS 19.6.1

Ethers Aliphatic ethers are boiled with concentrated HI. The alkyl iodide formed is volatile (relative to the boiling point of HI) when methoxy, ethoxy, and isopropoxy groups are the reactants. Higher alkyloxy entities are difficult or impossible to remove by distillation. 33

D. T. Englis and J. E. Reinschreiber, Anal. Chem. 21:602 (1949). W. Reiman, Ind. Eng. Chem., Anal. Ed. 15:325 (1943). 35 S. Siggia and J. G. Hanna, Quantitative Organic Analysis Via Functional Groups, 4th ed., Wiley, New York, 1977. 36 D. M. Smith, J. Mitchell, Jr., and A. M. Billmeyer, Anal. Chem. 24:1847 (1952). 37 J. A. Vinson, J. S. Fritz, and G. A. Kingsbury, Talanta 13:1673 (1966). 38 I. Hornstein and P. F. Crowe, Anal. Chem. 33:310 (1961); see also F. H. M. Nestler and D. F. Zinkel, ibid. 39:1118 (1967). 39 Hestrin, J. Biol. Chem. 180:249 (1949). 40 U. T. Hill, Ind. Eng. Chem., Anal. Ed. 18:317 (1946); Anal. Chem. 19:932 (1947). 34

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Technique and range

Procedure and reference

Unsaturated

Volumetric (1–5 meq)

Volumetric (0.1–0.2 meq)

Spectrophotometric

After reaction with dodecanethiol at room temperature, titrate excess mercaptan iodometrically [1].

Basic fuchsin (red) plus aqueous SO2 gives colorless leuco Schiff’s base which is coupled with aldehydes to give red dye [1–3]. Color formed from the reaction of 4-amino-3-hydrazino-5mercapto-1,2,4-triazole with simple aldehydes shows an absorption maximum at 520 to 550 nm. Reaction with Ag2O in column or flask; titration of Ag salts in eluate or unreacted Ag in flask with standard KSCN [+ Fe(III) indicator] [1,8].

Spectrophotometric (0–500 mg/mL)

Lower aliphatic

After reaction with 2-hydrazinobenzothiazole, develop color with K3[Fe(CN)6] plus KOH, measure absorbance [18]. Measure absorbance at 290–293 nm for 0.001M–1M, and at 180–200 nm for 10−6M–10−3M [1,16,17].

Spectrophotometric (0.01–30 mg/mL)

Aliphatic

Spectrophotometric

After hydrolysis with dilute HCl, the corresponding aldehyde is reacted with diethylamine and chloranil and color measured at 640 to 660 nm.

Spectrophotometric

(20–40 meq)

Volumetric (1–5 meq)

Spectrophotometric

Potentiometric titration (1–5 meq)

Polarographic (0.001M–0.1M)

Gravimetric (0.1–10 meq)

Fluorescence

Acetals

General

Type of aldehyde determined 1,2-Diaminonaphthalene reacts with aldehydes to give a fluorescent product. React with 3,5-dimethylcyclohexane-1,3-dione in solution buffered at pH 4.6, filter, dry and weigh precipitate [1]. Reduction at dme in aqueous 0.1M (CH3)4NOH solution. E1/2 = ca. –2.1 V vs. SCE [1,6]. Reaction with 1-dodecylamine in ethylene glycol–2-propanol medium followed by titration of excess amine with standard salicylic acid solution [14]. Details for colorimetric, fluorimetric, and phosphorimetric analysis of over 74 aldehydes given. Reaction product of an aldehyde, diethylamine and chloranil is measured at 640 to 660 nm. After reaction with excess Ag2O, add excess standard alkali and back-titrate with standard acid [1]. After reaction in neutral solution with Na2SO3, titrate with standard acid [1].

TABLE 19.9 Methods for the Determination of Aldehydes

For references, see list below Table 19.10.

Other unsaturated compounds, peroxides

Acids, esters

Higher aldehydes, some ketones, bases, oxidizing reagents

Other absorbing compounds; some ketones, aromatics, and unsaturates

Acrolein, nitromethane

Cyclohexanone, some other ketones if >10%, some organic acids, peroxides

Aromatic aldehydes, ketones, and formaldehyde do not interfere Acids, esters, peroxides

Strong acids, acyl anhydrides, acyl halides

Other reducible compounds

Acids, bases

Interferences

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.22

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Measure the C== O stretching band in the region 1834–1653 cm−1 [4,5]. Obtain mass spectrum and calculate concentration(s) from heights of characteristic peaks [10,18].

Infrared (10−3M–0.2M)

General methods

Volumetric (0.1–5 meq)

Volumetric (0–25 mg)

Spectrophotometric (0–100 mg/mL)

Polarographic (10–100 mg/mL)

Monitoring

Colorimetric

Mass spectrometry (>0.2 meq)

Thermometric titration

HPLC–fluorescence

Modified pararosaniline method for formaldehyde in air. NIOSH*-recommended procedure uses chromotropic acid and H2SO4. Two monitoring methods based on 2,4-dinitrophenylhydrazine and N-benzylethanolamine were evaluated. Obtain polarogram in aqueous 0.1M LiOH–0.01M LiCl; E1/2 = –1.6 V vs. SCE. After reaction with chromotropic acid (1,8-dihydroxy-3,6naphthalenedisulfonic acid) plus 18M H2SO4 at 100°C for 30 min, dilute and measure absorbance at 670 nm. After reaction with excess KCN, determine excess by titration with standard AgNO3; or destroy CN– with Br2, add excess KI, and titrate liberated I2 with standard Na2S2O3 [1]. After reaction with excess standard I2 in alkaline solution, make acid and titrate excess I2 with standard Na2S2O3 [1]. Review of methods for air and forest products.

Reaction with 4,5-dimethyoxy-1,2-diaminobenzene in dilute acid. Fluorescence is developed by addition of alkali and stabilized by the addition of 2-mercaptoethanol. Form fluorescent derivatives with 1,2-diamino-4,5-ethylenedioxybenzene and separate by reversed-phase HPLC. After reaction with 2,4-dinitrophenylhydrazine or N,Ndimethylhydrazine, titrate excess thermometrically with standard o-methoxybenzaldehyde in (1: 1: 23) sulfuric acid : water : isobutyl alcohol medium.

Fluorometric (10−8M–10−7M)

Spectrophotometric–flame atomic absorption

Spectrophotometric

Measure absorbance at 215, 265, 315, or 320 nm, depending on extent of conjugation [1,16,17]. Reaction with anthrone gives benzanthrone derivatives, detectable by ultraviolet spectroscopy but better detected by fluorescence spectroscopy. Tollen’s reagent method is used; the Ag is filtered, dissolved in HNO3 and measured at 328 nm by flame AAS.

Spectrophotometric (10−7M–10−4M)

* NIOSH = National Institute for Occupational Safety and Health.

Formaldehyde

Specific compounds

Aromatic

a,b-Unsaturated

Acetaldehyde, acrolein, EtOH, furfural, methyl ketones, 2-oxopropionic acid

>5% acetaldehyde, >50% acetone

Acetaldehyde and acrolein above limit determined by formaldehyde concentration Alcohols, formals, some ketones

Unaffected by phenol

Carboxylic acids, esters, ketones

Selective for aromatic aldehydes

Ketones

No interference from saturated aldehydes

Other absorbing compounds

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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Spectrophotometric

Ultraviolet

Mass spectrometry (>0.2 meq)

Spectrophotometric (0.02M–0.2M)

Diketones, aliphatic

Specific compounds

Acetone

Volumetric (1–10 meq)

Ultraviolet (10–3M–1M)

Infrared (10–3M – 0.2M)

Polarographic (10–5M–10–2M)

Polarographic

(10–4M–10–2M)

Technique

Aliphatic

General

Type of ketone

TABLE 19.10 Methods for the Determination of Ketones

Determine purple color after reaction with furfural plus H2SO4.

Obtain mass spectrum and use heights of characteristic peaks and calibration from pure compounds [24].

For a-diketones (10–3M–1M), measure absorbance at ca. 286 nm; for b-diketones (10–5M–10–2M), at ca. 270 nm [13,22].

Dissociation of dimer of dye cation from Brilliant Green occurs in presence of aliphatic ketones resulting in an increase in absorbance; detected eluants in reversed-phase HPLC [27].

For aliphatic ketones; aldehydes and other reducible compounds interfere

Obtain polarogram of N2H4 adduct in 0.1M N2H4 ⋅ H2SO4 plus 0.05M H2SO4 at E1/2 = −1.1 and –1.4 V vs. SCE [11]. After reaction with betaine hydrazide HCl, obtain polarogram in buffered alkaline chloride solution; E1/2 = –1.4 to –1.5 vs. internal Hg pool [11]. Measure absorbance at C== O stretching band at 1835–1653 cm–1 Measure absorbance at 279 to 289 nm for aliphatic and >289 nm for aromatic ketones [13,22]. Oxidize aldehydes selectively with Ag2O; react remaining ketones with excess H2NOH, and back-titrate excess with standard HCl solution [13].

Aliphatic aldehydes

Some aromatics interfere with b-diketones

Strong bases interfere

Carboxylic acids, aldehydes, esters interfere Aromatic compounds interfere

Aldehydes, other reducible compounds interfere

Remarks

Procedures and reference

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

After reaction with standard I2 in alkaline solution, titrate excess I2 with standard Na2S2O3 [13].

Acetaldehyde, acrolein, EtOH, furfural, methyl ketones, 2-oxopropionic acid

L. S. Bark and P. Bate, Analyst 96:881 (1971); L. S. Bark and P. Prachuabpaibul, Anal. Chem. Acta 84:207 (1976). W. F. Chao et al., Anal. Chim. Acta 215:259 (1988); S. Hara et al., ibid. 215:267 (1988). J. Chrastil and R. M. Reinhardt, Anal. Chem. 58:2848 (1986). D. L. DuVal, M. Rogers, and J. S. Fritz, Anal. Chem. 57:1583 (1985). K. Fung and D. Grosjean, Anal. Chem. 53:168 (1981). J. A. Gilpin and F. W. McLafferty, Anal. Chem. 29:990 (1957). L. Gollob and J. D. Willons, For. Prod. J. 30:27 (1980). M. F. Hawthorne, Anal. Chem. 28:540 (1956). N. W. Jacobsen and R. G. Dickinson, Anal. Chem. 46:298 (1974). S. E. Know and S. S. Q. Hee, Ind. Hyg. Assoc. J. 45:325 (1984). I. M. Kolthoff and J. J. Lingane, Polarography, 2d ed., Interscience, New York, 1952. B. Miller and N. D. Danielson, Anal. Chem. 60:622 (1988). J. Mitchell et al., eds., Organic Analysis, Interscience, New York, 1953, Vol. 1. M. Nakamura et al., Anal. Chim. Acta 134:39 (1982). S. I. Obtemperanskaya and E. K. R. Mohamed, Z. Anal. Khim. 35:1982 (1980); C.A. 93:230330g (1980). Y. Ohkura and K. Zaitsu, Talanta 21:554 (1974). P. J. Oles and S. Siggia, Anal. Chem. 46:911 (1974). D. G. Ollett, A. B. Attygalle, and E. D. Morgan, J. Chromatogr. 367:207 (1986). K. L. Olson and S. J. Swarin, J. Chromatogr. 333:337 (1985). E. Priha and I. Ahonen, Anal. Chem. 58:1195 (1986). E. L. Saier and R. H. Hughes, Anal. Chem. 30:513 (1958). E. Sawicki and T. R. Hauser, Anal. Chem. 32:1434 (1960). E. Sawicki and C. R. Sawicki, The Analysis of Organic Materials: Aldehydes Photometric Analysis, Academic, New York, 1975, Vols. 2 and 3. A. G. Sharkey, J. L. Shultz, and R. A. Friedel, Anal. Chem. 28:934 (1956). H. Siegal and F. T. Weiss, Anal. Chem. 26:917 (1954). S. Siggia and E. Segal, Anal. Chem. 25:640, 830 (1953). A. Trujilo, S. W. Kang, and H. Freiser, Anal. Chim. Acta 182:71 (1986). M. T. M. Zaki, Anal. Lett. 18:1697 (1985).

References for Tables 19.8 to 19.10

Volumetric (0.1–2 meq)

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19.26

SECTION NINETEEN

More recent procedures, especially for microamounts, use a volumetric version to complete the determination. In one procedure the alkyl iodide vapor is conducted into a glacial acetic acid solution of bromine with carbon dioxide or other inert gas. Before entering this solution, the vapor is washed to remove HI and I2. The bromine oxidizes the iodide portion of the alkyl iodide to iodate ions. On adding excess iodide ions, a sixfold amount of iodine atoms are liberated, making the procedure very sensitive. The liberated iodine is titrated with standard Na2S2O3 solution. In another procedure the distilled alkyl iodide is absorbed by pyridine and titrated with tetrabutylammonium hydroxide solution.41 In a third variation the alkyl iodides are absorbed in a known amount of benzene, which is then reacted with aniline; the aniline iodide formed is titrated with standard sodium methoxide solution.42 This latter method is suitable for the determination of the C4 to C20 alkyl iodides. Rather than distillation, the alkyl iodides produced from the higher alkoxyl groups can be extracted with cyclohexane and their ultraviolet absorption bands measured. All ethers that contain an a-hydrogen atom can be oxidized with bromine water; the excess of bromine is determined by iodometric titration.43 Polyglycol ethers are determined in a manner similar to the alkoxyl groups.44 A two-phase titration procedure for the determination of poly(oxyethylene) nonionic surfactants involves the extraction of the surfactant as a sodium tetraphenylborate complex, which is then (1) titrated with tetradecyldimethylbenzylammonium chloride,45 (2) determined in a two-phase system by titration with sodium tetrakis(4-fluorophenyl)borate,46 or (3) determined by ultraviolet spectroscopy after conversion into a red complex by reaction with Fe(SCN)3. Nonionic poly(oxyethylene) can be oxidized by V2O5 in H2SO4 solution and the excess V2O5 determined potentiometrically with Fe(II) ion.47 Ultra-trace levels of these nonionic surfactants have been determined in the presence of cationic surfactants in water by extraction with excess potassium picrate in dichloromethane; the potassium complex of the poly(oxyethylene) complex is concentrated in the dichloromethane layer.48 The oxygen ether content of coals and humic substances has been determined by carbon-13 NMR.49 If the alkyl iodides formed by hydrolysis with HI are absorbed in a 2-methylpyridine solution, 2,6-dimethylpyridine iodide is formed. For a spectrophotometric determination, this is reacted with an alkaline solution that contains 2,7-dihydroxynaphthol, K3[Fe(CN)6], and KCN [211].50 Gas chromatography easily separates the alkyl iodides formed by the reaction with HI because of their volatility. The column is tricresyl phosphate on Chromosorb R at 90°C; the eluant is passed through a thermal conductivity detector. Vinyl ethers are hydrolyzed with H2SO4 at room temperature for 15 to 30 min. The acetaldehyde formed is treated with NaHSO3 and the method finished volumetrically.51 The acetaldehyde can also be treated with excess hydroxylamine hydrochloride and the excess determined volumetrically. Infrared-absorption characteristics of various ethers will be found in Table 7.10. The Raman frequencies are given in Table 7.31. 19.6.2 The Epoxy Group (Oxiranes) A detailed description of the analysis of the epoxy group can be found in a monograph.52 On treatment with a nucleophilic reagent, such as hydrohalic acids, the oxirane ring of 1,2-epoxy compounds is opened and the corresponding chlorohydrin is formed. The methods consist of the addition of a 41

S. Ehrlich-Rogozinski and A. Patchornik, Anal. Chem. 36:849 (1964). J. Schole, Z. Anal. Chem. 193:321 (1963). N. C. Deno and N. H. Potter, J. Am. Chem. Soc. 89:3350 (1967). 44 P. W. Morgan, Ind. Eng. Chem., Anal. Ed. 18:500 (1946); S. Siggia et al., Anal. Chem. 30:115 (1958). 45 M. Tsubouchi and Y. Tanaka, Talanta 31:633 (1984). 46 M. Tsubouchi, N. Yamasaki, and K. Yanagisawa, Anal. Chem. 57:763 (1985). 47 C. Dauphin et al., Anal. Chim. Acta 149:313 (1983). 48 L. Favretto, B. Stancher, and F. Tunis, Int. J. Environ. Anal. Chem. 14:201 (1983). 49 T. Yoshida et al., Fuel 63:282 (1984); E. Bayer, Angew. Chem. 96:151 (1984). 50 R. F. Makeus, L. R. Rothringer, and A. R. Donia, Anal. Chem. 31:1265 (1959). 51 S. Siggia, Ind. Eng. Chem., Anal. Ed. 19:1025 (1947); S. Siggia, Quantitative Organic Analysis Via Functional Groups, 3d ed., Wiley, New York, 1963 pp. 98–101. 52 B. Dobinson, W. Hoffmann, and B. P. Stark, The Determination of Epoxide Groups, Pergamon, London, 1969. 42 43

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.27

known excess of the hydrohalic acid and the determination of the excess by titration with standard base. The use of various organic solvents makes possible the analysis of oxirane compounds insoluble in water. Table 19.11 gives methods for the determination of oxiranes. The reaction of oxiranes with sodium sulfide followed by interaction of the product with taurine and o-phthalic anhydride produces an intense fluorescence on excitation with a detection limit of 0.1 mmol/100 mL.53 A method based on proton NMR, and not subject to chemical interfering reactions, has been applied to epoxy resins.54 For water-soluble and reactive epoxy compounds, HCl saturated with MgCl2 is the reagent. The reaction time is 15 to 30 min at room temperature. On completion the excess HCl is titrated with standard NaOH to a methyl orange end point.55 An alcoholic MgCl2 solvent system with the same reagents can also be used; in this case the indicator is bromocresol green. In glacial acetic acid HBr reacts readily with the less reactive epoxy compounds. By adding HBr in known excess, the appropriate reaction time (15 to 60 min) can be selected. The volumetric finish involves titrating with a standard solution of sodium acetate until the color of the crystal violet indicator changes to blue-green. Chlorobenzene has been suggested as a superior solvent to glacial acetic acid. Epoxy compounds that form aldehydes on reaction with acids (such as styrene oxide) and those in which the epoxy ring contains a tertiary carbon atom cannot be determined by this method.56 Several other volumetric methods are based on the same general reaction scheme already outlined. The sample is added to an excess of HCl in 1,4-dioxane, allowed to stand at room temperature for 15 min, and the excess HCl is titrated with 0.1M NaOH in methanol to the cresol red end point. With pyridine as the solvent, the reaction time is 20 min at reflux; the back titration with NaOH is to the phenolphthalein end point.57 In yet another method, tetraethylammonium bromide and sample in chloroform, acetone, or benzene solution are titrated with standard perchloric acid dissolved in glacial acetic acid to the crystal violet end point or by potentiometry.58 The epoxide group can be cleaved by sulfuric acid to give glycols, which are in turn cleaved with excess H5IO6.59 A newer version, applicable to nanomole quantities of a variety of epoxides, 50% glyme is used as solvent but the excess H5IO6 is allowed to react with CdI2 to generate free iodine, which is then determined photometrically as its colored complex with starch.60

19.6.3 Peroxides The chemical methods for the determination of peroxides are outlined in Table 19.12. Peroxides will oxidize iodide ion to free iodine, arsenite ion to arsenate, and titanium(III) to titanium(IV). The liberation of iodine from potassium iodide is a rapid method, but it can be used only in a few organic solvents and cannot generally be used in the presence of unsaturated compounds. The arsenious oxide method suffers no interference from unsaturated compounds. Titanium(III) chloride will determine almost any peroxide; however, the reagent requires special handling and must be isolated from oxygen. Acyl peroxides in the presence of peresters, other peroxides, and hydrogen peroxide can be determined by reaction with hydroxylamine at pH 7 followed by formation of the colored iron(III) complex.61 By carrying out the hydroxylamine reaction at pH 14, both peroxoesters and diacyl peroxides can be determined.

53

A. Sana and S. Takitani, Anal. Chem. 57:1687 (1985). B. Davis, Anal. Chem. 49:832 (1977). 55 A. Elek, Ind. Eng. Chem., Anal. Ed. 11:174 (1939); Furter, Helv. Chem. Acta 21:873, 1144 (1938); W. Deckert, Z. Anal. Chem. 82:297 (1930). 56 A. J. Durbetaki, Anal. Chem. 28:2000 (1956). 57 G. King, Nature 164:706 (1949); S. Siggia, Quantitative Organic Analysis Via Functional Groups, 3d ed., Wiley, New York, 1963. 58 W. Selig, Mikrochim. Acta 1:112 (1980); R. R. Jay, Anal. Chem. 36:667 (1964). 59 F. E. Critchfield and J. B. Johnson, Anal. Chem. 29:797 (1957). 60 H. E. Mishmash and C. E. Meloan, Anal. Chem. 44:835 (1972). 61 N. A. Kozhikhova et al., Zh. Anal. Khim. 34:1217 (1979). 54

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1. 2. 3. 4. 5. 6. 7. 8. 9.

H2SO4, then excess H5IO6, CdI2 added 0.2M HCl in pyridine 0.2M HCl in pyridine–CHCl3

0.2M HCl in diethyl ether 0.2M HCl in 1,4-dioxane 0.1M HBr

0.1M HCl in saturated MgCl2 H2SO4 hydrolysis to glycol which is cleaved by H5IO6 0.5M HCl + MgCl2 in EtOH 0.2M HCl in Cellosolve (C2H5)4NBr

Reagent

Room

180

20 30 to 120

Reflux Reflux

Room (Styrene oxide and epoxy ring with a tertiary carbon atom in ring cannot be determined)

65°C

240

15 15–60

Room

Room

Temperature

30

15–30

Reaction time, min

Titrant

5 2

6 5, 9

Cresol red Crystal violet

Photometrically Phenolphthalein Phenolphthalein 0.1M NaOH in MeOH 0.1M NaOH in MeOH

Free I2 plus starch

0.1M NaOH in MeOH 0.1M NaOAc in HOAc

9

7, 8

Bromothymol blue Crystal violet or potentiometry Phenolphthalein

0.1M NaOH HClO4 in glacial acetic acid 0.1M NaOH

1

4, 9

Reference

Bromocresol green

Methyl orange

Indicator

0.5M NaOH

0.1M NaOH

Critchfield, F. E., and J. B. Johnson, Anal. Chem. 29:797 (1957). Durbetaki, A. J., Anal. Chem. 28:2000 (1956). Jay, R. R., Anal. Chem. 36:667 (1964). Kerchov, F. W., Z. Anal. Chem. 108:249 (1937). King, G., Nature 164:706 (1949). Mishmash, H. E., and C. E. Meloan, Anal. Chem. 44:835 (1972). Selig, W., Mikrochim. Acta 1:112 (1980). Siggia, S. and J. G. Hanna, Quantitative Organic Analysis Via Functional Groups, 4th ed., Wiley, New York, 1977. Dobinson, B., W. Hofmann, and B. P. Stark, The Determination of Epoxide Groups, Pergamon, London, 1969.

Pyridine Pyridine–CHCl3

50% Glyme

1,4-Dioxane Glacial HOAc or chlorobenzene

Cellusolve Chloroform (or acetone or benzene) Diethyl ether

Alcoholic MgCl2

Aqueous

Aqueous

Solvent system

TABLE 19.11 Methods for the Determination of Epoxides (Oxiranes)

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.28

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TABLE 19.12 Chemical Methods for the Determination of Peroxides Reducing agent

Reduction time, min

Temperature

Solvent

Final reagent employed

Reference

Volumetric NaI NaI KI Fe(NH4)2(SO4)2 FeSO4 As2O3

5–20 15 15–60 15

Room Reflux Room

Acetic anhydride 2-Propanol tert-BuOH + CCl4 HOAc Acetone–water (1 : 1)

Na2S2O3 Na2S2O3 Na2S2O3 K2Cr2O7 TiCl3 Excess I2 added and back-titrated with Na2S2O3

8 15 5 12 13 11

Colorimetric Fe(II) ions

0–5

Fe(II) ions N,N-Dimethyl-p-phenylenediamine sulfate

15 5

Room to incipient boiling Room Room

Absolute MeOH

SCN–

Benzene–MeOH MeOH

1,10-Phenanthroline None needed

14, 16 7 4

Miscellaneous techniques for various compounds Hydroxylamine, pH 7 (Acyl peroxides) Hydroxylamine, pH 14 (Peresters and diacyl peroxides) Triethylamine (Benzoyl peroxide)

Forms colored iron (III) complex Forms colored iron (III) complex Chemiluminescence measured None needed; color measured

N,N-Dimethyl-p-phenylenediamine (Peroxides derived from cyclohexanone) p-Tolyl methyl sulfide (Peracids)

p-Phenetidine (Peracids) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

KH2PO4

Gas chromatography of sulfoxide formed or unused sulfide Color measured

Blazheevskii, N. E., and V. K. Zinchuk, Chem. Abstr. 105:17620m (1986). Bowyer, J. R., and S. R. Spurlin, Anal. Chim. Acta 192:289 (1987). DiFuria, F., et al., Analyst 109:985 (1984). Dugan, P. R., Anal. Chem. 33:1630 (1961). Hartman, L., and M. D. L. White, Anal. Chem. 24:527 (1952). Kozhikhova, N. A., et al., Zh. Anal. Khim. 34:1217 (1979). Laitinen, H. A., and J. S. Nelson, Ind. Eng. Chem., Anal. Ed. 18:422 (1946). Nozaki, K., Ind. Eng. Chem., Anal. Ed. 18:583 (1946). Robey, R. F., and H. K. Wiese, Ind. Eng. Chem., Anal. Ed. 17:425 (1945). Sevast’yanova, E. M., and Z. S. Smirnova, Chem. Abstr. 106:143701x (1987). Siggia, S., Ind. Eng. Chem., Anal. Ed. 19:872 (1947). Tanner and Brown, J. Inst. Petrol. 32:341 (1946). Wagner, C. D., R. H. Smith, and E. D. Peters, Ind. Eng. Chem., Anal. Ed. 19:982 (1947). Wagner, C. D., H. L. Clever, and E. D. Peters, Ind. Eng. Chem., Anal. Ed. 19:980 (1947). Wagner, C. D., R. H. Smith, and E. D. Peters, Ind. Eng. Chem., Anal. Ed. 19:976 (1947). Young, C. A., R. R. Vogt, and J. A. Nieuwland, Ind. Eng. Chem., Anal. Ed. 8:198 (1936).

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6, 9 6 2 10

3

1

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.30

SECTION NINETEEN

Peracids in the presence of a large excess of hydrogen peroxide react selectively with p-tolyl methyl sulfide to produce p-tolyl methyl sulfoxide, which is determined by gas chromatography of the sulfoxide or the unreacted sulfide.62 The interaction of peroxides with amines forms the basis of peroxide analysis in several methods. Benzoyl peroxide can be determined by measuring the chemiluminescence arising from reaction with triethylamin.63 Peroxides derived from cyclohexanone were determined colorimetrically by reaction of the peroxide with N,N-dimethyl-p-phenylenediamine.64 Organic peracids in the presence of hydrogen peroxide were determined by measuring the absorbance of the reaction of the peroxide with p-phenetidine in KH2PO4.65

19.6.4 Quinones Quinones are moderately strong oxidizing agents (E1/2 = 0.7 V for quinone itself). About 100 mg of the sample is dissolved in 20 mL of ethanol. An ethanolic solution of 2.5% KI and 2.5M in HCl is added. The iodine formed is titrated with 0.1N Na2S2O3. Good results can also be obtained with stronger oxidants as titrants. Systems include cerium(IV), vanadium(V), dichromate(VI), and hexacyanoferrate(III) (with Zn). A critical summary of known procedures has been published.66

19.7 METHODS FOR THE DETERMINATION OF FUNCTIONAL GROUPS CONTAINING NITROGEN AND OXYGEN 19.7.1 Determination of Nitrates, Nitro, and Nitroso Compounds Using 3 to 6 mg of an aromatic nitro compound, the reduction is carried out at room temperature with 0.04N titanium(III) chloride (or sulfate) in a medium buffered with sodium citrate. Use titanium sulfate if dealing with easily chlorinated compounds. By performing the reaction in 12M HCl, nitroso compounds (except N-nitrosoamines) can be determined selectively in the presence of nitro compounds.67 An alternative procedure involves adding excess titanium(III) and then back-titrating the excess with iron(III) ions using thiocyanate as indicator. Primary nitroalkanes can be determined by reacting with nitrous acid. The formed nitro acids are then titrated with NaOH solution.68 Polarographic half-wave potentials for many nitro and nitroso compounds are given in Table 14.21. N-Nitrosoamines can be detected down to 3 × 10−8M using differential pulse voltammetry.69 Half-wave potentials become more negative as the organic content of the solvent is increased. A coulometric method using controlled-potential electroreduction will determine nitro and nitroso compounds in MeOH–LiCl solution.70 Aromatic nitroso compounds dissolved in ethanolic HCl can be titrated with SnCl2 in glycerol to an amperometric, potentiometric, or visual end point without interference by nitro compounds.71 The nitro group can be reduced with Fe(II) in a direct titrimetric procedure using alkaline sorbitol media with detection of the end point by potentiometric or amperometric means.72

62

F. DiFuria et al., Analyst 109:985 (1984). J. R. Bowyer and S. R. Spurlin, Anal. Chim. Acta 192:289 (1987). 64 E. M. Sevast’yanova and Z. S. Smirnova, Chem. Abstr. 106:143701x (1987). 65 N. E. Blazheevskii and V. K. Zinchuk, Chem. Abstr. 105:17620m (1986). 66 U. A. Th. Brinkman and H. A. M. Snelders, Talanta 11:47 (1964). 67 T. S. Ma and J. V. Early, Mikrochim. Acta 1959:129. 68 C. A. Reynolds and D. C. Underwood, Anal. Chem. 40:1983 (1968). 69 R. Samuelsson, Anal. Chim. Acta 108:213 (1979). 70 J. M. Kruse, Anal. Chem. 31:1854 (1959). 71 E. Ruzicka, M. Paleskova, and J. A. Jilek, Collect. Czech, Chem. Commun. 45:1677 (1980). 72 B. Velikov, J. Dolezal, and J. Zyka, Anal. Chim. Acta 94:149 (1977). 63

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19.31

DETERMINATION OF AMINES AND AMINE SALTS Amines with ionization constants equal to or greater than approximately 10–9 can be satisfactorily titrated in water or alcohol–water mixtures using either an indicator or a potentiometric procedure. Since aqueous acidimetric procedures are accurate and precise and usually require no special equipment, many are still used for the quality control of commercially available materials. However, these methods are much more limited in scope than are nonaqueous techniques. For example, a great number of amines that are water-insoluble or that are too weakly basic to be titrated in aqueous solution react as strong bases toward perchloric acid in acetic acid medium. In addition, mixtures of two or more amines can frequently be analyzed by differentiating titrations in water-free solvents. Tertiary amines can be determined in the presence of primary and secondary amines after acetylation, and secondary amines can be determined in the presence of primary amines by first treating the mixture with an aldehyde. Acid–base titrations in nonaqueous media are discussed in Sec. 4.3. The reader is referred to this material for the various organic solvent systems, the preparation and standardization of titrants, and procedures for selected titrations in nonaqueous media (Table 4.17). The leveling effect of glacial acetic acid for all types of bases has made this solvent an exceedingly useful reagent in nonaqueous titrimetry. However, this property is also a disadvantage in that it prevents the individual titration of two amines of widely different basic strengths. Differentiating titrations of certain amine mixtures are possible by the proper choice of solvents. Chloroform and acetonitrile have been suggested for this purpose, with perchloric acid in 1,4-dioxane as the titrant. This technique permits the analysis of binary mixtures of some aromatic and aliphatic amines as well as the determination of several aromatic amine mixtures. The latter also can be titrated with perchloric acid in acetic acid, since aromatic amines are usually too weak to be leveled by acetic acid. The following procedures can be used for these determinations. Procedure A: Dissolve 0.6 to 1.0 meq of sample (total amines) in 20 mL of acetonitrile, and titrate potentiometrically with 0.1M HClO4 in 1,4-dioxane. Perform a blank determination on 20 mL of each lot of solvent. Procedure B: Use the same sample size and solvent volume as in procedure A. Add six drops of eosin Y indicator and titrate with 0.1M HClO4 to a pale-yellow end point. Add two drops of methyl violet and 20 mL of HOAc and continue the titration until a blue-green end point is reached. Determine a blank by the method given in procedure A. Procedure C: Follow procedure A, but use 0.1M HClO4 in HOAc as titrant. Determine a blank on 20 mL of acetonitrile, using the same titrant. Subtract the blank from the first end point only. Table 19.13 lists a number of amine mixtures that have been successfully determined by these procedures. The titration curves of 55 amines reported by Hall73 are helpful in predicting whether other amine mixtures can be successfully titrated.

19.8.1 Determination of Primary Amines Primary amines react with salicylaldehyde to form Schiff bases, which are weaker bases than the parent compound. Salicylaldehyde does not react with tertiary amines and generally does not affect the basicity of secondary amines. This permits the determination of primary amines in the presence of other amines and also provides a procedure for the analysis of secondary plus tertiary amines in the presence of primary amines. Procedure: Pipette 25 mL of CHCl3 and 5 mL of salicylaldehyde into a glass-stoppered flask. Add 4 to 6 drops of bromocresol green indicator and weight into the flask an amount of amine mixture containing not more than 12.5 meq of primary and 12.5 meq of secondary plus tertiary amines. If the solution becomes turbid, add enough 1,4-dioxane to effect solution. After 15 min, titrate with 0.5M HClO4 in 1,4-dioxane just to the disappearance of the green color. Record the volume of titrant 73

N. F. Hall, J. Am. Chem. Soc. 52:5115 (1930).

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SECTION NINETEEN

TABLE 19.13 Typical Amine Mixtures That Can Be Analyzed by Differentiating Titrations in Nonaqueous Solvents Components of mixtures

pKb of amine in H2O

Procedure

Dibutylamine Pyridine

2.81 8.85

A

2-Phenylethylamine Aniline

4.17 9.42

B

Pyridine Caffeine

8.85 13.39

C

Aniline o-Chloroaniline

9.42 11.32

C

Aniline Sulfathiazole

9.42 11.64

C

used. Add 75 mL of 1,4-dioxane, 8 to 10 drops of congo red indicator, and titrate with the HClO4 solution to the appearance of a pure green color. The volume of titrant required for the first end point is a measure of the secondary plus tertiary amine content of the sample. The volume required for the second end point is a measure of the amount of primary amine.74 The method cannot be used for the analysis of mixtures of aromatic primary and secondary amines or of mixtures of primary aliphatic and primary aromatic amines. Morpholine, secondary alcohol amines, and certain polyethylene amines (such as diethylenetriamine and triethylenetetramine but not ethylenediamine) interfere with the determination. Ammonia does not react quantitatively and, if present, must be separated before analysis. In general, the method cannot be used for the determination of compounds containing both primary and secondary or tertiary amine groups in the same molecule. Primary aromatic amines can be diazotized with a known excess of sodium nitrite solution, and the excess back-titrated with p-nitroaniline solution. The method has been modified by using 4,4-sulfanilic dianiline as the reagent, the indicator being diphenylamine. On back-titration with 0.1N NaNO2 solution the color changes from red to yellow sharply at the end point.75 19.8.2 Determination of Secondary Amines Primary and secondary amines reach with carbon disulfide to form dithiocarbamic acids. These acids can be titrated quantitatively with a standard base without interference from ammonia and tertiary amines. Imines, formed by the reaction of primary amines with an aldehyde, do not react with carbon disulfide. Secondary aliphatio amines do not react with 2-ethylhexaldehyde and can be converted to dithiocarbamic acids in the presence of imines.76 Procedure: Pipette 10 mL of 2-ethylhexaldehyde into a flask and add 50 mL of 2-propanol. Weigh an amount of sample containing no more than 13 meq of secondary amine. After 5 min at room temperature, add additional solvent and cool the contents of the system to –10°C in a suitable bath (but not dry ice–acetone). Remove the flask from the cooling bath; add 5 mL of CS2 with a pipette and 5 to 6 drops of phenolphthalein indicator. Titrate immediately with 0.5M NaOH to the first definite pink color that persists for 1 min. During the titration keep the flask in a bath of crushed ice and MeOH. Aromatic amines and highly branched aliphatic amines cannot be determined by this method. 2Ethylhexaldehyde does not react quantitatively with primary alcohol amines, aromatic amines, highly 74 F. E. Critchfield and J. B. Johnson, Anal. Chem. 29:957 (1957); C. D. Wagner, R. H. Brown, and E. D. Peters, J. Am. Chem. Soc. 69:2611 (1947). 75 E. Szekely, A. Brande, and M. Flitman, Talanta 19:1429 (1972). 76 F. E. Critchfield and J. B. Johnson, Anal. Chem. 29:957 (1957).

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branched primary amines such as tert-butylamine and isopropylamine, and polyamines. These amines interfere in the determination, and so do materials that are acidic or basic under the titration conditions used. 19.8.3 Determination of Tertiary Amines The direct determination of tertiary amines involves acetylation of primary and secondary amines and NH3 with acetic anhydride, and then potentiometric titration of the unreacted tertiary amine with HClO4 in HOAc. The method is generally applicable to all aliphatic amines except certain sterically hindered secondary amines,77 and to aromatic amine mixtures.78 19.8.4 Determination of Primary Plus Secondary Amines Primary and secondary amines may be determined by acetylation and measurement of the excess acetic anhydride or by nonaqueous titration of the total amine content before and after acetylation. Ammonia, if present, interferes seriously in both procedures and should be removed before analysis. A specific method has been developed for the direct determination of the total primary and secondary amine content in the presence of NH3 and tertiary amines. An excess of CS2 is reacted with the primary or secondary amine in 2-propanol, alone or mixed with pyridine. The dithiocarbamic acid formed is titrated with a standard base using phenolphthalein as indicator.79 This procedure can be combined with a total base and tertiary amine determination to obtain, indirectly, the NH3 present in an amine–NH3 mixture. Interference with the foregoing procedure include compounds acidic or basic to phenolphthalein. However, suitable corrections for these materials can be made before addition of CS2. Aromatic amines, tert-butylamine, and diisopropylamine do not react quantitatively with the reagent and interfere in the analysis. 19.8.5 Determination of Secondary Plus Tertiary Amines The reaction of salicylaldehyde with an aliphatic primary amine is not only useful for the determination of primary amines, but was also proposed primarily for the determination of secondary plus tertiary amines. The azomethane formed in the reaction is a much weaker base than the secondary amine, and the total secondary and tertiary amine content of the reaction mixture may be titrated potentiometrically in a nonaqueous solvent. Neither NH3 nor H2O, if present alone in the amine mixture, interferes in the titration. However, if both NH3 and H2O are present in large amounts, NH3 must be removed. Procedure: Into a tall-form beaker containing 5 mL salicylaldehyde and 80 mL MeOH, weigh about 3.5 meq of sample. Cover the beaker, mix the contents thoroughly, and allow to stand at room temperature for 30 min. Titrate the mixture potentiometrically to the first end point with 0.5M HCl in 2-propanol.80 19.8.6 Determination of Primary, Secondary, and Tertiary Amines in Mixtures The tertiary amine is determined by the direct addition of acetic anhydride to a weighed sample, solution of the reaction mixture in 1 : 1 ethylene glycol–2-propanol, and finally titration with HCl dissolved in the same solvent. The primary amine in the mixture is determined by a total base titration before and after the addition of salicylaldehyde. The difference between the two titrations is a measure of the primary amine content. The secondary amine is determined from the titration value obtained after the addition of salicylaldehyde. This value is the tertiary plus secondary amine content. The amount of secondary amine is computed by subtracting the tertiary amine content from the tertiary plus secondary amine value.81 77

C. D. Wagner, R. H. Brown, and E. D. Peters, J. Am. Chem. Soc. 69:2609 (1947). S. Siggia, J. G. Hanna, and I. R. Kervenski, Anal. Chem. 22:1295 (1950). F. E. Critchfield and J. B. Johnson, Anal. Chem. 28:430 (1956). 80 C. D. Wagner, R. H. Brown, and E. D. Peters, J. Am. Chem. Soc. 69:2611 (1947). 81 S. Siggia, J. G. Hanna, and I. R. Kervenski, Anal. Chem. 22:1295 (1950). 78 79

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SECTION NINETEEN

N,N-Di-(2-hydroxyethyl)aniline cannot be determined by the tertiary amine procedure. Diphenylamine and triphenylamine also cannot be determined because these compounds are too weakly basic to be titrated in this nonaqueous system.

19.9 DETERMINATION OF AMINE AND QUATERNARY AMMONIUM SALTS The method described in Sec. 19.8.1 for the determination of total amines can also be used to determine many amine salts. Salts of organic bases and most acids (other than halogen and sulfonic) can be titrated directly with HClO4 provided the salt is soluble in the selected solvent. Halogen acid salts of organic bases can be titrated in HOAc after adding Hg(OAc)2 to the sample–solvent mixture due to the formation of slightly dissociated mercury halide salt and an equivalent amount of free acetate ion. Amine sulfates and nitrates can be titrated without the addition of Hg(OAc)2; the sulfate end point is reached when the sulfate is neutralized to hydrogen sulfate ion.82

19.10

DETERMINATION OF AMINO ACIDS A number of amino acids can be titrated in a differentiating solvent in which the basicity of the amino group is enhanced while the acidity of the carboxyl group is decreased. Amino acids can be titrated in glacial acetic acid solution with HClO4 in the presence of crystal violet indicator. Often it is better to dissolve the amino acid in a known excess of HClO4 and to back-titrate the excess with NaOAc dissolved in glacial acetic acid. If the amino group is masked with formaldehyde, the carboxyl group can be titrated with NaOH using phenolphthalein as indicator. An almost specific reagent for amino acids is ninhydrin, which reacts with all compounds that contain a free amino group with the formation of a colorless hydrindantin. At pH 3 to 4, an excess of ninhydrin reacts with hydrindantin and ammonia to form a blue product. The colored product is a useful spot test when developing thin-layer chromatographic (TLC) plates and can be used for the spectrophotometric determination of amino acids.

19.11 METHODS FOR THE DETERMINATION OF COMPOUNDS CONTAINING OTHER NITROGEN-BASED FUNCTIONAL GROUPS 19.11.1 Determination of Amides Amides have been determined by alkaline and acid hydrolysis, by hydroxamic acid formation and colorimetric estimation, by reaction with 3,5-dinitrobenzoyl chloride and titration of the excess benzoyl chloride, by several reductometric procedures, and by nonspecific total nitrogen procedures. Acetamide and a few other amides can be titrated as bases in nonaqueous solvents.83 However, it remained for Wimer84 to demonstrate the usefulness of these observations and to develop a general method for the titration of amides and acetylated and formylated amines. 19.11.1.1 Determination of Amides by Potentiometric Titration in Acetic Anhydride. Procedure: Weigh 6 to 9 mmol of sample into a 100-mL volumetric flask; dissolve in and dilute to volume with acetic anhydride. Transfer a 10-mL aliquot to a tall-form beaker, add 100 mL acetic anhydride and 82

C. W. Pifer and E. G. Wollish, Anal. Chem. 24:300 (1952). N. F. Hall and T. H. Werner, J. Am. Chem. Soc. 50:2367 (1928); J. S. Fritz and M. O. Fulda, Anal. Chem. 25:1837 (1953); A. F. Gremillion, ibid. 27:133 (1955). 84 C. D. Wimer, Anal. Chem. 30:77 (1958); see also T. Higuchi et al., ibid. 34:400 (1962). 83

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19.35

titrate potentiometrically with 0.1M HClO4 in acetic anhydride using a glass–calomel electrode pair. Replace the aqueous solution in the sleeve-type calomel with 0.1M LiClO4 in acetic anhydride. N-Phenyl- or a-phenyl-substituted amides, trifluoromethylformamide, cyanamide, and certain tertiary amides are too weakly basic to be titrated by this procedure. Unsaturated amides in which the double bond is conjugated with the carbonyl group appear to react with the anhydride but do not yield stoichiometric results. Diamides of dibasic acids, except malonamide and tetrasubstituted phthalamides, are too insoluble in acetic anhydride to be determined by this method. 19.11.1.2 Determination of Unsubstituted Amides by the Dinitrobenzoyl Chloride–Pyridine Method. The method of Mitchell and Ashby85 is based upon the reaction of 3,5-dinitrobenzoyl chloride with amides in pyridine followed by an acidimetric determination of the excess benzoyl chloride. Procedure: Transfer about 10 meq of accurately weighed sample to a 250-mL glass-stoppered flask containing 15 mL of 3,5-dinitrobenzoyl chloride and 5 mL of pyridine. Place the flask together with a blank in a water bath maintained at 60°C for 30 min. (For amides of dibasic acids use a reaction time of 1 h at 70°C.) Remove the flasks and cool in an ice-water bath. Add 2 mL of methanol to each flask, wait 5 min, and add 25 mL more. If the solutions are highly colored, add ethyl-bis(2,4dinitrophenyl) acetate indicator and titrate both sample and blank with 0.5M sodium methoxide. Phenolphthalein may be used as indicator if the solutions are only lightly colored. The net increase in acidity of the sample over the blank, after correction for free acid and water present in the sample, is equivalent to the amide content. Generally, secondary and tertiary amides do not interfere. Neither do amines and alcohols, beyond consuming some of the reagent. However, free acid and water do interfere. If present, these materials should be determined in the original sample and corrections applied to the results of the amide determination. 19.11.2 Methods for the Determination of Azo Compounds 19.11.2.1 Reduction with Titanium(III) Salts. For most azo compounds, four equivalents of titanium(III) are required. However, some chlorinated azobenzenes have been found to require only two equivalents of titanium(III). Furthermore, there is no one procedure that is completely satisfactory for all azo compounds. However, the following one can be applied to most samples. Procedure: Deaerate the reaction flask by adding several small pieces of dry ice or by passing CO2 into it for 5 min. Continue to pass the inert gas through the flask, and transfer to it a weight sample or an aliquot of a solution of the sample that will require about 2.5 meq of titrant. Dissolve the sample in 25 mL ethanol or glacial acetic acid. Add 25 mL 1 : 1 HCl and 50 mL 0.2M titanium(III) salt, and reflux for 5 to 10 min. With the CO2 still bubbling through the solution, cool the flask and contents to about room temperature, and titrate with 0.15M iron(III) ammonium sulfate solution until the purple color is very faint, add 10 mL 10% NH4SCN solution, and continue the titration until the pink color persists for 1 min. Run a blank. Nitro, nitroso, and many other compounds are also reduced by titanium(III) under the same conditions. Oxygen is a troublesome interferant and every precaution must be taken to keep it excluded. 19.11.2.2 By Reduction with Copper. A simple indirect gravimetric method for the determination of nitro, nitroso, and azo compounds is based upon the loss in weight of copper during reduction of the compound to the amine.86 Procedure: Add 20 mL of ethanol and 20 mL 3M H2SO4 to the reaction flask and deaerate the mixture by adding small pieces of dry ice. Weigh a sample of the azo compound that will cause 0.5 to 1.0 g Cu to dissolve during the reaction, and transfer it to the reaction flask. Add 6 to 15 g Cu and reflux the mixture for 2.5 h while passing a slow stream of CO2 through the condenser. Rinse rapidly 5 to 6 times with deaerated water, then 3 times with deaerated acetone. Dry the Cu under vacuum and weight. Run a blank. 85 86

J. Mitchell, Jr., and Ashby, J. Am. Chem. Soc. 67:161 (1945). Juvet, Twickler, and Afremow, Anal. Chim. Acta 22:87 (1960).

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.36

SECTION NINETEEN

The quantitative reduction of nitro and nitroso compounds also occurs. Halides interfere, as do oxidizing agents such as nitrate ion and iron(III). Aldehydes, ketones, carboxylic acids, phenols, alcohols, amines, and nitriles do not interfere.

19.11.3 Methods for the Determination of Hydrazines Procedures for the determination of hydrazine and its derivatives are based on their behavior as either weak bases or reducing agents. The available methods involve titration with acids or oxidation of the hydrazine group to nitrogen. Among the oxidants used are I2, Br2, ferricyanide, permanganate, bromate, iodate, cerium(IV), dichromate, periodate, vanadate, and chloramine T. In the presence of other bases an acidimetric procedure will not indicate the true hydrazine content. However, in combination with an oxidimetric method a measure can be obtained of both total base and hydrazine. Either an aqueous or a nonaqueous acidimetric procedure may be used for the titration of these bases. Water-insoluble hydrazines are best determined by titration with HClO4 in HOAc. In the presence of ≥4M HCl, hydrazine and monosubstituted hydrazines can be titrated directly with KIO3 to the ICl equivalence point. In the presence of H2SO4, the reduction of iodate proceeds to I2. Both of these procedures have been used for the determination of monosubstituted hydrazines and their hydrazones and acyl derivatives.87 Other derivatives of hydrazine, such as hydrazones, guanidines, triazoles, and tetrazoles, also react with iodate ion and interfere in the titration. Polysubstituted hydrazine undergo reproducible oxidations that vary from a two- to six-electron change per hydrazine group.

19.11.4 Determination of Primary Hydrazides Primary hydrazides are sufficiently basic to be titrated with HClO4 in HOAc. The procedures described for the determination of total primary, secondary, and tertiary amines is applicable to primary hydrazides.

19.11.5 Determination of Oxazolines Oxazolines can be titrated potentiometrically in glacial acetic acid with HClO4 if they behave as a strong base in HOAc.88

19.11.6 Determination of Isocyanates, Isothiocyanates, and Isocyanides Both aliphatic and aromatic isocyanates and isothiocyanates react quantitatively with amines (e.g., 0.3M butylamine in 1,4-dioxane) in about 45 min at room temperature to form a substituted carbamide (urea) or thiocarbamide (thiourea). Since the ureas are neutral compounds, the excess of added amine can be determined acidimetrically with 0.1N H2SO4 using methyl red as indicator. This provides the basis for the method developed by Siggia and Hanna,89 which was adapted to a micro and semimicro scale by Karten and Ma.90 Since water and alcohols also react with isothiocyanates and isocyanates, a nonhydroxylic solvent, 1,4-dioxane, is used as the reaction medium. Isocyanides (R −− N == C == ) are reacted with a known excess of HSCN to form a substituted triazine dithione. The excess of HSCN is titrated with triethylamine dissolved in ethyl acetate using a methanolic solution of methylene blue–neutral red mixed indicator.91 87

W. R. McBride, R. A. Henry, and S. Skolnik, Anal. Chem. 25:1042 (1953). P. C. Markunas and J. A. Riddick, Anal. Chem. 23:337 (1951). S. Siggia and J. G. Hanna, Ind. Eng. Chem., Anal. Ed. 20:1084 (1948). 90 B. S. Karten and T. S. Ma, Microchem. J. 3:507 (1959). 91 A. S. Arora, E. Hinrichs, and I. Ugi, Z. Anal. Chem. 269:9 (1974). 88 89

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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19.37

19.11.7 Determination of vic-Dioximes In the presence of Hg(OAc)2, I2 dehydrogenates vic-dioximes to furoxanes. The basis of this reaction forms a simple titrimetric method for the determination of aliphatic and alicyclic vic-dioximes.92 The reaction proceeds rapidly in chloroform medium, and back-titration of the excess I2 yields the amount of dioxime originally present. Materials easily oxidized by iodine, easily halogenated compounds, primary amines, ketoximes, aldoximes, and monoximes of vic-diketones interfere in the titration.

19.11.8 Determination of Hydroxylamine Hydroxylamine can be titrated with 0.1M HCl using methyl orange or bromophenol blue as indicator. Its salts can be titrated with 0.1M NaOH in the presence of phenolphthalein. However, a more accurate procedure uses the reduction of iron(III) ammonium sulfate by hydroxylamine in HCl medium; the iron(II) formed is titrated with KMnO4.

19.12

DETERMINATION OF NITRILES The determination of nitriles is possible by hydrolysis with a known excess of water in glacial acetic acid solution in the presence of BF3 catalyst. The remaining water is titrated with Karl Fischer solution.93 Some a ,b-unsaturated nitriles react with mercaptans; an excess is added and the excess is titrated by an iodometric or argentimetric method.94

19.13 METHODS FOR THE DETERMINATION OF PHOSPHORUS-BASED FUNCTIONS This subsection includes tables dealing with both the determination of phosphorus-based functional groups in organic molecules and the determination of phosphorus-based inorganic groups. Table 19.14 summarizes methods for the determination of phosphorus in the elemental form and in its inorganic compounds and ions and also includes some methods for the separation of these. Table 19.15 summarizes methods for the determination of organic phosphorus-based functional groups. References are given at the end of Table 19.15. Some additional methods are briefly discussed below. The determination of dibutylphosphoric acid in the presence of monobutylphosphoric acid and tributyl phosphate depends on its separation from the mixture by TLC. The separated analyte is mixed with a Th–Thoron complex and its concentration measured by the extent to which it reduces the color the complex.95 The analysis of diethyl dithiophosphate in aqueous solution is based on its conversion to the lead salt followed by an extraction of the salt into chloroform at pH 4 to 5. The concentration of the diethyl dithiophosphate is proportional to the UV absorbance of the CHCl3 extract.96 A technique for the determination of phosphonates in water involving supercritical fluid extraction with CO2 and supercritical fluid chromatography has been described.97 92

Banks and Richard, Talanta 2:235 (1959). D. H. Whitehurst and J. B. Johnson, Anal. Chem. 30:1332 (1958). W. D. Beesing et al., Anal. Chem. 21:1073 (1949). 95 S. C. Tripathi, Analyst 111:239 (1986). 96 H. Socio, P. Garrigues, and M. Ewald, Analusis 14:344 (1986). 97 J. Hedrick and L. T. Taylor, Anal. Chem. 61:1986 (1989). 93 94

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Ultraviolet

13

Oxidation with HNO3 + HClO4 and determine as orthophosphate. Dissolve in benzene, treat with Cu(NO3)2, oxidize with HNO3 + HClO4, and determine as orthophosphate. Extract from aqueous emulsion with benzene and measure absorbance at 290 nm against benzene reference.

Phosphorus, red

20

7

9 6 10

See Table 4.4

18

15

Phosphorus, white or yellow

Reaction with 0.5% 2-mercaptobenzimidazole in pyridine.

Treatment with excess KI in acid solution and titration of liberated I2, or titration with KMnO4 or Ce(IV). See orthophosphate above. See orthophosphate above. In aqueous solution P −− O −− O −− P absorbs at 230 nm; P −− O −− O −− H absorbs at 230 and also at 290 nm.

Precipitation with (NH4)2MoO4 from HNO3 solution; dissolve precipitate in excess standard NaOH, and titrate excess alkali with standard acid. Precipitate as MgNH4PO4, ignite, and weigh as Mg2P2O7. As reduced molybdate, molybdovanadate [9], or acetone molybdate [6] after precipitation of ammonium phosphomolybdate. Adsorption on Dowex 1-X8 resin, elution with KCl solution. Ascending technique, S&S No. 589. Solvent: 25 mL 20% (w/v) aqueous Cl3CCOOH, 5 mL H2O, 0.25 mL concentrated aqueous NH3 and 70 mL [5] or 2-propanol [14,23]. Spray solution: 1% (w/v) (NH4)2MoO4 in 0.12M HCl–0.60M HClO4. Develop blue bands under ultraviolet light or by spraying with 0.1% (w/v) SnCl2 in 0.1M HCl. In KBr pellet, PO 3− 4 shows no band that can be measured in presence of pyro-, trimeta-, and tri-polyphosphate.

Oxidation to P(III) with cerium(IV) or KBrO3; or oxidation to P(V) (see phosphite below). See phosphite below. Formation of blue color with molybdate iron. See phosphite below.

15

Reference

3, 11

Volumetric Paper chromatography

Procedure Oxidation with cerium(IV) in hot acid solution and determination of resulting orthophosphate (see below).

Titration of phosphate with KMnO4, I2, Ce(IV), or KBrO3. Ascending technique. S&S No. 589 paper. Solvent: 70 mL acetone +25 mL H2O + 5 mL concentrated aqueous NH3. Spray solution: see orthophosphate above.

Colorimetric

Phosphite, HPO 2− 3

Ion exchange Paper chromatography Ultraviolet

Volumetric

Infrared

Ion exchange Paper chromatography

Gravimetric Colorimetric

Volumetric

Phosphine, PH3

Peroxophosphates: mono- and di-

Orthophosphate, PO 3− 4

Gravimetric Colorimetric Paper chromatography

Volumetric

Phosphinate (hypophosphite) HPH2O2

Technique Volumetric

Hypophosphate, diphosphate(IV) [(HO)2OP]2

Substance determined

For references, see list below Table 19.15.

TABLE 19.14 Methods for the Determination of Inorganic Phosphorus Groups

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.38

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Volumetric Infrared Volumetric

Phosphorus trichloride, PCl3

Volumetric

Pyrophosphate, P2O 74−

Tripolyphosphate

Separation

Trimetaphosphate

Ion exchange Paper chromatography Infrared

Volumetric Colorimetric

Ion exchange Paper chromatography Infrared

Ion exchange Paper chromatography

Tetrametaphosphate

Colorimetric Ion exchange Paper chromatography Infrared

Paper chromatography

Infrared

Paper chromatography

Volumetric

Polyphosphates

P–S compounds, P4S3, P4S7, P4S10

Infrared

1 mol PCl5 plus solid KI gives 1 mol I2, P −− Cl band; see Table 7.17.

Infrared

Phosphorus pentachloride, PCl5

Like volumetric method for pyrophosphate. Measure absorbance at 455 nm of cobalt ammine solution before and after precipitation of tripolyphosphate at pH 3.6. See orthophosphate above. See orthophosphate above. KBr pellet. Phase I: 752 and 707 cm–1; phase II: 1015, 735, and 662 cm–1.

Precipitation with Ba(II); no other inorganic phosphate gives a Ba salt soluble above pH 9.0. See orthophosphate above. See orthophosphate above. KBr pellet, 772 cm–1.

See orthophosphate above. See orthophosphate above.

Adjust pH to 3.8, add excess ZnSO4, and titrate liberated H+ with standard alkali to pH 3.8. Indirect by bleaching of Fe(II) 1,10-phenanthroline color. See orthophosphate above. See orthophosphate above. KBr pellet, 1031 and 735 cm–1.

See orthophosphate above.

Dissolve in hot NaOH + H2O2, acidify, boiling with Br2 (giving H3PO4 + H2SO4), and determine products. Dissolve in CS2. Ascending technique using S&S No. 589 paper. Solvent: CS2; spray solution: 10% aqueous AgNO3. KBr pellet; see Table 7.17 for bands 714–625 cm−1.

After reaction with large excess of H2O (giving HCl + HPO(OH)2), determine as phosphite. P −− Cl band; see Table 7.17.

After reaction with large excess of H2O (giving HCl + H3PO4), determine as orthophosphate. Measure P −− Cl or P == O bands; Table 7.17.

Phosphorus chloride oxide, PClO

2 21

8

1

19

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

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Phosphonic compounds ROPH(O)OH, RP(O)(OR)(OH), RP(O)(OR)2, R may be Ar

Phosphites (RO)3P, (RO)2PH(== O), ROPH(O)OH

Phosphinous (P(III)) compounds, R2POH, R2POR (R may be Ar)

Phosphine oxides, R2P == O, As2P == O

Phosphines R3P, Ar3P, R2PH, Ar2PH, RPH2, ArPH2

Phosphates ROP(O)(OH)2 (RO)2P(O)OH (RO)3P == O

Substance determined

19.40

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Paper chromatography

Volumetric

Volumetric, gravimetric, colorimetric, infrared, ultraviolet

Gravimetric Infrared Ultraviolet

Volumetric

Infrared Ultraviolet

Infrared Ultraviolet

Volumetric

Gravimetric Paper chromatography Infrared Ultraviolet

Volumetric

Technique

Procedure

RP(O)(OR)OH + NaOH → RP(O)(OR)ONa + H2O; 1 equivalence point in acetone–H2O (9 :1, v/v). RP(O)(OH)2 + 2 NaOH → RP(O)(ONa)2 + 2 H2O; 2 equivalence points under same conditions. Ascending technique, S&S No. 589 paper. Solvent: 75 mL acetone +25 mL H2O + 5 mL concentrated NH3. For spray solution, see orthophosphate, Table 19.14. See phosphine compounds above.

See phosphonic compounds and phosphonous compounds.

Oxidize to P(V) by shaking 15 min with excess KBrO3–KBr in HOAc–HCl (25 : 10) volution; determine excess KBrO3 with KI and Na2S2O3. Add excess HgCl2 to hot acid solution of sample, dry and weigh precipitated Hg2Cl2. P −− O −− C and P −− O −− Ar bands; see Table 7.17. P −− O −− Ar at 260 to 280 nm; Ar2P at 220–240 nm.

P== O and Ar −− P bands. Ar3P, 220–240 nm.

Titrate in HOAc, using 1% methyl violet in HOAc as indicator. 1 mol R3P or Ar3P reacts with 2 mol HClO4; 1 mol R2PH or Ar2PH reacts with 1 mol HClO4. To 0.3 g sample is 50 mL EtOH, add 15 mL CCl4, mix, let stand 10 min, add 10 mL 1M NaOH, acidify, and titrated liberated Cl– with standard AgNO3. R2PH + 3 NaOH + 2 CCl4 → R2P(O)ONa + H2O + 3 CHCl3 + NaCl [Ar2PH behaves similarly]. RPH2 + 3 CCl4 + 5 NaOH → RP(O)(ONa)2 + 2 H2O + 3 CHCl3 + 3 NaCl [ArPH2 behaves similarly]. P −− H and Ar −− P bands; see Table 7.17. Ar −− P, 260 to 270 nm; Ar2P and Ar3P, 220 to 240 nm.

(RO)2P(O)OH + NaOH → (RO)2POONa + H2O; 1 equivalence point in acetone-H2O (9 :1, v/v). ROP(O)(OH)2 + 2NaOH → ROP(O)(ONa)2 + 2 H2O; 2 equivalence points in acetone–H2O (9 :1, v/v). If R = Aryl, or if R contains >6 C atoms, 2nd equivalence point can be detected by adding 15 mL 10% (w/v) BaCl2 at pH 9.0 to release HCl. ROP(O)(OH)2 gives Ba Salt insoluble above pH 7.0. See phosphonic compounds below. See Table 7.17. P −− O −− Ar absorbs at 260–280 nm.

TABLE 19.15 Methods for the Determination of Organic Phosphorus Groups Reference

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

Volumetric

Phosphorus-sulfur compounds Thiono groups, Pv ==S

Infrared

Gravimetric

Volumetric

Infrared Ultraviolet

Volumetric

Infrared

Volumetric

Gravimetric Colorimetric Infrared Ultraviolet

Volumetric

Thiol groups, Pv—SR

Phosphorus–sulfur compounds Thio acids

Phosphorus–nitrogen compounds

Phosphorus–halogen compounds

Phosphonous compounds RPH(O)OH, RPH(O)OR, RP(OR)2, R may be Ar

Shake 15 min with excess standard KBrO3–KBr in 5% HCl giving sulfate ion; determine excess bromate with KI and Na2S2O3. Reflux 15 to 20 min with 55% HNO3 and determine sulfate formed by precipitation with BaCl2 and weighing of BaSO4. P == S, 714 to 625 cm–1.

Shake 15 min with excess standard KBrO3–KBr in 5% aqueous HCl giving RSO2; determine excess bromate with KI and Na2S2O3.

Pv −− SH + Ag+ → Pv −− SAg + H+. Titrate 0.3 g sample in 100 mL acetone +5 mL HOAc with 0.05M AgNO3 using glass–Ag electrode system; equivalence point is at 0.0 mV. Pv −− SH + I2 → Pv −− S −− S −− Pv + 2 H+ + 2 I–. Shake 15 min with excess of standard KIO3–KI in 5% aqueous HCl, back titrate with standard Na2S2O3. S −− H band at 2500 cm–1. 220–240 nm.

See P—N compounds, Table 19.14.

Reaction with peroxide in alkaline solution giving peroxophosphate; determine excess peroxide. Hydrolysis with aqueous NaOH in acetone solution giving NaX; determine product. See P −− Cl compounds, Table 19.14.

RPH(O)OH + NaOH → RPH(O)ONa + H2O; one equivalence point in acetone– H2O (9:1, v/v). RPH(O)OR + NaOH → RPH(O)ONa + H2O. Reaction occurs in EtOH containing excess NaOH; let stand 15 min for R, 30 min for Ar. RPH(O)OR + 2 NaOH + CCl4 → RP(O)(OR)ONa + H2O + CHCl3 + NaCl; see phosphines above for procedure. RP(OR)2 + H2O → RPH(O)OR + ROH. Reaction occurs in 95% EtOH containing 5 mL 0.1M HCl; let stand 30 min to R, 60 min for Ar. See phosphinous compound below. −− PH(O) group forms color with trinitrobenzene in alkaline medium. See Table 7.17 Ar −− P, Ar −− O −− P, 260–280 nm.

(Continued)

16

17

5

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.41

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Technique

Paper chromatography Infrared

Volumetric

Procedure Reaction with peroxide in alkaline solution giving peroxophosphate; determine excess peroxide. Hydrolyze to orthophosphate by refluxing with pyridine + water (9:1), and determine phosphate formed. See phosphonic compounds above. See Table 7.17.

R. N. Bell, Ind. Eng. Chem., Anal. Ed. 19:97 (1947). R. N. Bell, A. R. Wreath, and W. T. Culess, Anal. Chem. 24:1997 (1952). D. N. Bernhart, Anal. Chem. 26:1798 (1954). D. N. Bernhart and W. B. Chess, Anal. Chem. 31:1026 (1959). D. N. Bernhart and K. H. Rattenbury, Anal. Chem. 26:1765 (1956). D. N. Bernhart and A. R. Wreath, Anal. Chem. 27:440 (1955). D. E. C. Corbridge and E. J. Lowe, Anal. Chem. 27:1383 (1955). W. B. Chess and D. N. Bernhart, Anal. Chem. 30:111 (1958). A. Gee and V. R. Dietz, Anal. Chem. 25:1320 (1953). J. A. Grande and J. Beukenkamp, Anal. Chem. 28:1495 (1956). R. T. Jones and E. H. Swift, Anal. Chem. 25:1272 (1953). E. Karl-Kroupa, Anal. Chem. 28:1091 (1956). R. A. Keeler, C. J. Anderson, and D. Satriana, Anal. Chem. 26:933 (1954). I. M. Kolthoff, Rec. trav. chim. 46:350 (1927). T. Moeller and G. H. Quinty, Anal. Chem. 24:1354 (1952). S. Sass et al., Anal. Chem. 32:285 (1960). S. Sass and J. Cassidy, Anal. Chem. 28:1968 (1956). A. P. Scancillo, Anal. Chem. 26:411 (1954). M. J. Smith, Anal. Chem. 31:1023 (1959). Vasak, Chem. Listy 50:1116 (1956). H. J. Weiser, Jr., Anal. Chem. 28:477 (1956).

References for Tables 19.14 and 19.15

Pyrophosphates

Substance determined

TABLE 19.15 Methods for the Determination of Organic Phosphorus Groups (Continued)

16

Reference

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.42

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DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.43

An HPLC method has been developed for the analysis of inositol phosphates using a strong anionexchange column, which avoids the hydrolysis of the esters on the column.98 The conditions for the ion chromatographic separation of poly(methylenephosphonic acids) have been investigated.99

19.14 METHODS FOR THE DETERMINATION OF SULFUR-BASED FUNCTIONAL GROUPS A monograph in three volumes discusses the analysis of sulfur compounds.100 The infraredabsorption frequencies of sulfur compounds are given in Table 7.11 and the Raman frequencies in Table 7.30. 19.14.1 Determination of Mercaptans (Thiols) and Hydrogen Sulfide The determination of both H2S (Table 19.16) and thiols (the mercapto group) (Table 19.17) is often necessary in technical analysis. Both are easily oxidized. Both react with certain metal ions [silver, copper(II), and mercury(II)] with the formation of insoluble metal mercaptides or sulfides. An old and simple method is the oxidation with iodine as titrant added in known excess (because the reaction is slow in dilute solution), and the excess of iodine is then titrated with sodium thiosulfate. 98

R. A. Minear et al., Analyst 113:645 (1988). G. Tschaebunin, P. Fischer, and G. Schwedt, Z. Anal. Chem. 333:117 (1989). 100 J. H. Karchmer, Analytical Chemistry of Sulfur and Its Compounds, Interscience, New York, 1969; M. R. F. Ashworth, The Determination of Sulfur-Containing Groups, Academic, New York, 1972. 99

TABLE 19.16

Methods for the Determination of Hydrogen Sulfide

Technique

Procedure

Range

Potentiometric titration

Dissolve sample in a mixture of C6H6, MeOH, NaOAc, and aqueous NH3. Titrate with alcoholic AgNO3 using Ag2S ISE and SCE.* Dissolve sample in EtOH containing NaOAc (or, if water soluble, in aqueous NaOH + NH3), and titrate with AgNO3, using Ag2S ISE and glass electrodes. Break at ca. −0.6 V is H2S end point; 1 mol H2S is equivalent to 2 mol of Ag+.†

Micro

Volumetric

Absorb in acidic CdSO4 solution, dissolve CdS precipitate in excess standard I2 solution and back-titrate with Na2S2O3.*

Micro

Spectrophotometric

Absorb and precipitate H2S as ZnS; dissolve and react this with p-aminodimethylaniline + FeCl3 giving methylene blue; measure at 670 nm and compare with standards.‡

3–500 mg/mL

Interferences Free H2S, RSH, CN–

* J. H. Karchmer, Anal. Chem. 30:80 (1958). † M. W. Tamele, L. B. Ryland, and R. N. McCoy, Anal. Chem. 32:1007 (1960). ‡ J. K. Fogo and M. Popowsky, Anal. Chem. 21:732 (1949).

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Procedure and reference If free S is absent, RSH and H2S can be determined simultaneously by methods given in Table 19.16. Break at ca. –0.3 V is RSH end point; 1 mol of RSH is equivalent to 1 mol Ag+ [1,2]. If free S is present, remove H2S with acidic CdSO4 solution, then dissolve sample in a mixture of C6H6, MeOH, HOAc, and NaOAc, and titrate as above. Use total titer even though two breaks may appear [1]. Titrate with 0.01N Hg(ClO4)2 solution using a bromide ISE or an ISE with a HgS matrix [10]. Thiols in water and DMF solution are titrated with HgCl2 [15]. The thiol is precipitated as silver salt with alcoholic AgNO3. After isolation of the precipitate, it is converted to Ag+ and analyzed by FAAS [16]. Dissolve 5 to 50 mL sample in 100 mL acetone +5 mL supporting electrolyte (NH4NO3 + aqueous NH3), and titrate with aqueous AgNO3, using rotating Pt and calomel electrodes [3]. The RSH content of biological liquids is titrated with 0.002M AgNO3 dissolved in tris(hydroxymethyl)methylamine [6]. Add excess AgNO3 and back-titrate with standard NH4SCN, using iron(III) alum indicator. Titrate with AgNO3 in ammoniacal alcoholic medium, using NH4 dithizonate indicator [4]. Titrate with Hg(ClO4)2 [7] directly, or titrate the HNO3 liberated from a known excess of Hg(NO3)2 [8]. Dissolve sample in toluene and titrate with 0.05N p-tolylmercury chloride in presence of EtOH and KOH, using thiofluorescein as indicator. o-Hydroxymercurybenzoic acid is an alternate titrant [9]. Tritrate with CrO2O2 by direct enthalpimetry [11]. Add sample to CCl4 solution of Ag dithiozonate and measure green color at 615 nm [4]. Based on color developed when an acetic acid solution of the thiol is treated with p-aminodimethylaniline in presence of FeCl3 and K3Fe(CN)6 [13].

Technique

Potentiometric titration

Conductometric titration

Atomic absorption flame

Amperometric titration

Volumetric

Spectrophotometric

TABLE 19.17 Methods for the Determination of RSH Groups Range

Interferences

0.002–0.08 mmol

Sulfide

Sulfide

0.01–1 mmol

>3 m g/mL

H2S, Cl−

H2S, CN−, I−

See Table 19.16

See Table 19.16

Micro

0–400 mg/mL

Micro

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J. H. Karchmer, Anal. Chem. 30:80 (1958). M. W. Tamele, L. B. Ryland, and R. N. McCoy, Anal. Chem. 32:1007 (1960). M. D. Grimes et al., Anal. Chem. 27:152 (1955). R. K. Kunkel, J. E. Buckley, and G. Gorin, Anal. Chem. 13:1098 (1959). W. P. Hoogendonk and F. W. Porsche, Anal. Chem. 32:941 (1960). S. K. Bhattachaya, Nature 183:1327 (1959). D. C. Gregg, P. E. Bonffard, and R. Barton, Anal. Chem. 32:269 (1961). B. Saville, Analyst 86:29 (1961). M. Wronsky, Z. Anal. Chem. 206:352 (1964). W. Selig, Mikrochim. Acta 1973:453. M. Wronsky and A. S. Abbas, Analyst 111:1073 (1986). Y. Nishikawa and K. Kuwata, Anal. Chem. 57:1864 (1985). V. B. Dorogova and V. A. Khomutova, Chem. Abstr. 99:10196b (1983). H. Knof, R. L. Arge, and G. Albers, Anal. Chem. 48:2130 (1976). L. M. Doane and J. T. Stock, Anal. Chem. 50:1891 (1978). J. S. Marhevka and S. Siggia, Anal. Chem. 51:1259 (1979).

Extract RSH compound (C1 to C7) from sample with K isobutyrate solution, acidify, and reextract into isooctane. Analzye isooctane solution by low ionizing voltage (or CI technique) [5]. Negative ion MS used in hydrocarbon solvents for a direct determination [14].

Mass spectrometric

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Low-molecular-weight thiols determined by HPLC after derivatization with 7-chloro-4-nitrobenz-2,1,3-oxadiazole [12].

HPLC

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SECTION NINETEEN

An extensive review of methods for the thiol group is available.101 An automated potentiometric titration system has been developed for the determination of thiols and sulfides in petroleum systems.102 Thiols in the presence of sulfides and disulfides in lubricating oils have been determined by a potentiometric titration with silver ammoniate solution.103 19.14.2 Determination of Thioethers (R −− S −− R) and Disulfides (R −− S −− S −− R) Thioethers easily form sulfoxides, and less easily sulfones, upon oxidation with bromine. The sample is oxidized in acetic acid solution, containing hydrochloric acid, with a known excess of 0.02N KBrO3–KBr reagent, and, after a time for completion of the reaction, the excess of bromine is backtitrated by iodimetry. Table 19.18 gives methods for thioethers. Disulfides can be reduced to mercaptans with zinc amalgam or sodium borohydride (tetrahydridoborate); the mercaptan formed is determined by argentimetry or iodimetry. Other methods include reducing disulfides in methanolic solution with triphenylphosphine,104 and a mercurimetric titration after reduction with butyllithium.105

19.14.3 Determination of Thioketones No reliable specific methods for the determination of thioketones are known. The methods listed below are useful for the qualitative detection of these compounds.

Procedure and reference Add NaN3 + I2. Disappearance of free I2 indicates presence of monomeric thiones.106 Treat sample with H2O2 + N2H4. Thiones give sulfate ion on oxidation by peroxide and hydrazones (with H2S evolution) on reaction with hydrazine; apply tests for these products.107

Interferences Divalent sulfur compounds

19.14.4 Determination of Sulfoxides and Sulfones A review contains 147 references describing methods for the determination of sulfoxides and sulfones.108 Since they are weak bases, sulfoxides can be titrated in acetic anhydride with HClO4 dissolved in glacial acetic acid. Sulfoxides can also be reduced with strong reducing agents, usually with a period of standing in contact with an excess of the reducing agent [titanium(II) or tin(II)], and the excess reductant ascertained by titration. Methods for the determination of sulfoxides are given in Table 19.19. Only very strong oxidizing agents will transform sulfones into sulfates. Sulfones can be reduced to the sulfide with metallic reductants and some can be reduced with lithium aluminum hydride. Procedures for sulfones are given in Table 19.19. 101 M. R. F. Ashworth, Determination of Sulfur-Containing Groups, Vol. 2: Analytical Methods for Thiol Groups, Academic, London, 1976. 102 S. W. Batson, G. J. Moody, and J. D. R. Thomas, Analyst 111:3 (1986). 103 N. G. Tyshchenko et al., Chem. Abstr. 92:44273v (1980). 104 R. E. Humprey and J. M. Hawkins, Anal. Chem. 36:1812 (1964). 105 S. Veibel and M. Wronsky, Anal. Chem. 38:910 (1966). 106 F. Feigl and Docorse, Chem. Abstr. 38:2585 (1944). 107 Kambara, Okita, and Tajima, Chem. Abstr. 46:1795i (1952). 108 M. R. F. Ashworth, in S. Patal et al. eds., Chemistry of Sulfones and Sulfoxides, Wiley, Chichester, UK, 1988.

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19.47

TABLE 19.18 Methods for the Determination of R −− S −− R Groups Technique Spectrophotometric (for aliphatic and cyclic sulfides)

Chemical

Volumetric

Electrokinetic

1. 2. 3. 4. 5. 6. 7. 8.

Procedure and reference

Range

Add isooctane–I2 solution to sample; measure absorbance at 308 nm [1,2].

Micro

Prepare sample–I2 blend and measure absorbance at 310 nm against CCl4 reference [3].

Macro

Remove RSH and H2S with 0.05M AgNO3 and elemental S with Hg. Determine total S by lamp (O2 combustion, Sec. 18.4). Extract with solid HgNO3, and again determine total sulfur. Difference approximates aliphatic and cyclic sulfides [4]. Remove RSH and H2S with 0.05M AgNO3, free S with Hg, and aliphatic sulfides with HgNO3. Determine total S by lamp (see Sec. 18.4). Extract with solid Hg(NO3)2, and again determine total S. Difference approximates aromatic sulfides [4]. Extract with aqueous Hg(OAc)2 solution (21.2 g per 100 mL). Suitable for separation of alkyl and cyclic sulfides and isolation of each from petroleum fractions [5].

Micro and macro

Oxidize with saturated aqueous Br2 giving sulfoxides and sulfones; titrate acid formed with standard NaOH solution using bromocresol purple indicator [6]. Direct titration with standard (0.1N ) KBrO3–Br– solution. R2S + Br2 → R2SBr2; R2SBr2 + H2O → R2SO + 2 HBr [7].

Interferences Slight interference from aromatic sulfides See above

Micro or macro

Thiophenes

Macro

Thiols and olefins

Macro

Thiols and olefins

Macro

Olefins, thiols, and disulfides

Alkyl phenyl sulfides and alkyl benzyl sulfides are separated by an electrokinetic method in a micellar solution (aqueous–MeOH solution containing sodium dodecyl sulfate) [8].

S. R. Hastings, Anal. Chem. 25:420 (1953). S. R. Hastings and B. H. Johnson, Anal. Chem. 27:564 (1955). H. V. Drushel and J. F. Miller, Anal. Chem. 27:495 (1955). J. S. Ball, U.S. Bur. Mines Rept. Invest. 3591 (1941). Birch and McAllan, J. Inst. Petrol. 37:443 (1951). J. H. Sampey, K. H. Slagle, and E. E. Reid, J. Am. Chem. Soc. 52:2401 (1932). S. Siggia and B. L. Edsberg, Anal. Chem. 20:938 (1948). K. Otsuka, S. Terabe, and T. Ando, Chem. Abstr. 105:12648r (1986).

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Other oxidizing agents interfere.

Macro range

1. 2. 3. 4. 5. 6. 7. 8.

D. D. Wimer, Anal. Chem. 30:77, 2026 (1958). C. A. Streuli, Anal. Chem. 30:997 (1958). E. Barnard and K. R. Hargrove, Anal. Chim. Acta 5:536 (1951). E. Glynn, Analyst 72:248 (1947). S. Allenmark, Acta Chem. Scand. 20:910 (1968). R. R. Legault and K. Groves, Anal. Chem. 29:1495 (1957). W. Ciesielski, Talanta 35:969 (1988). Simpson, Intern. J. Leprosy 17:208 (1949).

In blood or urine: Mix 1 mL of oxalated blood or suitable dilution of urine with 6 mL H2O, 5 mL 2M HCl, and 4 mL 12% Cl3CCOOH, and filter. To 10 mL of filtrate add 3 drops fresh 0.3% NaNO2 solution; 3 min later add 3 drops 1.5% NH4 sulfamate solution; 2 min later add 3 drops 0.1% N-(1-naphthyl) ethylenediamine hydrochloride, let stand 20 min in the dark, compare with standards and correct for a blank [8].

Sulfones

Other oxidizing agents interfere.

0.7 to 1.0 mequiv

Colorimetric

At end point, solution is a reddish-blue color, which does not deepen further on addition of excess iron(III) solution. A blank is essential.

Mean error < 1%

To 20 mL of air-free aqueous or alcoholic solution of samples under CO2, add 10 mL of SnCl2 solution (27 g/L SnCl2 in 0.9M HCl, standardized against I2 solution) plus 10 mL hot 12M HCl. Boil 15 min, add 25 mL H2O + 5 mL 12M HCl and boil 30 min. Add 20 mL 12M HCl + 10 mL H2O + 5 to 10 drops 1% K indigotrisulfonate solution. Cool and titrate in artificial light with 0.1M FeNH4(SO4)2 in 0.5 M H2SO4 very slowly as end point is approached [4]. In a N2 atmosphere, treat sample with 15 mL 0.1M TiCl3 solution. Let stand 1 h at 80°C. Oxidize excess Ti(III) with boiling Fe(III) solution, let stand 30 s, cool rapidly, add 10 mL 2.5M H3PO4–3M H2SO4, extract sulfides twice with BuOH and 3 times with 15 mL portions of CCl4. Add 55 mL water and titrate Fe(II) with 0.05N K2Cr2O7 using diphenhylaminesulfonic acid indicator [3,6]. Add KI and acetyl chloride to an acetic acid solution of the sulfoxide. After 2 to 5 min, dilute the solution with HCl and titrate the iodine formed with Na2S2O3 solution [5]. Treat sample with excess KI in a trifluoroacetic acid–acetone medium to produce I2 which is then titrated with Na2S2O3 [7]. In a N2 atmosphere, treat a solution of the sample in glacial acetic acid with excess standard TiCl3 solution + 1.5 mL 3M NH4SCN. Let stand 1 h in water bath at 80°C, and titrate while hot with standard 0.05M iron(III) alum solution [3].

Volumetric

Remarks

Sulfides, sulfones, and diethyl sulfite do not interefere.

Range

Dissolve about 1 mmol of sulfoxide in 75 mL of acetic anhydride and titrate with freshly standardized HClO4 in 1,4-dioxane, using glass and calomel electrodes [1,2].

Sulfoxides

Procedure and reference

Potentiometric titration

Technique

TABLE 19.19 Methods for the Determination of Sulfoxides and Sulfones

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Exchange the hydrogen of the sulfinic acid group with iodine, and the iodine subsequently replaced with potassium by treatment with KOH to give HIO. In presence of H2O2 the HIO is reduced and one molecule of O2 is liberated and gas evolved is measured [3].

Gasometric

1. 2. 3. 4. 5. 6. 7. 8.

I. Ackerman, Ind. Eng. Chem., Anal. Ed. 18:243 (1946). P. Allen, J. Org. Chem. 7:23 (1942). S. Krishna and Q. B. Das, J. Indian Chem. Soc. 4:367 (1927). V. Z. Deal and G. E. A Wild, Anal. Chem. 27:47 (1955). S. Siggia and J. G. Hanna, Quantitative Organic Analysis Via Functional Groups, 4th ed., Wiley, New York, 1977. J. J. Kirkland, Anal. Chem. 32:1388 (1960). A. Amer, E. G. Alley, and C. U. Pittman, Jr., J. Chromatogr. 362:413 (1986). H. Oka and T. Kojima, Chem. Abstr. 91:203867q (1979).

General

Evaporate sample with H2SO4, then determine the sulfate (usually alkyl sulfate) formed [5].

Sulfonic acids and sulfonates can be transformed with thionyl chloride into sulfonyl chloride and then determined by gas chromatography [6,7]. Sulfonic acids are converted to sulfonyl chlorides with PCl5 followed by reduction to thiols with LiAlH4; the thiols were determined by GC using a 3% OV-1 column [8].

Gas

chromatography

Titrate with standard NaOH in aqueous solution using phenolphthalein as indicator. Less strong acids are titrated in nonaqueous medium (ethanol, ethylene glycol, or pyridine) using potentiometric end-point detection [4].

Volumetric

Sulfonic acids

Make sample (ca. 0.06M sulfinic acid) slightly alkaline with NaOH. Remove SO 2− 3 , if present, by digestion with excess BaCl2 solution, filtration, and dilution to known volume. Use this to titrate a mixture of 50 mL 0.03M NaOCl (standardized at 2 h intervals vs. standard 0.05M NaAsO2), 50 mL 10% Na2CO3 and 100 mL H2O at 15°C using starch–iodide paper as external indicator [1]. Titrate with standard NaOH, using cresol red, thymol blue, or phenolphthalein as indicator. In the presence of sulfonic acids, titrate potentiometrically in acetic acid medium. Titrate directly to sulfonic acid with either NaOCl [1] or KMnO4 solution [2].

Sulfinic acids

Procedure and reference

Volumetric

Technique

TABLE 19.20 Determination of Sulfinic and Sulfonic Acids

Macro (ca. 2.5 g)

Range

Alkali sulfates must be absent.

Although an old method, it still finds use.

Suitable for assay of pure compounds, but sulfonic and many other acids interfere.

Sulfonates do not interfere.

Remarks

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If sulfate ion is present originally, run second aliquot but omitting NaNO2. No interference from H2SO4.

Ultraviolet. Measure absorbance at 301 nm in 50% EtOH containing 0.05M KOH [4]. Spectrophotometric. Dissolve sample in dilute aqueous HOAc and extract a measured aliquot into diethyl ether. Evaporate to dryness, dissolve residue in 1 mL 10% NaOH, dilute to 100 mL with water. Take an aliquot containing 50–100 mg saccharin, evaporate to dryness at 100°C, and fuse residue 2 h at 140°C with 2 mL phenolsulfonic acid. Cool, dissolve in water, dilute to 50 mL, add 10 mL 10% NaOH, dilute to 100 mL, and measure the phenol red color at 480 nm [5]. Gravimetric. Add dilute HNO3 or HClO4 + excess NaNO2 (giving sulfate ion) and determine as BaSO4 [6]. Potentiometric titration. Under N2 atmosphere, titration a solution of sample in pyridine with standard (butyl)4 NOH using glass–calomel electrodes [7]. Volumetric. To 100 mL sample in an iodine flask, add 20 mL 1:1 H2SO4, then excess standard 0.01M NaNO2, stopper, and let stand 30 min. Add excess 0.02M Ce(IV) sulfate and back titrate with standard Fe(NH4)2(SO4)2 using 1,10-phenanthroline indicator [8]. See under sulfonyl chlorides. See under sulfonyl chlorides.

Dixanthogen

Ethyl xanthate

Saccharin

Sulfamate

Sulfamic acids

Sulfenyl chlorides

Sulfinyl chlorides

HNO3 does not interfere.

Caffeine and vanillin interfere. Separate caffeine by extraction from first NaOH solution with CHCl3; separate vanillin by extraction from first residue with light petroleum.

Volumetric. To 10 mL of solution of sample in acetone, add 5 mL 10% (w/v) aqueous NH4NO3, 10 mL 2% aqueous KCN, and 10 mL acetone. Heat 25 min at 40 to 50°C. Extract twice with benzene, add 10 mL 1:4 H3PO4 to aqueous solution and heat 30 min at 100°C. Cool, add Br2 dropwise to permanent yellow color. Decolorize by careful addition of FeSO4, add 3 g KI and a little NaHCO3, let stand 10 min in the dark, and titrate liberated I2 with Na2S2O3 [3].

Diethyldithiocarbamate

5–30 mg

Remarks Remove H2S. Dimethyl sulfide, thiophene, and mercaptans should not interfere. Thiocarbonyls react similarly.

Range 0–50 mg CS2

Procedure and reference Colorimetric. (To determine CS2 in air.) Absorb CS2 in a solution prepared by adding 1 mL (C2H5)2NH + 20 mL triethanolamine + 50 mg Cu(OAc)2 to 1 L 85% to 100% EtOH (or methyl Cellusolve) and letting stand until clear. Measure absorbance of golden-yellow color at 420 nm [1,2].

Functional group determined

TABLE 19.21 Methods for the Determination of Specific Substances

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Titrimetric. Titrate an acetone–water (2 : 3 v/v) solution, 0.02–0.1M in RSO2Cl with standard Na2S (a) to first persistent yellow color, (b) to a two-electrode amperometric and point at 0.15 V, or (c) to a constant-current potentiometric end point using Pt electrodes [9,10]. Volumetric. Add a small excess of pyridine, aniline, or 8-hydroxyquinoline to sample, cool, add a few drops of water, let stand a few minutes, dilute with 30 to 50 mL water, make acid with HNO3, add excess standard AgNO3, and back titrate with standard KSCN, using Fe(III) as indicator [11]. Volumetric. Titrate with standard NaOH, using phenolphthalein indicator. Volumetric. To an alkaline solution of sample, add excess I2, let stand, acidify, and back titrate excess I2 with standard NA2S2O3 [12]. Potentiometric. In aqueous solution the sample is titrated with AgNO3 using a silver ion selective electrode [23]. Volumetric. Dissolve 30 mg in hot water, add excess standard ICl, let stand 15 min. Extract I2 into CHCl3, was with water and titrated the combined aqueous solutions with thiosulfate after adding excess KI [13]. Colorimetric. To 1 mL 2-propanol containing 0.05–5 mmol sample, add 1 mL 0.05M N-ethylmalemide in 2-propanol, then add 1 mL 0.25M KOH in 2-propanol. Let stand 10–20 min, and measure absorbance at 515 nm [14]. Volumetric. Dissolve 20 to 80 mg sample in 15 mL HOAc, dearate with CO2, add 2 mL air-free saturated aqueous KI, stir 2 min, and titrate with thiosulfate using starch indicator (continue to exclude air) [15]. Volumetric. Dissolve 1 mmol sample in 5 mL neutral EtOH, add 2 mL 20% (w/v) thiophenol in EtOH, swirl, and titrate RSO2H with 0.1N NaOH using bromophenol blue indicator [15]. Volumetric. Let stand 12 h at room temperature in the dark with excess alcoholic AgNO3 solution. Filter, wash, and dissolve precipitated Ag2S by warming with 6M HNO3. Determine silver content by titration with KSCN using Fe(III) as indicator [16]. Volumetric. The sample in aqueous acetonitrile is treated with excess Cu(NO3)2; protons released are titrated with a base [24].

Sulfonyl chlorides

Thio acids

Thioamide, —C(==S)NH2

Thiol esters

Thiolsulfinate

Thiolsulfonates

Thione esters

Thiosemicarbazones

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(Continued)

Applicable in presence of thiol esters, which react only when heated.

Interference from thiocyanates, thiols, and other reducible compounds.

Other acidic compounds interfere. Use intermediate salt bridge if done potentiometrically.

Reaction mixture gives low results if warmed. Interference from compounds with easily replaceable halogens, and may be determined similarly.

Sulfenyl and sulfinyl chlorides interfere and may be similarly determined.

DETERMINATION OF FUNCTIONAL GROUPS IN ORGANIC COMPOUNDS

19.51

Potentiometric. Titrate an alkaline solution of the sample with standard AgNO3 using an Ag indicator electrode [20].

Xanthate

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Colorimetric. To a neutral solution of sample, add excess NaHCO3 solution (pH 9 to 10), Na2[Fe(NO)(CN)5], NH2OH, and Br2. Compare resulting color to standards [18]. Volumetric. To 35 mL neutral solution of sample, add 1 g powdered KBr, then 20 mL 12M HCl, warm to 50°C, add 1 mL 0.1% AuCl3 solution, and titrate with 0.0167M KBrO3 potentiometrically [19].

Thiourea

Viles, J. Ind. Hyg. Toxicol. 22:188 (1940). F. F. Morehead, Ind. Eng. Chem., Anal. Ed. 12:373 (1940). Shankaranarayana and S. Patal, Analyst 86:98 (1961). Maurice and Mulder, Mikrochim. Acta 1957:661. Whittle, Analyst 69:45 (1944). Barnard and Hargrave, Anal. Chim. Acta 5:536 (1951). Cundiff and Markunas, Anal. Chim. Acta 20:506 (1959). Whitman, Anal. Chem. 29:1684 (1957). Ashworth, Walisch, and Kronz-Dienhart, Anal. Chim. Acta 20:96 (1959). Walish, Hertel, and Ashworth, Chim. Anal. 43:234 (1961). Drahowzal and Klamann, Monatsh. 82:470 (1951). Wojahn and Wempe, Arch. Pharm. 285:375 (1952). Carson and Wong, Nature 183:1673 (1959). Rapaport, Chem. Abstr. 53:15856h (1959). Barnard and Cole, Anal. Chim. Acta 20:540 (1959). Karjala and McElvain, J. Am. Chem. Soc. 55:2966 (1933). Hennert and Merlin, Chem. Anal. 39:429 (1957). Grote, J. Biol. Chem. 93:25 (1931). Szbelledy and Madia. Z. Anal. Chem. 114:253 (1938). Plaksin et al., Zav. Lab. 22:28 (1956). Makins, J. Am. Chem. Soc. 57:405 (1935). Matuszak, Ind. Eng. Chem., Anal. Ed. 4:98 (1932). H. Sikorska-Tomicka and G. Strutynsaka, Chem. Anal. (Warsaw) 30:547 (1985). M. Argueso et al., Microchem. J. 31:74 (1985).

Volumetric. To a neutral solution of sample, add a small excess of standard 0.1N AgNO3, then immediately add Fe(III) indicator and back titrate with KSCN [21]. Volumetric. Add 1M HOAc until just acid to phenolphthalein, then starch indicator and titrated with 0.01–0.2N I2 solution. Cooling to 0°C before titration improves the end point [22].

Volumetric. Reflux with KOH solution (giving sulfite ion), acidify with H3PO4, distill SO2 into excess standard I2, and back-titrate with thio-sulfate [17].

Procedure and reference

Thionyl

Functional group determined

TABLE 19.21 Methods for the Determination of Specific Substances (Continued)

0.05 to 3 mmol

Applicable to 10–5M – 10–4M

1 to 3 meq

Range

Remarks

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19.14.5 Determination of Sulfinic and Sulfonic Acids The sulfinic acid group can be easily oxidized to the sulfonic acid group. Since sulfonic acids are strong acids, they can be determined by an acidimetric titration. Less strong sulfonic acids can be titrated in nonaqueous medium. Methods for sulfinic and sulfonic acids are given in Table 19.20.

19.14.6 Determination of Miscellaneous Sulfur-Based Functional Groups Table 19.21 is a collection of methods for sulfur-based function groups that have not been covered in earlier tables of this subsection. Compounds or functional groups are listed alphabetically. In a review, the instrumental methods available for the determination of linear alkylbenzenesulfonates in sewage, sludges, soils, and sediments have been discussed.109 Of all the methods used for the determination of a-olefins sulfonates, reversed-phase HPLC was found to be the most valuable since it provided more qualitative and quantitative information about the analyte.110 A critical review of the methods used in the analysis and separation of sulfonates has been published.111 109 110 111

H. DeHenau, E. Mathijs, and W. D. Hopping, Int. J. Environ. Anal. Chem. 26:279 (1986). V. Castro, J. P. Canseller, and J. L. Boyer., Commun. Jorn. Com. Esp. Deterg. 16:373 (1985). M.-S. Kuo and H. A. Mottola, CRC Crit. Rev. Anal. Chem. 9:297 (1980).

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 20

ANALYSIS OF PESTICIDES AND HERBICIDES 20.1 INTRODUCTION 20.2 SAMPLE EXTRACTIONS AND CLEANUP 20.2.1 Aqueous Samples Table 20.1 Pesticides/Herbicides Extraction Processes 20.2.2 Fruits and Vegetables 20.3 ORGANOCHLORINE PESTICIDES 20.3.1 Methods of Analysis Table 20.2 Formulas and Synonyms of Some Organochlorine Pesticides Table 20.3 Characteristic Mass Ions of Some Chlorinated Pesticides 20.3.2 Mass Spectrometric Analysis 20.4 ORGANOPHOSPHORUS PESTICIDES 20.4.1 Structural Features 20.4.2 Extraction and Analysis Table 20.4 Common Organophosphorus Pesticides and Their Formulas and Synonyms 20.5 CARBAMATE PESTICIDES 20.5.1 Extraction and Analysis: General Outline 20.5.2 HPLC Postcolumn Derivatization and Fluorometric Detection 20.5.3 GC/MS Analysis Table 20.5 Characteristic Mass Ions of Some Organophosphorus Pesticides 20.6 UREA-TYPE HERBICIDES 20.6.1 Methods of Analysis Table 20.6 Characteristic Mass Ions of Some Common Carbamates 20.7 TRIAZINE HERBICIDES Table 20.7 Characteristic Mass Ions of Some Common Urea-Type Herbicides 20.8 CHLOROPHENOXY ACID HERBICIDES 20.8.1 General Discussion 20.8.2 Extraction, Derivatization, and Analysis Table 20.8 Names, Synonyms, and Charcteristic Mass Ions of Triazine Herbicides Figure 20.1 Esterification of 2,4-D: (a) Methylation; (b) Pentafluorobenzylation Table 20.9 Primary and Secondary Mass Ions of Herbicide Acids and Esters Table 20.10 SPE/HPLC Analysis of Herbicides Bibliography

20.1

20.1 20.2 20.2 20.3 20.5 20.5 20.5 20.6 20.7 20.8 20.8 20.8 20.8 20.9 20.11 20.11 20.11 20.11 20.12 20.13 20.13 20.13 20.14 20.14 20.15 20.15 20.15 20.15 20.16 20.17 20.17 20.18

INTRODUCTION The terms pesticides and herbicides refer to a wide class of substances used for pest and weed control, respectively, and include several thousand formulations. Because of their wide-scale uses in household pest control, as well as in garden, crop, and other agricultural applications, trace residues of many pesticides are susceptible to persist in the environment and may be found in many wastewaters, groundwaters, soils, and sediments and also in crops, fruits, and other food products. 20.1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

ANALYSIS OF PESTICIDES AND HERBICIDES

20.2

SECTION TWENTY

Although these substances vary widely in their chemical structures and properties, many common pesticides and herbicides fall under the following categories: 1. 2. 3. 4. 5. 6.

Organochlorine pesticides Organophosphorus pesticides Carbamate pesticides Triazine herbicides Chlorophenoxy acid herbicides Urea-type herbicides

Except for the organochlorine pesticides, compounds of each of these classes may have some similar structural features. Their general structures and methods of analyses are discussed below. The most common instrumental methods for trace analyses of pesticides and herbicides involve gas chromatography (GC), mass spectrometry (MS), and high-performance liquid chromatography (HPLC). Other techniques, such as enzyme immunoassay, are also known for a limited number of compounds, but such methods are susceptible to interference from other pesticides of similar structures. The GC, GC/MS, and HPLC methods are also susceptible to interference. Therefore, it is always recommended that once a pesticide is detected in a sample extract, its presence should be confirmed on an alternative column (GC or HPLC) or by an alternative technique, such as, derivatization or by mass spectrometry. Sample extraction is probably the most important step in pesticide analysis. Extraction not only transfers the analytes into a suitable organic solvent for chromatographic determination but also increases their concentrations by several order of magnitude and thereby lowers their detection levels accordingly. Also, extraction processes separate interfering substances from analytes. Various extraction methods are highlighted below.

20.2

SAMPLE EXTRACTIONS AND CLEANUP Sample extractions constitute the most critical steps in the analysis of pesticides and herbicides. Such steps involve extractions of the analytes from their bulk matrices into an appropriate solvent and then removal of potentially interfering substances from the solvent extracts by various cleanup procedures and concentrating the extracts to small volumes—usually 1–2 mL—before analysis. Often the extraction and cleanup steps can be achieved simultaneously, in part or fully, by many techniques. For example, supercritical fluid extraction (SFE) can extract and separate the analytes in a single step by appropriately controlling the pressure, modifier, or flow. Such techniques, however, are not commonly applied in pesticides analyses. The common and traditional approach is to carry out extractions and cleanup separately in multiple stages. While the method of extraction usually depends on the sample matrix, the cleanup procedure may depend on the nature of the interfering substances that may be present in the extract. For example, for the GC analysis of certain organochlorine pesticides, such as aldrin or lindane, using an electron-capture detector, interfering sulfur that may mask these peaks in the chromatogram are removed from the extract by treatment with copper powder.

20.2.1

Aqueous Samples Trace residues of pesticides that have very low solubilities in water may be extracted by (1) liquid– liquid extraction (LLE) or (2) solid-phase extraction (SPE).Various extraction techniques are highlighted in Table 20.1. 20.2.1.1 Liquid–Liquid Extraction. In liquid–liquid extraction, a 1-L aliquot of the aqueous sample is repeatedly extracted with three 50- to 60-mL portion of methylene chloride in a 2-L separatory

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20.3

TABLE 20.1 Pesticides/Herbicides Extraction Processes Extraction process Liquid–liquid extraction (LLE)

Solid-phase extraction (SPE)

Supercritical fluid extraction (SFE)

Technique

Matrix and methods

Separatory funnel

All aqueous samples; usually 1-L volume, manually shaken with a solvent heavier than water; extract concentrated to 1–2 mL

Continuous

All aqueous samples; 1-L or larger volume extracted continuously by intimate mixing with a suitable immiscible solvent heavier than water; no manual shaking but longer extraction time; extract concentrated to 1–2 mL

Microextraction

All aqueous samples; much smaller volume, usually 25–35 mL in a 40-mL vial; shaken with 2 mL hexane or other solvent lighter than water; a few microliters of top layer injected directly onto GC

Cartridge or column adsorption

Aqueous samples, biological fluids, or fruits and vegetables (extracted with water-miscible solvents) filtered through SPE cartridge or column to retain either pesticides or interfering substances; pesticides retained on cartridge eluted with an appropriate extracting solvent; selection of solvent based on polarity and Kow of analyte molecules; if interference removed on cartridge, sample passed through is subjected to further extraction

Disk-type device

Aqueous samples or food extracts passed through disk-type device (i.e., glass fiber filter with C-18 or C-8 sorbent) under pressure or suction; disk removed from device and shaken with an appropriate solvent; hydrophobic disks (C-18, C-8) require activation by passing a small volume of a water-miscible solvent

SPE microextraction

Water, juices, and biological fluids; a fiber coated with extractant or a droplet of extractant suspended in sample, transferred to a GC for analysis

Static or dynamic mode (or both)

Soils, sediments, solid wastes, fruits, vegetables, oil, meats and animal tissues; a supercritical fluid such as CO2 or N2O is pumped (under supercritical temperature and pressure conditions) through sample matrix in an extraction vessel to transfer the analytes from the matrix into a collection device, usually either an organic solvent or a SPE device

Soxhlet extraction

Soil, sediment, and animal tissues; pesticides extracted by an appropriate solvent by intimate mixing of solvent and sample in several cycles of batch processes

Passive sampling device (PSD)

Air, water, and soil; the sampling device is left in the environmental medium to sorb analyte, which is then extracted separately by a variety of techniques

Microwave-assisted solvent extraction (MASE)

Solid samples; analytes removed from sample by heating with a solvent under microwave energy.

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20.4

SECTION TWENTY

funnel. All nonpolar trace residues of pesticides partition from the aqueous phase into methylene chloride at the bottom immiscible phase. The methylene chloride extract is then concentrated down to a much smaller volume, usually 1 mL, by slowly and carefully evaporating the solution on a water bath. Many concentrators, such as Kuderna-Danish type, are commercially available. Thus, a 1-L sample is extracted and concentrated down to a volume of 1 mL, thereby lowering the method detection limit of the pesticides by almost 1000 times. Continuous liquid–liquid extraction units are commercially available that eliminate the need for manual extraction using a separatory funnel. Such units are designed to extract a large number of samples and in larger volumes. It may be noted that the selection of the solvent for extraction is important. Methylene chloride or other chlorinated solvents cannot be used if the GC analysis involves the use of an electroncapture detector. Chlorinated solvents, however, may readily be employed if the pesticides are to be detected by mass spectrometry. Also, solvent selection may depend on other factors, such as the density, volatility, and of course the solubility of the pesticides in the solvent. Solvents that are nonpolar or have low polarity are effective for extracting analytes that are nonpolar or have low polarity. Also, when a separatory funnel is used in LLE, the solvent should have a density greater than that of the water, so that the water-immiscible, denser solvent can be drained out conveniently from the separatory funnel and separated from the top aqueous phase for repeated extraction. Many hydrocarbon solvents have proven to be effective in the extraction of organochlorine pesticides such as aldrin, lindane, and DDT. Such solvents include n-hexane, isooctane, and toluene. The former is a solvent of choice in LLE microextraction for potable water. In such extractions, a much smaller volume of aqueous sample, i.e., 35 mL, is placed in a 40-mL vial with a Teflon cap. To this sample, 1 or 2 mL of n-hexane is added. The mixture is shaken vigorously for 1 min and the solution is allowed to stand. Hexane-soluble pesticide residues partition into the lighter solvent layer on the top, from which a few microliters of the extract are carefully withdrawn for injection onto the GC. Although such extraction is simple and faster than that using a separatory funnel or a continuous extractor, the extent of sample concentration is much lower than with the latter types of extraction. The octanol–water partition coefficient Kow may serve as a guidance for selecting a solvent, its volume, and the number of repetitive extractions. For example, if the Kow of the pesticide falls in the range 104–106, any of the solvents mentioned above may provide a good extraction. If the Kow is ~103 or less, certain additional steps may be necessary to achieve the same degree of extraction as that with a Kow > 104. In such a case the degree of extraction may be increased by (1) increasing the volume of the solvent or (2) by performing multiple extractions. Alternatively, the solubility of the pesticide in the water may be decreased by (1) adding a salt, such as sodium chloride, to “salt out” the analyte, (2) choosing a slightly more polar solvent, or (3) adjusting the pH to ~3 for acidic analytes and to ~10 for basic analytes. 20.2.1.2 Solid-Phase Extraction. Both solid-phase extraction (SPE) and solid-phase microextraction (SPME) are widely applied for extracting many types of pesticides from aqueous matrix. SPE devices offer certain advantages over LLE methods: 1. The speed of extraction is faster than with LLE. 2. The technique is more versatile and can be applied to a wide array of pesticides and herbicides of varying molecular structures and polarities. 3. Interference substances are also separated from the analytes during extraction. Thus, the SPE technique may be applied for sample cleanup. 4. SPE or SPME reduces or often eliminates the use of much larger volumes of organic solvents that may be toxic, environmentally polluting, or expensive. 5. The SPE device may be carried to the field for on-site analysis. Solid-phase extraction is carried over SPE cartridges, columns, or disks, which are commercially available. The sample is passed through the cartridge or column. While the analytes are retained on the cartridge or the column, the interfering substances are allowed to pass through. The analytes are subsequently eluted from the cartridge or column bed using a suitable solvent or solvent mixture.

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20.5

Another approach is to retain the interfering substances on the cartridge, allowing the pesticides to pass though for subsequent analysis. Selection of cartridge materials and elution solvents depends on the polarity of the analyte. For nonpolar and low-polar analytes, hydrophobic octadecyl (−C18), octyl (−C8), and cyclohexyl (−C6H11) cartridges are commonly used, in which the analytes are retained on the adsorbent bed and latter eluted out for analysis. Alternatively, such cartridges may be employed for the removal of nonpolar or lowpolar interfering substances from moderately polar pesticides. Other packing materials that are used in solid-phase extraction and cleanup include phenyl, cyanoprohyl, diol, aminopropyl, carboxymethyl, and sulfonylpropyl groups. Silica, alumina, and florisil cartridges are known to be effective for many polar pesticides or interfering substances. All types of cartridges and columns can be readily activated, loaded, washed, and eluted. Empore disks were introduced in the 1990s for solid-phase extraction of pesticides and other pollutants from aqueous matrix. These disks constitute reverse-phase bonded HPLC silica particles held tightly into a porous Teflon or glass fiber disk. Disks are commercially available with octadecyl, octyl, and other bonded phases on silica and plastic bead particles. The disks are placed in a filtration apparatus and the sample is passed through. Vacuum is then applied for complete drying of the disk. The disk is then shaken in a solvent to elute out the pesticides adsorbed over it. Hydrophobic disks containing C18- or C8-bound silica particles require activation. This is achieved by passing a small volume of methanol through the disk. Disks containing hydrophilic particles do not require activation. The SPE disk technique has both advantages and disadvantages relative to the cartridge and column devices. The major advantages include faster flow rates of samples without breakthrough of pesticides, rapid elution from the disk, and the ease of storage and transportation of samples. A disadvantage, however, is possible plugging of the disk when extracting dirty samples. Many solvents and combinations in varying proportions have accomplished excellent elution of pesticides from SPE cartridges, columns, and disks. Such solvents include acetonitrile, methanol, hexane, diethyl ether, petroleum ether, and methylene chloride.

20.2.2 Fruits and Vegetables Fruits and vegetables consist largely of water. The pesticide residues in such plant extracts are therefore extracted with a water-miscible solvent, such as acetone or acetonitrile. A convenient method of extraction involves the use of a SPE cartridge. The method is suitable for pesticides having moderate degree of polarity and Kow values ranging between 100 and 1000. The water content of the extract is carefully adjusted by adding an appropriate volume of extracting solvent. Hydrophobic −C18, −C8, or -phenyl-bonded SPE cartridges are used for the purpose. Aqueous solutions of plant extracts containing oils, waxes, lipids, sugars, and other interfering substances are filtered through the cartridge. While such interferences are retained on the hydrophobic cartridge, the pesticide extract solution passes through. Furthermore, retention of pesticides of low polarity on the SPE cartridge or column may be avoided or minimized by adjusting the pH and the water content of the extracting solvent mixture. The extract solution is then analyzed for pesticides by HPLC methods. Polar interferences in the plant extract may be removed in a similar manner by using a polar SPE cartridge. The packing materials in such cartridges include aminopropyl, cyanopropyl, quaternary methylamine, and diol groups.

20.3

ORGANOCHLORINE PESTICIDES

20.3.1 Methods of Analysis Some chlorinated pesticides with their alternative names and chemical formulas are listed in Table 20.2. Pesticides referred to under this broad class do not have any common specific structural features. They all contain chlorine atoms in their molecules and therefore may be determined by GC using a

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20.6

SECTION TWENTY

TABLE 20.2 Formulas and Synonyms of Some Organochlorine Pesticides Pesticide

Molecular formula

Alachlor Aldrin a-BHC

C14H20ClNO2 C12H8Cl6 C6H6Cl6

b-BHC

C6H6Cl6

g-BHC

C6H6Cl6

d-BHC

C6H6Cl6

Captafol a-Chlordane g-Chlordane

C10H9Cl4NO2S C10H6Cl8 C10H6Cl8

Chlorobenzilate Chloropropylate Chlorothalonil DBCP 4,4¢-DDD 4,4¢-DDE 4,4¢-DDT Dieldrin Endosulfan I Endosulfan II Endosulfan sulfate Endrin Endrin aldehyde Endrin ketone Heptachlor Heptachlor epoxide Hexachlorobenzene Hexachlorocyclopentadiene Isodrin Kepone Methoxychlor Mirex Nitrofen Permathrin Perthane Propachlor Strobane

C16H14Cl2O3 C17H16Cl2O3 C8Cl4N2 C3H5Br2Cl C14H10Cl4 C14H8Cl4 C14H9Cl5 C12H8Cl6O C9H6Cl6O3S C9H6Cl6O3S C9H6Cl6O4S C12H8Cl6O C12H8Cl6O C12H8Cl6O C10H5Cl7 C10H5Cl7O C6Cl6 C5Cl6 C12H8Cl6 C10Cl10O C16H15Cl3O2 C10Cl12 C12H7Cl2NO3 C21H20Cl2O3 C18H20Cl2 C11H14ClNO A mixture of terpene isomers C10H10Cl8 C13H16F3N3O4

Toxaphene Trifluralin

Some other names Metachlor, Lasso, Alanox Aldrex, Octalene, Seedrin a-Benzene hexachloride, a-hexachlorocyclohexane, a-hexachloran b-Benzene hexachloride, b-hexachlorocyclohexane, g-Benzene hexachloride, g-hexachlorocyclohexane d-Hexachlorocyclohexane, d-benzene hexachloride Difolatan, Folcid, Mycodifol cis-Chlordan, a-Chlordan trans-Chlordan, trans-g-Chlordane, b-Chlordane Acar, Benzilan, Folbex Chlormite, Rospin, Acaralate Daconil, Sweep, Nopcocide Nemagon, Fuzmazone, Nemabrom p,p¢-DDD, Rothane, Dilene p,p¢- DDE, DDT dehydrochloride p,p¢-DDT, Chlorphenotane, Neocid Dieldrite, Termitox, Aldrin epoxide a-Endosulfan, a-Thiodan b-Endosulfan, b-Thiodan Thiodan sulfate Endrex, Mendrin, Hexadrin Endrex aldehyde d-Ketoendrin Rhodiachlor, Hetamul, Drinox Epoxyheptachlor, HCE Sanocide, Amatin, HCB Graphlox, perchloro-1,3-cyclopentadiene Latka 711, SD 3418 Chlordecone, Merex Metox, Marlate, Methoxcide Dechlorane, Paramex Nitrochlor, TOK, NIP Elimite, Nix Ethylan Bexton, Ramrod, Satecid Terpene polychlorinates, Dichloricide aerosol Camphechlor, Alltex, Octachlorocamphene Nitran, Trifloran, Trefanocide

halogen-specific detector. Many environmental samples, after extraction into a suitable nonhalogen solvent such as hexane or isooctane, and after the solvent extract is concentrated down to a small volume, are analyzed by GC-ECD (electron-capture detector). These detectors are most sensitive to organochlorine pesticides. After the compound is identified by GC, its presence in the sample may be confirmed

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20.7

by mass spectrometry using GC/MS or LC/MS. HPLC and enzyme immunoassay are the other methods of detecting these pesticides. Some of these methods are briefly discussed below: GC/ECD analyses may be performed using either packed or capillary columns. A fused-silica, open-tubular capillary column provides improved resolution, increased sensitivity, and better selectivity than a packed column. Both types of columns are commercially available. A 30-m-length capillary column having an inside diameter (ID) of 0.53 or 0.32 mm and a film thickness ranging between 0.83 and 1.5 mm should exhibit high resolution in separating pesticides in mixtures. Such columns under different trade names include DB-5, DB-17, Rxt-5, SPB-608, and DB-608. Helium and nitrogen may be used as carrier and makeup gases, respectively. Alternatively a 1.8-m × 4-mm-ID glass column packed with 1.5% SP-2250, 1.95% SP-2401, or 3% OV-1 on Supelcoport or equivalent may be used as a packed column. The chromatographic conditions for such packed column separation of pesticides are outlined below to serve as a rough guideline. Other conditions may be employed as appropriate. TABLE 20.3 Characteristic Mass Ions of Some Chlorinated Pesticides Pesticides

Primary ion

Secondary mass ions

Alachlor Aldrin a-BHC b-BHC g-BHC d-BHC Captafol a-Chlordane g-Chlordane Chlorobenzilate Chloropropylate Chlorothalonil DBCP 4,4¢-DDD 4,4¢-DDE 4,4¢-DDT Dieldrin Endosulfan I Endosulfan II Endosulphan sulfate Endrin Endrin aldehyde Endrin ketone Heptachlor Heptachlor epoxide Hexachlorobenzene Hexachlorocyclopentadiene Isodrin Kepone Methoxychlor Mirex Nitrofen Permathrin Perthane Propachlor Toxaphene Trifluralin

45 66 181 109 181 181 79 39 373 251 139 266 57 235 246 235 79 195 39 272 81 67 67 100 353 284 237 193 272 227 272 283 183 223 120 197 306

160, 188, 146, 237, 118, 77, 132 263, 79, 91, 101, 265, 65, 39, 293 183, 219, 111, 217, 51, 221, 185, 109 183, 181, 111, 51, 219, 85, 217, 73 183, 219, 111, 109, 217, 51, 221, 185 183, 219, 111, 109, 217, 51, 218, 221 77, 80, 78, 39, 27, 107, 51 65, 75, 27, 49, 373, 375, 51, 109 375, 109, 272, 119, 377, 75, 117, 235 139, 253, 111, 141, 75, 29, 252 251, 253, 111, 141, 43, 252, 75 264, 268, 267, 270, 109, 124, 231 157, 75, 155, 28, 39, 27, 77, 41 237, 165, 236, 239, 199, 178, 238 318, 248, 316, 320, 176, 105, 123 237, 165, 236, 212, 199, 28, 282 81, 263, 277, 279, 77, 345, 380 241, 197, 243, 207, 277, 75, 69, 339 48, 41, 49, 63, 50, 85, 195 274, 229, 387, 239, 227, 422 39, 83, 67, 263, 53, 113, 281, 345 345, 250, 347, 66, 173, 252, 243 27, 39, 66, 87, 79, 55, 147 65, 272, 39, 102, 274, 135, 237, 337 355, 351, 81, 357, 263, 237, 388 286, 282, 288, 142, 249, 107, 71 239, 235, 95, 130, 60, 272, 119 195, 263, 66, 261, 197, 265, 147, 91 274, 270, 237, 276, 239, 143, 355 229, 152, 169, 212, 115, 274, 344 274, 270, 237, 239, 235, 276, 143, 119 285, 202, 50, 63, 75, 139, 162 163, 165, 77, 91, 184, 127, 51 224, 167, 179, 236, 193, 306 77, 176, 93, 43, 51, 169, 196, 211 125, 159, 75, 99, 195, 100, 83, 233, 343 264, 43, 335, 290, 248, 206, 151

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20.8

SECTION TWENTY

Carrier gas: 5% methane/95% argon Flow rate: 60 mL/min Temperature: Oven 200ºC (isothermal) Injector 250ºC, ECD 320ºC Many target analytes may co-elute even on a 30-m capillary column. For example, perthane and endrin or methoxychlor and dicofol co-elute on a DB-5 column. Different co-eluting pesticides that may co-elute similarly on other columns are also known. Dual-column analysis using two different columns is therefore preferred over single-column analysis. Also, narrow-bore capillary columns of film thickness 0.32 m m or less give better resolution of pesticides than a wide-bore column having a film thickness of 0.53 m m. The latter, however, has a greater loading capacity and is therefore recommended for dirtier environmental samples.

20.3.2

Mass Spectrometric Analysis Although the mass-selective detector (MSD) is not as sensitive in detecting pesticides at the level that ECD does, such compounds may be identified by mass spectrometry at relatively higher concentration levels. The solvent extract of the sample must be appropriately concentrated. The pesticides may be quantitated from the area response of their primary mass ions. Table 20.3 gives the primary and secondary mass ions of some selected organochlorine pesticides. For many compounds, seven to eight secondary ions are presented in the table, to help distinguish between isomers or different compounds that are structurally similar. The secondary ions shown in Table 20.3 are usually listed in order of decreasing abundance. For some compounds the molecular ions, even with low abundance, are included in the table to aid in identifying a compound. The characteristic ions for these and other pesticides in this section are obtained under electron impact ionization mode.

20.4 20.4.1

ORGANOPHOSPHORUS PESTICIDES Structural Features Organophosphorus pesticides are esters of phosphoric acid with structures containing the (RO)2 P(== O or S)−− O(or S) unit, where R is an alkyl or aryl group. Some common pesticides of this class and their alternative names are listed in Table 20.4.

20.4.2

Extraction and Analysis Aqueous samples are extracted with methylene chloride at neutral pH by liquid–liquid extraction using a separatory funnel or a continuous liquid–liquid extractor. Oils, fats, and nonaqueous liquid wastes may be appropriately diluted in a suitable solvent and analyzed. Solid environmental samples, such as soils, sediments, hazardous wastes, or food and agricultural products, may be extracted by Soxhlet extraction using 1:1 methylene chloride/acetone or other solvent mixtures. Such solvent mixtures should be a combination of miscible nonpolar and polar organic solvents that can extract the analytes that are of nonpolar and polar-type phosphate esters from solid matrices. Fruits and vegetables may be extracted by solid-phase extraction (SPE). A detailed procedure is given in Sec. 20.5. Organic phosphates in biological fluids may be extracted into an appropriate solvent by liquid–liquid extraction or waste dilution methods, depending on to what degree the sample is miscible in the solvent. If the sample is expected to contain elemental sulfur, the latter may be removed by treating the sample

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20.9

TABLE 20.4 Common Organophosphorus Pesticides and Their Formulas and Synonyms Pesticide Abate Acephate Akton* Aspon Azinphos methyl Azinphos ethyl Bolstar sulfone Bomyl Carbofenthion Chlorfenvinphos Chlorofos Coumaphos Crotoxyphos Demeton-O Demeton-S Diazinon Dicapthon Dichlofenthion Dichlorvos Dicrotophos Dimethoate Dioxathion Disulfoton Dursban EPN Ethion Ethephon Famophos Fenamiphos Fensulfothion Fenthion Folithion Fonofos Isazophos Isofenphos Leptophos Malathion Merphos Methamidophos Methidathion Mevinphos Monocrotophos Montrel Naled Oxydemeton-methyl Oxydemetonmethyl sulfone Parathion-ethyl Parathion-methyl Paraoxon Phenamiphos Phorate Phosalone Phosfolan

Molecular formula C16H20O6P2S3 C4H10NO3PS C12H14Cl3O3PS C12H28O5P2S2 C10H12N3O3PS2 C12H16N3O3PS2 C12H19O4PS3 CHOP C11H16ClO2PS3 C12H14Cl3O4P C4H8Cl3O4P C14H16ClO5PS C14H19O6P C8H19O3PS2 C8H19O3PS2 C12H21N2O3PS C8H9ClNO5PS C10H13Cl2O3PS C4H7Cl2O4P C8H16NO5P C5H12NO3PS2 C12H26O6P2S4 C8H19O2PS3 C9H11Cl3NO3PS C14H14NO4PS C9H22O4P2S4 C2H6ClO3P C10H16NO5PS2 C13H22NO3PS C11H17O4PS2 C10H15O3PS2 C9H12NO5PS C10H15OPS2 C9H17ClN3O3PS C15H24NO4PS C13H10BrCl2O2PS C10H19O6PS2 C12H27PS3 C2H8NO2PS C6H11N2O4PS3 C7H13O6P C7H14NO5P C12H19ClNO3P C4H7Br2Cl2O4P C6H15O4PS2 C6H15O5PS2 C10H14NO5PS C8H10NO5PS C10H14NO6P C13H22NO3PS C7H17O2PS3 C12H15ClNO4PS C7H14NO3PS2

Alternative Names Difenphos, Temefos, Bithion Acetamidophos, Orthene Axiom ASP 51, Propyl thiopyrophosphate Gusathion, Guthion, Carfene Gusathion ethyl, Ethyl guthion, Bionex — GC 3707 Trithion, Hexathion, Acarithion Birlane, Enolofos, Sapecron Metrifonate, Anthon, Trichlorfon Muscatox, Asuntol, Baymix Ciodrin, Decrotox Mercaptophos, Thiodemeton Isosystox, Thioldemeton Basudin, Dimpylate, Neocidol Isochlorthion, Chlorthion, Dicaptan Nemacide, Mobilawn, Bromex Chlorvinphos, Cyanophos, Atgard Carbicron, Ektafos, Bidrin Fosfotox, Cygon, Phosphamid Navadel, Ruphos, Delnatex Dithiodemeton, Glebofos, Dithiosystox Chlorpyrifos, Lorsban, Eradex Santox, EPN 300 Rodocide, Phosphotox E, Ethanox Camposan, Ethrel Famphur, Warbex, Cyflee Nemacur, Phenamiphos Terracur P, Desanit, Hexazir Baycid, Mercaptophos, Baytex Fenitrothion, Metathion, Nitrophos Dyphonate, Difonate Miral Oftanol, Amaze Phosvel, Abar, MBCP Fosfothion, Carbofos, Malafos, Cythion Folex, Tributyl trithiophosphite ENT, Tamaron, Monitor Supracide, Medathion, Ultracide Phosdrin, Duraphos, Mevinex Azodrin, Bilobran Crufomate, Amidophos, Ruelene Dibrom, Bromex, Dibromfos Metasystox R, Metaisosystox sulfoxide Metasystox R sulfone Parathion, Thiophos, Foliodol Metaphos, Azofos, Metron, Nitrox Phosphacol, Diethyl paraoxon Nemacur Thimate, Aastar, Granutox Rubitox, Zolone, Benzophosphate Cyolane, Cylan (Continued)

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ANALYSIS OF PESTICIDES AND HERBICIDES

20.10

SECTION TWENTY

TABLE 20.4 Common Organophosphorus Pesticides and Their Formulas and Synonyms (Continued) Pesticide

Molecular formula

Alternate Names

Phosphamidon Phosmet Profenofos Prophos Sulfotepp TEPP Terbufos Tetrachlorvinphos Tokuthion Trichloronate Zinophos

C10H19ClNO5P C11H12NO4PS C11H15BrClO3PS C8H19O2PS2 C8H20O5P2S2 C8H20O7P2 C9H21O2PS3 C10H9Cl4O4P C11H15Cl2O2PS2 C10H12Cl3O2PS C8H13N2O3PS

Famfos, Dimecron, Apamidon Imidan, Decemthion, Safidon Polycron, Selecron Ethoprophos, Mocap, Ethoprop Dithiophos, Thiotepp, Pirofos Fosvex, Nifos, Hexamite Counter, ST-100 Stirofos, Rabon, Gardona Protothiophos Agritox, Phytosol, Phenophosphon Cyanophos, Menafos, Thionazine

extract with tetrabutylammonim sulfite solution. Many interfering substances may be removed from sample extracts by Florisil column cleanup procedure. However, such cleanup steps may also remove certain organic phosphates and therefore should not be applied for analyzing such substances. The extracts may be analyzed directly, without any cleanup, if a flame photometric detector (FPD) or a mass spectrometer is used. The organic phosphates are best measured by a gas chromatograph with a flame photometric or a nitrogen–phosphorus detector (NPD). Both these detectors are operated in phosphorus mode to minimize interference from materials that do not contain phosphorus or sulfur, especially when using an FPD, or that do not contain phosphorus when using an NPD. The compounds may be separated on a 30-m-long capillary column with 0.32-mm ID. Shorter columns with 0.25-mm or 0.53-mm ID may also be used, depending on the resolution desired. Several such columns are commercially available that contain various chemically bonded polysiloxane mixtures in varying proportions. They include phenyl methyl polysiloxane, trifluoropropyl polysiloxane, methyl polysiloxane, and phenyl polysiloxane under different trade names, such as DB-1, DB-5, DB-608, DB-210, SPB-1, SPB-5, SPB-608, RTx-1, RTx-5, RTx-35, and RTx-1701. Although a 15-m × 0.53-mm fused-silica capillary column gives adequate resolution of most organic phosphates, a few compounds, such as ethyl azinphos, crotoxyphos, dicrotophos, famphur, fonofos, leptophos, terbufos, and zinofohs, are not well separated from compounds that have closely related structures in phosphate mixtures using such columns. A 30-m or longer column provides better resolution of these and certain other phosphates. Once a compound is detected from its retention time, its presence is confirmed on an alternate column or using another detector. The most common approach is to use two fused-silica opentubular columns of different polarities connected to an injection tee, with each connected to a detector. On the other hand, if two detectros are to be used, FPD and NPD are the ideal choice. FPD is more sensitive and selective to phosphorus- or sulfur-containing compounds. Both the FPD and NPD are flame ionization-type detectors. The performance of the FPD may be optimized by selecting the proper optical filter and by adjusting the flows of air and hydrogen to the flame. Although FPD is more sensitive than NPD, an advantage of the latter is that it can also measure triazine herbicides and other nitrogen-containing substances in sample extracts. As mentioned earlier, presence of elemental sulfur can interfere with FPD analysis and therefore must be removed by treating the sample extract with tetrabutylammonium sulfite solution. Many organophosphorus pesticides, such as dichlorvos, trichloronate, ronnel, coumaphos, chlorpyrifos, stirophos, and naled, contain halogen atoms in their molecules. Such compounds, marked with asterisks in Table 20.4, may also be determined by a halogen-specific detector, such as electrolytic conductivity or microcoulometric detector. Also, many organic phosphates may be determined by ECD, although the latter may not be as specific as FPD or NPD. Among other analytical techniques, HPLC and mass spectrometry are as common as GC-FPD and GC-NPD. HPLC measurement usually requires a mobile phase of acetonitrile:water solvent

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ANALYSIS OF PESTICIDES AND HERBICIDES

20.11

mixture (e.g., 60:40 mixture) at a flow rate such as 1 mL/min at ambient temperature using a UV detector at 214 nm. Pesticides in fruits and vegetables are commonly extracted by SPE or SPME and separated on columns such as Discovery RP-AmideC16 and analyzed by HPLC-UV techniques. Although the MSD is the least sensitive of all detectors, unknown compounds can be identified from their mass spectra. The characteristic mass ions (both the primary and the secondary mass ions) of some selected organic phosphates are given in Table 20.5.

20.5

CARBAMATE PESTICIDES

20.5.1 Extraction and Analysis: General Outline Carbamate pesticides/herbicides are nitrogen-containing substances having the following structural feature: R1 O R2

N−−C−−O−−(leaving group)

where R1 and R2 are alkyl or aryl groups or hydrogen. These compounds can be detected by GCNPD in nitrogen mode, GC-FID, GC/MS, LC/MS, and HPLC. Carbamates are extracted from sample matrices by various extraction procedures discussed earlier. Most common methods of extraction involve the SPE and SPME techniques, which may be applied to all aqueous samples including wastewaters, drinking waters, biological fluids, and water-soluble plant extracts. Among instrumental techniques, HPLC using postcolumn derivatization and UV detection are the most common methods. Numerous methods are reported in the literature, a few of which are outlined below. 20.5.2 HPLC Postcolumn Derivatization and Fluorometric Detection Carbamates in sample extracts are separated on a C-18 reverse-phase HPLC column (e.g., 25-cm × 4.6-mm stainless steal packed with 5-mm Beckman Ultrasphere, 15-cm × 2.9-mm stainless steel packed with 4-mm Nova Pac C-18, 25-cm × 4.6-mm stainless steel packed with 5-mm Supelco LC-1, or equivalent). The separated carbamates are then hydrolyzed with 0.05N NaOH to convert them to their methyl esters. The esters are then reacted with o-phthalaldehyde and 2-mercaptoethanol at 40°C (at a flow rate of 0.7 mL/min and a residence time of 35 s) to give highly fluorescent derivatives which are detected by a fluorescent detector. The excitation wavelength for flourometric detection is 330 nm (cutoff filter). The solvents A and B are reagent-grade water acidified with phosphoric acid and 1:1 methanol/acetonitrile mixture, respectively, and the flow rate is 1 mL/min. The derivatization reagent o-phthalaldehyde is prepared by adding a 10-mL aliquot of 1% o-phthalaldehyde in methanol to 10 mL of acetonitrile containing 100 mL of 2-mercaptoethanol. The mixture is then diluted to 1 L with 0.05N sodium borate solution. Carbamates can also be determined by HPLC without derivatization. A number of LC columns are commercially available. They include Discovery C18, 15 m × 4.6-mm ID, 5-mm particles (Supelco, Inc.), Luna 5m CN 150 × 4.6 mm, or Luna 3m C18, and such columns provide excellent separation of carbamate, triazine, urea, and other types of pesticides and herbicides using a mobile phase of acetonitrile/water under varying gradient programs. The compounds may be detected by a UV detector at 214 nm. 20.5.3 GC/MS Analysis Although UV detection is commonly employed for analyzing carbamates, other detectors are equally versatile. Mass spectrometry is the single most confirmatory test. Either a GC or an LC is interfaced

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ANALYSIS OF PESTICIDES AND HERBICIDES

20.12

SECTION TWENTY

TABLE 20.5 Characteristic Mass Ions of Some Organophosphorus Pesticides Compound

Primary ion

Secondary mass ions

Azinphos-ethyl Azinphos-methyl Bolstar sulfone Carbofenthion Chlorfenvinphos Chlorofos Coumaphos Crotoxyphos Demeton-O Demeton-S Diazinon Dicapthon Dichlofenthion Dichlorvos Dicrotophos Dimethoate Dioxathion Disulfoton Dursban EPN Ethion Ethephon Fampohos Fenamiphos Fensulfothion Fenthion Fonofos Leptophos Malathion Merphos Methamidophos Methidathion Mevinphos Monocrotophos Montrel Naled Oxydemetonmethyl sulfone Parathion-ethyl Parathion-methyl Paraoxon Phenamiphos Phorate Phosalone Phosfolan Phosphamidon Phosmet Profenofos Prophos Sulfotepp TEPP Terbufos Tetrachlorvinphos Tokuthion Trichloronate Zinophos

132 160 188 157 267 109 362 127 88 88 179 262 279 109 127 87 97 88 97 157 231 82 218 303 293 278 109 171 173 57 15 85 127 127 256 15 169 97 109 109 303 75 182 92 127 160 139 158 322 99 57 329 43 109 107

77, 160, 105, 104, 65, 29, 76 77, 132, 104, 105, 93, 51, 50 43, 113, 312, 63, 141, 354 45, 97, 121, 159, 199, 342 323, 269, 325, 81, 109, 295, 170 185, 79, 47, 145, 187, 220 109, 226, 97, 210, 364, 334, 228 105, 193, 166, 104, 77, 79, 67 60, 29, 115, 171, 143, 45, 258 89, 60, 171, 97, 115, 28, 29, 258 137, 152, 199, 304, 29, 135, 276 125, 79, 47, 109, 93, 63, 216 97, 223, 251, 88, 162, 281, 109 185, 79, 47, 145, 187, 220 67, 72, 193, 109, 44, 237 93, 125, 79, 58, 47, 229, 172 125, 270, 73, 45, 65, 197, 153 29, 89, 60, 97, 142, 125, 153, 274 197, 199, 29, 314, 316, 258, 125 169, 63, 141, 185, 77, 29, 323 153, 97, 125, 121, 65, 384, 93 81, 109, 27, 28, 65, 47, 91, 145 93, 125, 28, 44, 109, 63, 282 154, 217, 44, 288, 260, 80, 122 308, 141, 97, 125, 153, 109, 265 125, 109, 153, 169, 93, 79, 279, 280 137, 246, 110, 81, 28, 63, 174 377, 28, 375, 77, 155, 60, 379 127, 93, 158, 99, 29, 143, 79, 256 209, 41, 298, 153, 88, 56, 55, 242 94, 47, 95, 141, 45, 64, 30 145, 93, 58, 47, 125, 63, 302 192, 109, 67, 43, 164, 193, 79 67, 97, 58, 192, 109, 43, 193 108, 276, 182, 169, 278, 41, 291 109, 145, 79, 47, 18, 29, 31 109, 125, 168, 79, 110, 142 291, 109, 137, 139, 125, 29, 186, 65 125, 263, 79, 42, 61, 92, 28 29, 149, 81, 275, 139, 65, 99, 75 154, 288, 44, 80, 260, 43, 304 121, 260, 97, 28, 47, 65, 93 121, 97, 184, 154, 65, 111, 367 140, 196, 60, 168, 81, 29, 255, 227 72, 264, 138, 109, 28, 67, 193 61, 76, 77, 133, 104, 161, 50 97, 208, 206, 125, 339, 337, 63 43, 97, 139, 126, 93, 41, 200, 242 202, 97, 266, 65, 174, 121, 238, 294 155, 43, 127, 81, 109, 82, 111 231, 97, 29, 153, 41, 103, 233 109, 331, 333, 79, 240, 93 27, 29, 113, 267, 63, 309, 162, 155 297, 269, 299, 28, 93, 271, 137 96, 106, 97, 143, 248, 140, 79, 68

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ANALYSIS OF PESTICIDES AND HERBICIDES

ANALYSIS OF PESTICIDES AND HERBICIDES

20.13

TABLE 20.6 Characteristic Mass Ions of Some Common Carbamates Carbamates

Synonyms

Primary ion

Secondary ions

Aldicarb Aldicarb sulfone Aldicarb sulfoxide Banol Barban Bendiocarb Benomyl Carbaryl Carbofuran Chlorpropham Matacil Methiocarb Methomyl Oxamyl Pirimicarb Promecarb Propham Propoxur Swep Terbucarb

Temik Aldoxycarb, Standak — Carbanolate Barbamate, Carbyne Ficam Benlate Sevin, Arylam Furadan Chloro IPC Aminocarb Mesurol Lannate — Pirimor Carbamult IPC Baygon, Aprocarb — Terbutol

41 86 86 156 222 151 191 144 164 43 151 168 58 72 166 135 93 110 219 205

58, 86, 76, 144 143, 85, 41 41, 28, 58, 143 121, 91, 158, 141, 65 51, 87, 143, 153, 224, 257 126, 166, 51, 223 159, 40, 105, 42, 132 115, 116, 28, 89, 201 149, 122, 57, 221 127, 213, 171, 129, 154, 41 136, 77, 120, 77, 208 153, 225, 109 105, 32, 42, 88, 91, 45 44, 32, 30, 162, 58, 115, 88, 145 72, 238, 138 150, 91, 58, 41, 39 43, 179, 137, 120, 65 152, 81, 27, 43, 39 221, 174, 59, 89, 187, 176 57, 220, 58, 206, 41, 105

to a mass spectrometer. The pesticides are identified from their characteristic mass ions and retention times. The characteristic mass ions of some selected carbamates under electron-impact ionization are listed in Table 20.6.

20.6

UREA-TYPE HERBICIDES

20.6.1 Methods of Analysis Urea-type herbicides, like carbamates, are all nitrogen-containing organic compounds. The structural features are also similar to those of carbamates, except that the terminal oxygen atom of carbamate is replaced by a nitrogen atom in urea-type substances. Thus, the common structural feature in all urea herbicides is R1 R2

O N−−C−−N

(leaving group)

Trace residues of these herbicides in environmental samples, biological fluids, and food products may be determined by HPLC using a UV detector, GC using a NPD in nitrogen mode, or by mass spectrometry. Among these methods, HPLC-UV and LC/MS are the most common techniques. Urea-type herbicides are polar compounds. They may be effectively extracted by solid-phase extraction. Traditional LLE is also applicable for extracting aqueous samples. SPE tubes containing ENVICarb carbon-based packing or charcoal/celite packing have been reported to provide superior and more uniform recovery of such polar analytes over the octyl (C8)- or octadecyl (C18)-bonded silica packing. Alternatively, these herbicides may be extracted by solid-phase microextraction using a SPME fiber coated with 100-mm film of polydimethylsiloxane.

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ANALYSIS OF PESTICIDES AND HERBICIDES

20.14

SECTION TWENTY

TABLE 20.7 Characteristic Mass Ions of Some Common Urea-Type Herbicides Name

Synonyms

Primary ion

Secondary ions

Chlorbromuron Chloroxuron Diuron Fenuron Fluometuron Linuron Metobromuron Monuron Monuron TCA Neburon Siduron Tebuthiuron

Maloran Tenoran Karmex, Herbatox Dybar, Beet-kleen Cotoran, Lanex Sinuron, Cephalon Patoran Telvar Urox Kloben Tupersan Tebulan, Perflan

61 72 72 72 72 61 61 72 72 57 93 156

46, 294, 206, 124 245, 290, 44, 182 232, 234, 44, 73, 187, 124 164, 44, 119, 77, 65, 91 232, 44, 42, 28, 145, 187 46, 248, 250, 160, 62, 162 46, 91, 258, 260, 172, 32 198, 38, 153, 200, 61 82, 81, 198, 127, 98, 153, 270 44, 114, 41, 58, 125, 274 55, 56, 41, 39, 77, 232 171, 74, 41, 88, 57, 157

Two typical extraction procedures are given below as examples for fruits and vegetables and water samples, respectively. Other e efficient. For fruits and vegetables, 50 g of chopped sample are mixed with 100 mL of acetonitrile and homogenized in a mixer for 5 min. To this solution, 10 g of sodium chloride are added and the resulting solution is homogenized for another 5 min. About 13 mL of acetonitrile solution is transferred from the top layer to a 15-mL graduated centrifuge tube. To the latter is then added about 3 g of anhydrous sodium sulfate to bring it up to the 15-mL mark. The solution is then shaken to remove water from the acetonitrile. It is then centrifuged for 5 min. A 10-mL aliquot of this solution is transferred to a clean 15-mL tube. The solution is evaporated on a water bath at 35°C under nitrogen flow to a final volume of 0.5 mL. The latter is then transferred onto an SPE tube containing 500 mg (6-mL tube) ENVI-Carb packing. The urea and carbamate herbicides are then eluted with 20-mL of 3 : 1 acetonitriletoluene mixture. The extract is concentrated down to a volume of 2 mL in a rotary evaporator and then solvent-exchanged to acetone by adding 10 mL acetone successively. The solution is then analyzed by a suitable technique. As mentioned earlier, HPLC-UV and LC/MS are commonly employed in analyzing urea herbicides in sample extracts. Compound can be separated on many types of LC columns, and several such columns are commercially available. They include C-18 Novapk (Waters Corp.), Pinnacle II C8 (Resteck), Discovery C18 (Supelco, Inc.), Luna 3m Phenyl-Hexyl, and many other equivalent columns. The mobile phase usually consists of acetonitrile/water under varying gradient conditions. The analysis may be carried out at ambient temperature using a UV detector at wavelengths ranging between 220 and 240 nm. Identification of compounds by mass spectrometry is the most confirmative test. Characteristic mass ions of some common urea-type herbicides are presented in Table 20.7.

20.7

TRIAZINE HERBICIDES Triazine herbicides are compounds containing a triazine ring as follows: N

N N

These heterocyclic nitrogen compounds usually exhibit low toxicity. They may be determined by GC-NPD in nitrogen mode, by HPLC, or by GC/MS. Although the latter is not as sensitive as the

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ANALYSIS OF PESTICIDES AND HERBICIDES

ANALYSIS OF PESTICIDES AND HERBICIDES

20.15

TABLE 20.8 Names, Synonyms, and Charcteristic Mass Ions of Triazine Herbicides Compound

Synonyms(s)

Primary ion

Secondary ions

Ametryne Anilazine Atraton Atrazine Cyanazine Dipropetryne Metribuzine Procyazine Prometon Prometryne Propazine Simazine Simetryne Terbuthylazine Terbutryne

Ametrex, Gesapax Dyrene, Triasyn Primatol, Gesatamin Gesaprim, Zeazine Bladex, Cyanazone Sancap, Cotofer Sencor, Lexone Cycle Primatol, Gesafram Gesagard, Caparol Gesamil, Milogard Tafazine, Herbazin Gybon Gardoprim Igran, Perbane

227 239 196 58 68 43 198 41 210 241 58 201 213 43 226

212, 58, 44, 170, 98, 185, 71 241, 178, 143, 62, 75, 274, 276 58, 211, 169, 44, 69, 91, 154, 141 43, 44, 200, 68, 71, 69, 215, 173 44, 225, 43, 173, 198, 240, 96 58, 68, 255, 85, 113, 152, 184, 240 41, 57, 74, 61, 144, 103, 199 28, 39, 68, 81, 237, 210, 170 168, 225, 58, 43, 183, 141, 69, 98 58, 184, 226, 43, 68, 199, 69 214, 229, 43, 172, 187, 68, 69, 104, 152 44, 186, 173, 68, 28, 96, 138 170, 155, 68, 198, 43, 96 214, 173, 229, 58, 216, 68, 100 185, 241, 170, 43, 83, 106

GC and HPLC methods, it is the most confirmatory technique. Certain triazines may also be determined by colorimetric methods. The common trade names, synonyms, and the characeristic mass ions of some triazine herbicides for identification by mass spectrometry are listed in Table 20.8.

20.8

CHLOROPHENOXY ACID HERBICIDES

20.8.1 General Discussion Chlorophenoxy acid herbicides are used in various forms, such as acids, salts, or esters. The acid forms of these herbicides are strong organic acids that can readily react with basic compounds and may be lost during analysis. Because of this, because of their occurrence in various forms, and because of potential interference from other chlorinated organic acids, phenolic compounds, and phthalate esters, the most crucial steps in the analysis of chlorophenoxy acid herbicides are sample extraction, cleanup of the solvent extracts, and derivatization of the chlorophenoxy acids. Complete analysis of these herbicides in all forms (as acids, salts, and esters) involves several tedious steps, including hydrolysis and esterification and repeated extractions with diethyl ether in between. The extraction and analytical steps are briefly outlined below.

20.8.2 Extraction, Derivatization, and Analysis The aqueous samples are acidified with HCl to a pH below 2 to convert any salt or ester forms of herbicides present in the samples into their acids. The acidified sample is then extracted with diethyl ether (peroxide free and stabilized with butylated hydroxytoluene) in a separatory funnel. Chlorophenoxy acids, being soluble in ether, partition into ether. The aqueous phase, containing water-soluble organic interfering substances, is discarded. The ether extract containing the herbicides in acid form is heated carefully with an aqueous solution of potassium hydroxide. This converts the herbicide acids into their water-soluble potassium salts. The product mixture again is repeatedly extracted with ether. While most organic contaminants dissolve in ether, the potassium salts of herbicide acids pass into the aqueous phase. The ether extract, which may contain many types of organic contaminants that are soluble in ether, is discarded. The aqueous solution is then acidified to a pH below 2 to convert the potassium salts of the herbicides back

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ANALYSIS OF PESTICIDES AND HERBICIDES

20.16

SECTION TWENTY

(a)

O Cl

CH2COOH + CH3OH

Cl (2,4-D)

(b)

Cl (2,4-D)

Cl

CH2COOCH3 + H2O

Cl

(2,4-D-methyl ester)

O Cl

O

(methanol)

CH2COOH +

C6F5CH2Br (pentafluorobenzyl bromide)

O Cl

CH2COOCH2C6H5 + HBr

Cl

(2,4-D-pentafluorobenzyl ester)

FIGURE 20.1 Esterification of 2,4-D: (a) methylation; (b) pentafluorobenzylation.

again into their acids. The acidified solution is then repeatedly extracted with diethyl ether. The acid herbicides partition into ether. The aqueous phase is now discarded. The ether extract is concentrated down to a small volume by carefully evaporating the solvent. After removing the interfering substances in the above steps and concentrating the ether extract of the acids to a small volume, the chlorophenoxy acids are converted into their methyl esters. Esterification is carried out either by treatment with boron trifluoride in methanol solution or with diazomethane. In the former, BF3 serves as a catalyst for methylation. The esterification is carried out by heating the mixture for a few minutes in a water bath. The excess methanol and BF3, both water-soluble, are removed from the organic solvent by treatment with water. The methyl esters of chlorophenoxy acids are solvent-exchanged to toluene or hexane. An aliquot of the organic solvent from the top immiscible layer is then injected onto a GC equipped with an ECD for analysis of the methyl ester derivatives. If diazomethane is used for esterification it may be generated in situ from Diazald (N-methylN-nitroso-p-toluenesulfonamide) or other equivalent kits that are commercially available. Alternatively, carbitol (diethylene glycol monoethyl ether) may be employed to produce alcohol-free diazomethane. The sample extracts should be dry before methylation, or the recoveries will be poor. Also, the diethyl ether used for extraction should be stabilized with BHT (butylated hydroxytoluene) and not ethanol. The latter can form ethyl esters of herbicides, lowering the yield of methyl esters. Alternatively, the chlorophenoxy herbicides in their acid forms may be esterified to their pentafluorobenzyl derivatives instead of methyl esters. Derivatization can be carried out with 2,3,4,5,6pentafluorobenzyl bromide. The reactions are shown in Fig. 20.1 for methylation and perfluorobenzylation with 2,4-D as an example. The ester derivatives are determined by GC-ECD or GC/MS. The latter is a confirmatory test, although it is much less sensitive than the GC-ECD. The characteristic mass ions of chlorophenoxy acids and their esters for their identification by GC/MS under electron-impact ionization is listed in Table 20.9. The methyl esters also can be determined by GC-FID. Fused-silica capillary columns containing a stationary phase made of phenyl silicone, methyl silicone, and cyanopropyl phenyl silicone in varying compositions provide excellent resolution. Such columns, commercially available, include, DB5, DB-608, DB-1701, SPB-5, SPB-608, SPB-608, SPB-1701, HP-608, BP-608, AT-1701, Rtx-5, and equivalent. The herbicides are determined as acids, stoichiometrically calculated from the concentration of their esters as follows: Concentration of acid =

concentration of ester × molecular wt. of acid molecular wt. of ester

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ANALYSIS OF PESTICIDES AND HERBICIDES

ANALYSIS OF PESTICIDES AND HERBICIDES

20.17

TABLE 20.9 Primary and Secondary Mass Ions of Herbicide Acids and Esters Chlorophenoxy acids and esters 2,4-D 2,4-D methyl ester 2,4-D butoxyethyl ester 2,4-DB 2,4-DB butyl ester Dicamba Dicamba methyl ester Dichlorprop MCPA MCPA methyl ester MCPA isooctyl ester MCPB MCPP MCPP methyl ester 2,4,5-T 2,4,5-T butyl ester 2,4,5-T isopropyl ester 2,4,5-T methyl ester 2,4,5-TB 2,4,5-TP (Silvex) Silvex methyl ester Silvex isooctyl ester

Primary ion

Secondary ions

162 199 57 57 87 173 203 28 141 141 57 142 142 169 196 57 209 233 87 196 196 57

164, 220, 222, 161, 133, 63, 175 175, 234, 111, 147, 133, 161, 201 29, 56, 85, 220, 145, 320 29, 41, 43, 185, 111, 220, 276 41, 43, 29, 143, 69, 57, 162, 231 175, 220, 222, 174, 203, 191, 97 205, 188, 97, 234, 190, 160 162, 164, 234, 45, 98, 189 200, 77, 125, 143, 51, 155, 202 214, 77, 155, 125, 89, 143, 45 43, 41, 29, 200, 125, 155, 312 107, 43, 45, 87, 144, 77, 51 214, 77, 107, 45, 141, 169, 216 142, 107, 77, 228, 141, 59, 89 198, 254, 256, 200, 209, 167, 97 41, 40, 43, 109, 196, 312 211, 196, 198, 74, 109, 145, 296, 298 235, 45, 268, 270, 73, 209, 145 196, 198, 45, 43, 40, 284, 282 198, 97, 270, 268, 169, 167, 45 198, 59, 225, 223, 55, 87, 200, 284 43, 71, 55, 41, 196, 198, 69, 223

Source: NIST Mass Spectral Library.

Herbicides in their acid form may also be determined by HPLC techniques. Aqueous samples are extracted by SPE methods using ENVI-Carb or equivalent sorbent. Polymeric-coated silica-based HPLC specialty columns are commercially available. A photodiode array detector is interfaced to HPLC for determination of such compounds. No esterification or derivatization is required for analysis when the compounds are present in the samples in their acid form. To determine herbicides in all forms, the sample may be acidified, upon which the esters and salts of chlorophenoxy acids are converted to their acids and then analyzed by HPLC as described above. Typical SPE/HPLC analytical conditions for measuring herbicide acids in water are given as an example in Table 20.10. TABLE 20.10 SPE/HPLC Analysis of Herbicides Extraction: Column: Mobile phase: Flow rate: Gradient program:

Temperature: Detector:

SPE, ENVI-Carb, 6 mL, 250 mg Polymeric-coated silica-based PAH specialty column, 20 cm ⫻ 3-m m ID, 5-µm particles Gradient, A ⫽ water/0.05%H3PO4, B ⫽ acetonitrile 0.5 mL/min Time (min) %B 2.5 40 5.0 60 13.0 60 13.5 40 50°C Photodiode array at 210 and 225 nm; peak width 0.320 s (0.053-min sampling interval)

Source: Supelco Catalog for Chromatography Products for Analysis and Purification, 2003.

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ANALYSIS OF PESTICIDES AND HERBICIDES

20.18

SECTION TWENTY

Bibliography American Public Health Association, American Water Works Association and Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Association, Washington, DC, 1998. Fong, W. G., H. N., Moye, J. N., Seiber, and J. P. Toth, Pesticide Residues in Foods, Wiley, New York, 1999. Milne, G. W. A., ed., CRC Handbook of Pesticides, CRC Press, Boca Raton, FL, 1995. Patnaik, P., Handbook of Environmental Analysis, CRC Press, Boca Raton, FL, 1997. U.S. Environmental Protection Agency, Test Methods for Evaluating Solid Waste, SW-846, National Technical Information Service, Washington, DC, 1997. U.S. Food and Drug Administration, Pesticide Analytical Manual, National Technical Information Service, Washington, DC, 1975.

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 21

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT 21.1 INTRODUCTION 21.2 SAMPLING 21.2.1 Sample Containers, Preservatives, and Holding Time 21.3 SAMPLE EXTRACTION AND DIGESTION 21.3.1 Solid Samples 21.3.2 Nonaqueous Liquid Samples 21.3.3 Aqueous Samples Table 21.1 Sample Containers, Preservatives, and Holding Times 21.4 CLEANUP OF SAMPLE EXTRACTS 21.4.1 Acid–Base Partitioning 21.4.2 Alumina Column Cleanup Table 21.2 Separation of Organic Substances on Alumina Columns 21.4.3 Silica Gel Cleanup 21.4.4 Florisil Column Cleanup 21.4.5 Gel Permeation Cleanup 21.4.6 Permanganate–Sulfuric Acid Cleanup 21.4.7 Sulfur Cleanup Table 21.3 Some Common Cleanup Methods in Organic Analysis 21.5 DETERMINATION OF TRACE ORGANIC POLLUTANTS BY INSTRUMENTAL TECHNIQUES Table 21.4 Characteristic Mass Ions for Identifying Purgeable Organics Table 21.5 Characteristic Mass Ions for Identifying Some Semivolatile Organics 21.6 ANALYSIS OF INORGANIC ANIONS 21.6.1 Colorimetric Methods 21.6.2 Titrimetry Table 21.6 Gas-Phase Group Frequencies of Various Classes of Compounds 21.6.3 Capillary Ion Electrophoresis 21.6.4 Ion Chromatography Table 21.7 Most Intense IR Peak for Compound Identification and Quantitation 21.7 DETERMINATION OF GENERAL AND AGGREGATE PROPERTIES OF SAMPLES 21.7.1 pH Table 21.8 Measurement of Inorganic Anions at Trace Concentrations by Various Analytical Techniques 21.7.2 Acidity Table 21.9 Determination of Inorganic Anions by Colorimetric Methods 21.7.3 Alkalinity Table 21.10 Determination of Common Inorganic Anious by Titrimetric Methods Table 21.11 Alkalinity Relationships 21.7.4 Conductivity 21.7.5 Solids 21.7.6 Hardness 21.7.7 Chemical Oxygen Demand

21.2 21.2 21.3 21.3 21.3 21.3 21.3 21.4 21.7 21.7 21.7 21.7 21.8 21.8 21.8 21.8 21.8 21.9 21.9 21.10 21.13 21.16 21.16 21.16 21.17 21.18 21.18 21.19 21.20 21.20 21.21 21.24 21.25 21.26 21.27 21.27 21.28 21.28 21.28 21.29 21.1

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21.2

SECTION TWENTY-ONE

21.7.8 Biochemical Oxygen Demand 21.7.9 Organic Carbon 21.7.10 Oil and Grease 21.7.11 Kjeldahl Nitrogen 21.8 DISSOLVED GASES IN WATER 21.9 METALS 21.9.1 Flame and Furnace Atomic Absorption Spectrophotometry Table 21.12 Determination of Dissolved Gases in Water Table 21.13 Recommended Wavelength, Flame Type, and Technique for Flame Atomic Absorption Analysis 21.9.2 Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) Table 21.14 Recommended Wavelength and Instrument Detection Level for ICP-AES 21.9.3 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) 21.9.4 Sample Digestion Table 21.15 Acid Combinations for Sample Preparation 21.9.5 Chelation Extraction Method 21.9.6 Hydride Generation Method 21.9.7 Cold Vapor Method for Measuring Mercury Bibliography

21.1

21.29 21.30 21.30 21.30 21.31 21.31 21.31 21.32 21.34 21.34 21.35 21.35 21.35 21.36 21.36 21.36 21.36 21.36

INTRODUCTION The analysis of trace pollutants in environmental matrices, such as wastewaters, groundwaters and surface waters, soils, sediments and solid wastes, and the ambient and atmospheric air, has grown into a major, fully pledged discipline of analytical science in recent years. This may be attributed to growing concern about pollution of the environment and numerous legislations and regulatory requirements that have been adopted to abate this problem. The first step in understanding the problem and subsequently controlling it rests on the identification and accurate measurements of toxic pollutants and their degradation products at trace levels. This involves measuring various organic substances, metal ions, inorganic anions, radionuclides, microorganisms, and certain physicochemical properties of environmental waters and solid wastes by various physical, chemical, and instrumental methods. A full discussion of the subject is beyond the scope of this text.

21.2

SAMPLING If water from a faucet is to be collected, allow the water to run into the sink for a few minutes before collecting the sample. If water is to be sampled from a river or a lake, collect the samples from an appropriate depth from the middle of the river or the lake or select the locations where the levels of pollutants need to be monitored. If the sample is taken in a single lot for analysis, it is called a grab sample. However, it may often be necessary to collect a more representative sample than a grab sample. In such cases, several samples are collected at different but nearby locations or are taken from the same locations at different intervals. Such samples are homogenized and mixed thoroughly. They are termed composite samples. Samples, especially soils, sediments, and solid wastes, are often composited for analysis. For such analysis, any stone, rock, grass, or any foreign material such as a piece of wood that may be found in the soil or the sediment but does not represent the sample matrix, should be removed from the sample before homogenizing.

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21.3

21.2.1 Sample Containers, Preservatives, and Holding Time Samples are collected in wide-mouthed glass or plastic bottles with screw caps. Substances in the matrix that are susceptible to photochemical degradation must be collected in amber glass bottles or vials. Usually, glass containers are suitable for all types of sampling; in certain cases, however, such as trace analysis of boron metal, borosilicate glass containers must not be used. Plastic containers are suitable for collecting samples, for most inorganic analytes but not for measuring total organic carbon (TOC) or any individual organic analytes. Many substances are susceptible to decomposition at ambient conditions because of oxidation, volatilization, or microbial degradation. Such degradation can significantly affect the accuracy of measurement at trace concentrations. To prevent such loss of certain types of analytes from the samples, preservatives may be added to the samples or to the sample container bottles before sampling, to retain the integrity of the analytes. For example, samples for metal analysis are acidified to prevent any loss of metals due to possible ion exchange. This acidification enhances the stability of the samples and their holding times. Even with preservatives added, samples tend to degrade over time and therefore must be tested for the respective analytes within their holding times. All samples must be placed in a refrigerator at a temperature of 4°C or below to minimize any volatilization, if tests cannot be performed immediately. The samples must be brought back to room temperature before analysis. The nature of the sample containers, the preservatives to be added, and the holding times of samples for various tests are summarized in Table 21.1.

21.3

SAMPLE EXTRACTION AND DIGESTION Most analyses of trace pollutants require extracting the pollutants into an appropriate solvent and concentration of the solutions into smaller volumes before analysis. This is essential for practically all soil, sediment, and solid-waste analysis and for many types of water testing, especially those pertaining to organic analytes. Also, reduction of the volume of the solvent extracts by careful evaporation can lower the detection limits of trace pollutants by severalfold. Some common extraction methods are briefly outlined below.

21.3.1 Solid Samples An accurately weighed quantity of a solid sample, such as soil, sediment, or waste, is mixed with a solvent and subject to sonication or Soxhlett extraction. Alternatively, the sample may be shaken with solvent on a mechanical shaker or stirred using a magnetic stirrer. If the analytes are soluble in water, the latter may be used as the solvent for extraction. For the extraction of organic pollutants, acetone, methanol, methylene chloride, and hexane are some of the common solvents that are used. 21.3.2 Nonaqueous Liquid Samples Many nonaqueous liquid samples, such as waste oil, may simply be diluted with a solvent in which the liquid is soluble. Dilution is used in determining certain types of organic pollutants such as polychlorinated biphenyls (PCBs). Since fuel oils and other petroleum products are essentially hydrocarbon in nature, a hydrocarbon solvent such as n-hexane, isooctane, or toluene may be used to dilute liquid wastes before their analysis. 21.3.3 Aqueous Samples Trace organic pollutants are extracted from aqueous samples into appropriate organic solvents by several methods. These techniques fall primarily into two categories, liquid–liquid extraction (LLE)

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.4

SECTION TWENTY-ONE

TABLE 21.1 Sample Containers, Preservatives, and Holding Times

Analyte

Container

Maximum holding time

Preservation

Inorganics and microbial tests Acidity Alkalinity Bacterias, coliform (total and fecal)

P, G P, G P, G

Biochemical oxygen demand Bromide Chloride Chlorine, residual

P, G

Cool, 4°C Cool, 4°C Cool, 4°C; add 0.008% Na2S2O3 if residual chlorine is present Cool, 4°C

14 d 14 d 6h

P, G P, G P, G

None required None required None required

Chemical oxygen demand Color Cyanide

P, G

Cool, 4°C, H2SO4 to pH < 2

P, G P, G

Fluoride Hardness Iodine

P P, G P, G

Cool, 4°C Cool, 4°C, pH > 12, 0.6 g ascorbic acid None required pH < 2 with HNO3 or H2SO4 None required

Kjeldahl nitrogen Metals (except chromium-VI, boron and mercury) Chromium-VI Mercury Boron Nitrate Nitrite Odor

P, G P, G

Cool, 4°C, pH < 2 with H2SO4 HNO3 to pH < 2

P, G P, G P P, G P, G G

Cool, 4°C HNO3 to pH < 2 HNO3 to pH < 2 Cool, 4°C, H2SO4 to pH < 2 Cool, 4°C None required

Oil and grease Oxygen, dissolved

G G (BOD bottle)

Cool, 4°C, H2SO4 or HCl to pH < 2 None required

pH

P, G

None required

Phenolics Phosphorus Elemental Orthophosphate Total Residue Total Filterable Nonfilterable (TSS) Settleable Volatile Silica Specific conductance Sulfate

G

Cool 4°C, H2SO4 to pH < 2

24 h 28 d 28 d 28 d 48 h Analyze immediately 28 d Analyze immediately Analyze immediately 28 d

G P, G P, G

Cool, 4°C Cool, 4°C Cool, 4°C, H2SO4 to pH < 2

48 h 48 h 28 d

P, G P, G P, G P, G P, G P P, G P, G

Cool, 4°C Cool, 4°C Cool, 4°C Cool, 4°C Cool, 4°C Cool, 4°C Cool, 4°C Cool, 4°C

7d 7d 7d 48 h 7d 28 d 28 d 28 d

48 h 28 d 28 d Analyze immediately 28 d 48 h 14 d 28 d 6 mo Analyze immediately 28 d 6 mo

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.5

TABLE 21.1 Sample Containers, Preservatives, and Holding Times (Continued )

Analyte

Container

Preservation

Maximum holding time

Inorganics and microbial tests Sulfide

P, G

Cool, 4°C, zinc acetate plus NaOH to pH > 9 None required

Sulfite

P, G

Surfactants Taste Temperature Total organic carbon Total organic halogen

P, G G P, G G G (amber bottles)

Turbidity

P, G

Purgeable halocarbons

G (Teflon-lined septum)

Purgeable aromatics

G (Teflon-lined septum)

Pesticides, chlorinated

G (Teflon-lined cap)

PCBs

G (Teflon-lined cap)

Cool, 4°C

Phthalate esters

G (Teflon-lined cap)

Cool, 4°C

Nitroaromatics

G (Teflon-lined cap)

Nitrosamines

G (Teflon-lined cap)

Polynuclear aromatic hydrocarbons

G (Teflon-lined cap)

Haloethers

G (Teflon-lined cap)

Cool, 4°C (add 0.008% Na2S2O3, if residual chlorine present), store in dark Cool, 4°C (add 0.008% Na2S2O3, if residual chlorine present), store in dark Cool, 4°C (add 0.008% Na2S2O3, if residual chlorine present), store in dark Cool, 4°C (add 0.008% Na2S2O3 if residual chlorine present)

Cool, 4°C Cool, 4°C None required Cool, 4°C, HCl or H2SO4 to pH < 2 Cool, 4°C, store in dark, HNO3 to pH < 2, add Na2SO3 if residual chlorine present Cool, 4°C

7d Analyze immediately 48 h 24 h Analyze 28 d 14 d

48 h

Organics tests Cool, 4°C, no headspace (add 0.008% Na2S2O3 if residual chlorine is present) Cool, 4°C, no headspace (add 0.008% Na2S2O3 if residual chlorine is present), HCl to pH < 2 Cool, 4°C, pH 5–9

14 d

14 d

7 d until extraction; 40 d after extraction 7 d until extraction; 40 d after extraction 7 d until extraction; 40 d after extraction 7 d until extraction 40 d after extraction 7 d until extraction; 40 d after extraction 7 d until extraction; 40 d after extraction 7 d until extraction; 40 d after extraction (Continued)

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.6

SECTION TWENTY-ONE

TABLE 21.1 Sample Containers, Preservatives, and Holding Times (Continued )

Analyte

Container

Preservation

Maximum holding time

Organics tests Phenols

G (Teflon-lined cap)

Cool, 4°C (add 0.008% Na2S2O3 if residual chlorine present)

Dioxins and dibenzofurans

G (Teflon-lined cap)

Cool, 4°C (add 0.008% Na2S2O3 if residual chlorine present)

7 d until extraction; 40 d after extraction 7 d until extraction; 40 d after extraction

Note: P, polyethylene; G, glass. If there is no residual chlorine in the sample, the addition of Na2S2O3 may be omitted. Source: P. Patnaik, Handbook of Environmental Analysis, CRC Press, Boca Raton, Fla., 1997.

and solid-phase extraction (SPE). Extraction units are commercially available for continuous liquid–liquid extraction. In the LLE method, extraction may be carried out manually using a separatory funnel. In this procedure, an aqueous sample, 1 L in volume, is put into a 2-L separatory funnel. The sample is then repeatedly shaken with three 50–60-mL aliquots of methylene chloride. The solvent extracts are then combined and the solution is evaporated slowly in a water bath into a small volume, usually 1–2 mL. The solvent extract is then passed through a thin bed of anhydrous Na2SO4 to remove any water present in it. Thus a 1-L sample is concentrated down to a volume of 1 mL, thereby lowering the detection limits for trace pollutants by a factor of 1000. Alternatively, LLE may be carried out using a continuous liquid–liquid extractor. Such apparatus are commercially available. Although the process is automatic, proceeds continuously, and does not involve manual shaking of the samples with the extraction solvent in a separatory funnel, the process takes much longer and requires a relatively large volume of solvent. In recent years, solid-phase extraction has been slowly replacing the LLE methods. The advantages of solid-phase extraction are low cost, faster speed, and ease of operation. In SPE methods, sample aliquots, usually in much smaller volumes than the amounts employed in the LLE methods, are passed through small cartridges packed with adsorbent materials (such as C-18 or other hydrocarbons, often functionalized). The packing material is designed to trap the pollutants or interfering substances in the samples. Analytes retained on the cartridges are eluted out with an appropriate solvent or a solvent mixture. In contrast to separatory funnel extraction, microextraction processes are very rapid and may be applied to extract organic analytes in potable and nonpotable waters. In such extractions about 30 mL of sample are treated with 1–2 mL of hexane in a 40-mL vial. The vial is capped and shaken for 2–3 min. The solution is then allowed to stand until the immiscible hexane layer on top separates out from the denser water layer at the bottom. A few microliters of solution from the top hexane layer are then carefully withdrawn and injected onto a gas chromatograph (GC). Although microextraction in environmental analysis usually involves separation of anlytes into a lighter, water-immiscible solvent such as hexane, heavier, water-immiscible solvents, such as methylene chloride, have been found to be effective as well. In using the latter, a small aliquot of the solvent extract is carefully withdrawn from the bottom layer and allowed to pass through anhydrous Na2SO4. Water removal from the solvent extract is essential if methylene chloride is used in such a microextraction.

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21.4

21.7

CLEANUP OF SAMPLE EXTRACTS Sample extracts often contain interfering substances, which may be concentrated along with the analytes. Such substances may co-elute or produce extraneous peaks in the chromatograms of the analytes, affecting the resolution efficiency of the column. Also, they may cause loss of detector sensitivity and shorten the life of the column. Solvent extracts obtained from aqueous or nonaqueous samples may be purified by one or more of the following techniques.

21.4.1 Acid–Base Partitioning Acid–base partitioning is used to separate acidic pollutants from basic pollutants or vice versa. Some U.S. Environmental Protection Agency (EPA) methods, such as Methods 625, 8250, and 8270, have adopted acid–base partitioning techniques for the extraction of organic pollutants from aqueous matrices. The method applies to separate acidic pollutants from basic and neutral pollutants during sample extraction. The technique may also be applied for cleanup purposes based on the acidic or basic properties of the interfering substances. In such cleanup procedures, the solvent extract is shaken with water that is highly basic. The acidic analytes, such as phenols, partition from the organic phase into the basic aqueous phase. The organic phase now contains the basic and neutral analytes. The aqueous solution is then acidified with HCl to a pH below 2 and shaken with ethylene chloride. The acidic analytes from the aqueous phase partition into ethylene chloride, separating out from the acidified water. Thus, we now have two separate ethylene chloride solutions, one containing basic analytes and the other acidic compounds only.

21.4.2 Alumina Column Cleanup Alumina column cleanup is based on separating analytes from interfering substances in the solvent extracts by virtue of differences in polarity. The column is packed with highly porous granular aluminum oxide. The latter is available in three pH ranges—acidic, neutral, and basic. All three grades of alumina are used in column cleanup. The acidic form of alumina, which has a pH of 4–5, is usually used to separate strong acids and acid pigments. Basic alumina, pH 9–10, is applied to separate basic compounds, such as alkaline solutions, amines, and alkaloids. The neutral form of alumina is less active than the basic form and may be used to separate aldehydes, ketones, and esters. Some applications of various grades of alumina are summarized in Table 21.2. Alumina of various activity grades can be selectively prepared by adding water to dehydrated alumina. The latter is obtained by heating alumina above 400°C until no more water is lost.

TABLE 21.2 Separation of Organic Substances on Alumina Columns Alumina grades Basic alumina (pH 9–10) Neutral alumina (pH 6–8) Acidic alumina (pH 4–5)

Organic compounds Alkaloids, amines, pyridine nitrosamines, steroids, and basic pigments Aldehydes, ketones, alcohols, alkyl esters, phthalates, lactones, and petroleum waste Carboxylic acids, chlorophenoxy acids, phenolics, and acid pigments

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21.8

SECTION TWENTY-ONE

Alumina is covered under anhydrous Na2SO4 in the column before loading the extract. Analytes are eluted with a suitable solvent or solvent mixtures, leaving behind interfering substances adsorbed onto the column.

21.4.3 Silica Gel Cleanup Silica gel is a weakly acidic form of amorphous silica. For cleanup purposes it may be prepared by treating sodium sulfate with H2SO4. An activated form of silica gel may be made by heating the latter at 150°C for several hours. Incorporating 10–20% water may produce a deactivated form of silica gel. The latter is used to separate alkaloids, steroids, terpenoids, dyes, plasticizers, sugars, esters, and lipids. Activated silica gel may be used to separate hydrocarbons.

21.4.4 Florisil Column Cleanup Florisil, a form of magnesium silicate, is used to remove interfering substances from sample extracts in the analysis of many types of pesticides, phthalates, haloethers, nitrosamines, and chlorinated hydrocarbons. Like silica gel, florisil exhibits weak acid properties. In addition to cleanup of sample extracts, florisil is also used to separate esters, ketones, alkaloids, steroids, glycerides, and certain types of carbohydrates.

21.4.5 Gel Permeation Cleanup Gel permeation cleanup (GPC) is applicable for separating macromolecules such as proteins, lipids, polymers, and steroids from sample extracts. The gel is porous and hydrophobic, having a pore size greater than the size of the molecules to be separated. The solvent extract is loaded onto the GPC column and the analytes are eluted out using a suitable solvent.

21.4.6 Permanganate–Sulfuric Acid Cleanup Permanganate–sulfuric acid cleanup is applied to separate oxidizable interfering substances in the sample extract from analytes such as polychlorinated biphenyls, which are highly stable under strong oxidizing conditions. The extract is heated with a strong oxidant, such as a mixture of KMnO4 and H2SO4, for a short time, during which interfering substances in the extract are destroyed.

21.4.7 Sulfur Cleanup Sulfur occurs in many industrial wastewaters, sludges, sediments, and marine algae. Its presence can mask the regions of interest. Also, its mass spectra, corresponding to the mass ions S2, S4, and S6, can interfere in the determination of compounds that produce similar primary or secondary characteristic mass ions. In addition, the solubility of sulfur in most organic solvents is similar to that in some of the organochlorine and organophosphorus pesticides. It therefore cannot be removed by florisil cleanup. Sulfur present in the organic solvent extract of a sample can be effectively removed by shaking the extract with a small quantity of either copper (powder or turning) or mercury, which settles to the bottom and leaves a clean extract solution on the top that can be injected onto a GC column. Alternatively, sulfur may be separated from the extract by vigorously shaking the extract with tetrabutyl ammonium–sodium sulfite reagent. Some common cleanup methods in trace organic analysis are summarized in Table 21.3.

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.9

TABLE 21.3 Some Common Cleanup Methods in Organic Analysis Technique Acid–base partitioning GPC

Alumina

Silica gel Florisil Sulfur cleanup Permanganate–sulfuric acid

Analyte groups Amines, imines, amides, nitrosamines, phenols, carboxylic acids, chlorophenoxy acid herbicides Polynuclear aromatics, nitroaromatics, nitrosamines, phenols, phthalate esters, chlorinated hydrocarbons, PCBs, organophosphorus pesticides, organochlorine pesticides, cyclic ketones Amines, alkaloids, nitrosamine aldehydes, ketones, esters, alcohols, phenols, carboxylic acids, chlorophenoxy acid herbicides, polynuclear aromatics Polynuclear aromatics, PCBs, certain chlorinated pesticides, derivatized phenolic compounds Organochlorine pesticides, organophosphorus pesticides, PCBs, phthalates, nitrosamines, nitroaromatics, haloethers, chlorinated hydrocarbons PCBs PCBs and compounds that are chemically stable under strong oxidizing conditions

21.5 DETERMINATION OF TRACE ORGANIC POLLUTANTS BY INSTRUMENTAL TECHNIQUES Trace organic pollutants extracted from sample matrices into an appropriate solvent as outlined above are then analyzed by one or more of the following instrumental techniques. Because of the concentration of the solvent extract to a much smaller final volume and the high sensitivity of the instrument, a detection level below 1 ng/L (ppt) may be obtained for many types of pollutants. The detection levels of analytes depend on the instrument and the detector used. Gas chromatography and high-performance liquid chromatography (HPLC) are most commonly employed in environmental analysis. While the most sensitive GC detectors for determining halogenated organics are the electron-capture detector (ECD) and the Hall electrolytic conductivity detector (HECD), the photoionization detector (PID) is sensitive to substances containing double bonds, such as olefins and aromatics, the nitrogen–phosphorus detector (NPD) is sensitive to nitrogen- or phosphorus-containing organics in N or P mode, respectively, and the flame photometric detector (FPD) can accurately determine trace organics containing sulfur or phosphorus. If an analyte is suspected to be present in the sample extract based on its retention time on the GC column, determined from injecting its standard solution, its presence must be confirmed either on an alternative GC column or preferably by mass spectrometry (MS). In laboratories that are not equipped with a mass spectrometer, a dual-column, dual-injector GC should be adequate to carry out these organic analyses. In all such analyses, an aliquot of the sample extract is simultaneously injected onto the second (or confirmatory) GC column. Mass spectrometry is by far the best confirmatory test for the presence of a substance in a sample. A mass spectrometer is interfaced to a GC or a liquid chromatography (LC) column to identify products after their separation on the GC or LC column. The presence of a compound is determined from its characteristic mass spectra, as well as from its retention time. The characteristic mass ions of various types of common pollutants are listed in Tables 21.4 and 21.5. The mass ions of pesticide and herbicide classes of compounds are not presented in these tables; they are listed separately in Sec. 20. Table 21.4 lists the primary and secondary mass ions of volatile organic compounds (VOCs) that may be purged from an aqueous sample with an inert gas for identification. Table 21.5 lists the characteristic mass ions of nonvolatile or semivolatile substances.

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21.10

SECTION TWENTY-ONE

TABLE 21.4 Characteristic Mass Ions for Identifying Purgeable Organics Compounds Alcohols Allyl alcohol 1-Butanol 2-Butanol Isobutyl alcohol Ethanol Propargyl alcohol

Primary ion (m/z)

Secondary ions (m/z)

57 56 74 43 31 55

58, 39 41 43 41, 42, 74 45, 27, 46 39, 38, 53

Aldehydes and ketones Acetone Acrolein Methyl ethyl ketone Methyl isobutyl ketone 2-Hexanone

58 56 72 100 43

43 55, 58 43 43, 58, 85 58, 57, 100

Aromatics Benzene n-Butylbenzene sec-Butylbenzene tert-Butylbenzene Ethylbenzene Isopropylbenzene n-Propylbenzene Styrene Toluene o-Xylene m-Xylene p-Xylene p-Isopropyltoluene 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene

78 91 105 119 91 105 91 104 92 106 106 106 119 105 105

— 92, 134 134 91, 134 106 120 120 78 91 91 91 91 134, 91 120 120

Esters Ethyl acetate Ethyl methacrylate Methyl acrylate Methyl methacrylate Vinyl acetate

88 69 55 69 43

43, 45, 61 41, 99, 86, 114 85 41, 100, 39 86

Nitriles Acetonitrile Acrylonitrile Malononitrile Methacrylonitrile Propionitrile

40 53 66 41 54

40, 39 52, 51 39, 65, 38 67, 39, 52, 66 52, 55, 40

76 91 136 156 128 83 173

41, 39, 78 126, 65, 128 43, 138, 93, 95 77, 158 49, 130 85, 127 175, 254

Halogenated organics Allyl chloride Benzyl chloride Bromoacetone Bromobenzene Bromochloromethane Bromodichloromethane Bromoform

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

TABLE 21.4 Characteristic Mass Ions for Identifying Purgeable Organics (Continued) Compounds

Primary ion (m/z)

Secondary ions (m/z)

Halogenated organics Bromomethane Chlorobenzene 1-Chlorobutane Chlorodibromomethane Chloroethane 2-Chloroethyl vinyl ether 2-Chloroethanol Chloroprene 2-Chlorotoluene 1,2-Dibromo-3-chloropropane Dibromochloromethane 1,2-Dibromoethane Dibromomethane 1,2-Dichlorobenzene 1,3-Dichlorobenzene 1,4-Dichlorobenzene cis-1,4-Dichloro-2-butene trans-1,4-Dichloro-2-butene Dichlorodifluoromethane 1,1-Dichloroethane 1,2-Dichloroethane 1,1-Dichloroethene cis-1,2-Dichloroethene trans-1,2-Dichloroethene 1,2-Dichloropropane 1,3-Dichloropropane 2,2-Dichloropropane 1,3-Dichloro-2-propanol 1,1-Dichloropropene cis-1,3-Dichloropropene trans-1,3-Dichloropropene Epichlorohydrin Hexachlorobutadiene Hexachloroethane Methylene chloride Methyl iodide Pentachloroethane 1,2,3-Trichlorobenzene 1,2,4-Trichlorobenzene 1,1,1,2-Tetrachloroethane 1,1,2,2-Tetrachloroethane Tetrachloroethene 1,1,1-Trichloroethane 1,1,2-Trichloroethane Trichloroethene Trichlorofluoromethane 1,2,3-Trichloropropane Vinyl chloride

94 112 56 129 64 63 49 53 91 75 129 107 93 146 146 146 75 53 85 63 62 96 96 96 63 76 77 79 75 75 75 57 225 201 84 142 167 180 180 131 83 164 97 83 95 151 75 62

96 77, 114 49 208, 206 66 65, 106 44, 43, 51, 80 88, 90, 51 126 155, 157 127 109, 188 95, 174 111, 148 111, 148 111, 148 53, 77, 124, 89 88, 75 87 65, 83 98 61, 63 61, 98 61, 98 112 78 97 43, 81, 49 110, 77 77, 39 77, 39 49, 62, 51 223, 227 166, 199, 203 86, 49 127, 141 130, 132 182, 145 182, 145 133, 119 131, 85 129, 131, 166 99, 61 97, 85 97, 130, 132 101, 153 77 64

Miscellaneous compounds Carbon disulfide

76

78 (Continued)

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21.11

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.12

SECTION TWENTY-ONE

TABLE 21.4 Characteristic Mass Ions for Identifying Purgeable Organics (Continued) Primary ion (m/z)

Secondary ions (m/z)

Miscellaneous compounds Nitrobenzene Ethylene oxide 1,4-Dioxane n-Propylamine Pyridine bis(2-Chloroethyl) sulfide 2-Picoline b-Propiolactone Diethyl ether Methyl-tert-butyl ether

123 44 88 59 79 109 93 42 74 73

51, 77 43, 42 58, 43, 57 41, 39 52 111, 158, 160 66, 92, 78 43, 44 45, 59 57

Internal standards/surrogates Benzene-d6 Bromobenzene-d5 Bromochloromethane-d2 1,4-Difluorobenzene Chlorobenzene-d5 1,4-Dichlorobenzene-d4 4-Bromofluorobenzene Dibromofluoromethane Toluene-d8 Pentafluorobenzene Fluorobenzene

84 82 51 114 117 152 95 113 98 168 96

83 162 131 — — 115, 150 174, 176 — — — 77

Compounds

Another technique to confirm the presence of trace organic pollutants is GC interfaced with Fourier-transform infrared spectrometry (FT-IR). This method may serve as a useful complement to GC/MS analysis. The analytes in the sample extracts are separated by capillary GC and the target analytes are detected and quantified by FT-IR. The compound classes are determined from infrared group absorption frequencies. The possible presence of a particular compound class may be inferred from the presence of an infrared band in the appropriate group frequency region. In this technique, a temperature-programmable gas chromatograph equipped with a capillary column is interfaced to an FT-IR spectrometer. The infrared spectrum of the analyte is visually compared with the search library spectrum of the most promising online library search hits. The five most intense, sharp, and well-resolved IR bands of the analytes are compared with the corresponding bands in the library spectrum. The FT-IR stretching frequencies of the compound groups in the gas phase may be 0–30 cm−1 higher in frequency than those of the condensed phase. Additionally, as a faster confirmation of the presence of a compound in the sample extract, the relative retention time of the analyte can be compared with an authentic standard of the same compound. Quantitation may be done by two methods: (1) integrated absorbance technique or (2) maximum absorbance IR band technique. In both these methods, the concentrations of compounds in the samples are determined from standard calibration curves. The standard calibration curves are constructed by plotting concentrations versus integrated infrared absorbance or maximum infrared band intensity. The working range of analyte concentration should fall within the calibration curve, which should span at least one order of magnitude. A medium-band mercury–cadmium–tellurium (MCT) detector that can reach 650 cm−1 should be suitable for the purpose.

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

TABLE 21.5 Characteristic Mass Ions for Identifying Some Semivolatile Organics Compound Acenaphthene Acenaphthylene Acetopheonone 2-Acetylaminofluorene 1-Acetyl-2-thiourea 2-Aminoanthraquinone Aminoazobenzene 4-Aminobiphenyl 3-Amino-9-ethylcarbazole Aniline o-Anisidine Anthracene Benzidine Benzoic acid Benz(a)anthracene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(g,h,i)perylene Benzo(a)pyrene p-Benzoquinone Benzyl alcohol bis(2-Chloroethoxy)methane bis(2-Chloroethyl)ether bis(2-Chloroisopropyl)ether bis(2-Ethylhexyl)phthalate 4-Bromophenyl phenyl ether Bromoxynil Butyl benzyl phthalate Carbofuran 4-Chloroaniline 1-Chloronaphthalene 2-Chloronaphthalene 2-Chlorophenol 4-Chloro-1,2-phenylenediamine 4-Chlorophenyl phenyl ether Chrysene p-Cresidine 2-Cyclohexyl-4,6-dinitrophenol 2,4-Diaminotoluene Dibenz(a,j)acridine Dibenz(a,h)anthracene Dibenzofuran Dibenzo(a,e)pyrene Di-n-butyl phthalate Dichlorobenzene (all isomers) 3,3′-Dichlorobenzidine Dichlorophenol (2,4- and 2,6-) Diethyl phthalate Diethyl stilbestrol Diethyl sulfate Dihydrosaffrole

Primary ion (m/z)

Secondary ions (m/z)

154 152 105 181 118 223 197 169 195 93 108 178 184 122 228 252 252 276 252 54 108 93 93 45 149 51 277 149 164 127 162 162 128 142 204 228 122 231 121 279 278 168 302 149 146 252 162 149 268 139 135

153, 152 151, 153 71, 51, 120 180, 223, 152 43, 42, 76 167, 195 92, 120, 65, 77 168, 170, 115 210, 181, 127 66, 65 80, 123, 52 176, 179 92, 185 105, 77 229, 226 253, 125 253, 125 138, 277 253, 125 108, 82, 80, 52 79, 77 95, 123 63, 95 77, 121 167, 279 77, 248, 250, 50, 63 279, 88, 275, 168 91, 206 149, 131, 122 129, 65, 92 127, 164 127, 164 64, 130 80, 114, 144 206, 141 226, 229 94, 137, 77, 93 185, 41, 193, 266 122, 94, 77, 104 280, 277, 250 139, 279 139 151, 150, 300 150, 104 148, 111 254, 126 164, 98 177, 150 145, 107, 239, 121, 159 45, 59, 99, 111, 125 64, 77 (Continued)

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21.13

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.14

SECTION TWENTY-ONE

TABLE 21.5 Characteristic Mass Ions for Identifying Some Semivolatile Organics (Continued) Compound

Primary ion (m/z)

Secondary ions (m/z)

3,3′-Dimethoxybenzidine Dimethylaminoazobenzene 7,12-Dimethylbenz(a)anthracene 3,3′-Dimethylbenzidine 2,4-Dimethylphenol Dimethyl phthalate 1,2-Dinitrobenzene 1,3-Dinitrobenzene 1,4-Dinitrobenzene 4,6-Dinitro-2-methylphenol 2,4-Dinitrophenol 2,6-Dinitrophenol Dinitrotoluene (all isomers) Diphenylamine 5,5-Diphenylhydantoin 1,2-Diphenylhydrazine Di-n-octyl phthalate Ethyl carbamate Ethyl methanesulfonate Fluoranthene Fluorene Hexachlorobenzene Hexachlorobutadiene Hexachlorocyclopentadiene Hexachloroethane Hexachlorophene Hexachloropropene Hexamethylphosphoramide Hydroquinone Indeno(1,2,3-cd)pyrene Isodrin Isophorone Isosafrole Maleic anhydride 3-Methylcholanthrene 4,4′-Methylenebis(2-chloroaniline) Methyl methanesulfonate 2-Methylnaphthalene Methylphenol (all isomers) Naphthalene 1,4-Naphthoquinone 1-Naphthylamine 2-Naphthylamine Nicotine 5-Nitroacenaphthene 2-Nitroaniline 3-Nitroaniline 5-Nitro-o-anisidine Nitrobenzene 4-Nitrobiphenyl Nitrophenol (all isomers)

244 225 256 212 122 163 168 168 168 198 184 162 165 169 79 77 149 62 79 202 166 284 225 237 117 196 213 135 110 276 193 82 162 54 268 254 80 142 107 128 158 143 143 84 199 65 138 168 77 199 139

201, 229 120, 77, 105, 148, 62 241, 239, 120 106, 196, 180 107, 121 194, 164 50, 63, 74 76, 50, 75, 92, 122 75, 50, 76, 92, 122 51, 105 63, 154 164, 126, 98, 63 63, 89 168, 167 77, 80, 107 105, 182 167, 43 44, 45, 74 109, 97, 45, 65 101, 203 165, 167 142, 249 223, 227 235, 272 201, 199 198, 209, 211, 406, 408 211, 215, 117, 106, 141 44, 179, 92, 42 81, 53, 55 138, 227 66, 195, 263, 265, 147 95, 138 131, 104, 77, 51 98, 53, 44 252, 253, 126, 134, 113 134, 253, 210, 118 79, 65, 95 141 108, 77, 79, 90 129, 127 104, 102, 76, 50, 130 115, 89, 63 115, 116 133, 161, 162 152, 169, 141, 115 92, 138 108, 92 79, 52, 138, 153, 77 123, 65 152, 141, 169, 151 109, 65

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

TABLE 21.5 Characteristic Mass Ions for Identifying Some Semivolatile Organics (Continued) Primary ion (m/z)

Secondary ions (m/z)

5-Nitro-o-toluidine Nitroquinoline-1-oxide N-Nitrosodi-n-butylamine N-Nitrosodiethylamine N-Nitrosodimethylamine N-Nitrosomethylethylamine N-Nitrosodiphenylamine N-Nitrosodi-n-propylamine N-Nitrosomorpholine N-Nitrosopiperidine N-Nitrosopyrrolidine Octamethyl pyrophosphoramide 4,4′-Oxydianiline Pentachlorobenzene Pentachloronitrobenzene Pentachlorophenol Phenacetin Phenanthrene Phenobarbital Phenol 1,4-Phenylenediamine Phthalic anhydride 2-Picoline Piperonyl sulfoxide Pronamide Propylthiouracil Pyrene Pyridine Resorcinol Safrole Strychnine 1,2,4,5-Tetrachlorobenzene 2,3,4,6-Tetrachlorophenol Tetraethyl pyrophosphate Thiophenol Toluene diisocyanate o-Toluidine 1,2,4-Trichlorobenzene 2,4,6-Trichlorophenol 2,4,5-Trimethylaniline Trimethyl phosphate 1,3,5-Trinitrobenzene

152 174 84 102 42 88 169 70 56 114 100 135 200 250 237 266 108 178 204 94 108 104 93 162 173 170 202 79 110 162 334 216 232 99 110 174 106 180 196 120 110 75

77, 79, 106, 94 101, 128, 75, 116 57, 41, 116, 158 42, 57, 44, 56 74, 44 42, 43, 56 168, 167 42, 101, 130 116, 86, 30, 42 42, 55, 56, 41 41, 42, 68, 69 44, 199, 286, 153, 243 108, 171, 80, 65 252, 108, 248, 215, 254 142, 214, 249, 295, 265 264, 268 180, 179, 109, 137 176, 179 117, 232, 146, 161 65, 66 80, 53, 54, 52 76, 50, 148 66, 92 135, 105, 77 175, 145, 109, 147 142, 114, 83 200, 203 52, 51 81, 82, 53, 69 104, 77, 103, 135 335, 333 214, 179, 108, 143, 218 131, 230, 166, 234, 168 155, 127, 81, 109 66, 109, 84 145, 173, 146, 132, 91 107, 77, 51, 79 182, 145 198, 200 135 79, 95, 109, 140 74, 213, 120, 91, 63

Internal standards and surrogates 1,4-Dichlorobenzene-d4 (IS) Naphthalene-d8 (IS) Acenaphthene-d10 (IS) Phenanthrene-d10 (IS) Chrysene-d12 (IS) Perylene-d12 (IS) 2-Fluorophenol (surr)

152 136 164 188 240 264 112

150, 115 68 162, 160 94, 80 120, 236 260, 265 64

Compound

(Continued)

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21.15

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.16

SECTION TWENTY-ONE

TABLE 21.5 Characteristic Mass Ions for Identifying Some Semivolatile Organics (Continued) Compound Internal standards and surrogates 2-Fluorobiphenyl (surr) Nitrobenzene-d5 (surr) Phenol-d6 (surr) Terphenyl-d14 (surr) 2,4,6-Tribromophenol (surr)

Primary ion (m/z) 172 82 99 244 330

Secondary ions (m/z) 171 128, 54 42, 71 122, 212 332, 141

The gas-phase group frequencies of various types of compounds containing different functional groups are presented in Table 21.6. The most intense IR peak for some common base/neutral- and acid-extractable pollutants for quantitation are tabulated in Table 21.7.

21.6

ANALYSIS OF INORGANIC ANIONS The anions in aqueous and solid samples can be measured by several techniques; the most common methods involve colorimetry, capillary ion electrophoresis, ion chromatography, ion-selective electrode methods, and titrimetry. Some of these techniques are discussed briefly in the following sections and highlighted in Table 21.8.

21.6.1 Colorimetric Methods Colorimetric methods are widely employed in environmental wet analysis. Aqueous samples and the aqueous extracts of soils, sediments, and hazardous wastes are treated with various reagents to form colored complexes with the anions. The absorbance or the transmittance of the solutions after adding the color-forming reagents are then measured by a spectrophotometer or a filter photometer. Within a narrow range of concentrations, the absorbance or the transmittance of the colored complexes formed in the solutions should be proportional to the concentrations of specific anions in the solutions, within the limits of Beers’ law. The concentrations of analytes in samples are determined directly from standard calibration curves constructed by plotting the absorbance or transmittance of calibration standard solutions against their concentrations. The colorimetric methods, however, have certain limitations, the major ones being that they cannot be applied to dirty samples and that the presence of other substances in the samples may interfere in the tests. For example, the presence of chloride ion at a concentration greater than 200 mg/L in the sample may interfere in the analysis of iodide ion when using a colorless indicator, such as 4,4′,4′′-methylidynetris (N,N-dimethylaniline also known as leucocrystal violet). Dilution of samples, or pretreatments such as distillation, may reduce the effects of interfering substances in samples. With these limitations, colorimetric procedures nevertheless offer certain advantages over other methods because of low cost, simplicity, and low detection limits. Table 21.9 outlines briefly one or two colorimetric methods each for some common anions found in environmental matrices. 21.6.2

Titrimetry Certain inorganic anions in aqueous samples may conveniently be determined by titration. Titrimetric procedures often are short and simple, although the detection limits are not as low as

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

TABLE 21.6 Gas-Phase Group Frequencies of Various Classes of Compounds Functional group Acid

Compound type Aliphatic Aromatic

Alcohol

Primary aliphatic

Secondary aliphatic Tertiary aliphatic Aldehyde

Aliphatic

Aromatic

Alkane

Frequency range n, cm−1 3574–3580 1770–1782 3574–3586 1757–1774 3630–3680 1206–1270 1026–1094 3604–3665 1231–1270 3640–3670 1213–1245 1742–1744 2802–2877 2698–2712 1703–1749 2820–2866 2720–2760 2930–2970 2851–2884 1450–1475 1355–1389

Alkyne

Aliphatic

3323–3329

Amide

Substituted acetamide

1710–1724

Amine

Aliphatic Primary aromatic Secondary aromatic

760–785 3480–3532 3387–3480

Benzene

Monosubstituted

675–698 735–790 831–893 1470–1510 1582–1630 1707–1737

Ester

Unsubstituted aliphatic Aromatic Monosubstituted acetate

1748–1761 1703–1759 1753–1788

Ether

Alkyl, aryl Alkyl, benzyl Dialkyl Diaryl Alkyl, vinyl

1215–1275 1103–1117 1084–1130 1238–1250 1204–1207 1128–1142

Aliphatic (acyclic) Aromatic

1726–1732 1638–1699 1701–1722

Aliphatic Aromatic

2240–2265 2234–2245

Ketone

a,b-Unsaturated

Nitrile

(Continued)

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21.17

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.18

SECTION TWENTY-ONE

TABLE 21.6 Gas-Phase Group Frequencies of Various Classes of Compounds (Continued) Functional group Nitro

Compound type Aliphatic

Aromatic Phenol

1,2-Disubstituted 1,3-Disubstituted

1,4-Disubstituted

Frequency range n, cm−1 1566–1594 1548–1589 1377–1408 1327–1381 1535–1566 1335–1358 3582–3595 1255–1274 3643–3655 1256–1315 1157–1198 3645–3657 1233–1269 1171–1190

those attained by colorimetric and electrode methods. Because of the relatively higher detection limits for analytes, applications of titrimetric techniques in environmental sample analyses are very limited. Also, the presence of certain substances in the samples may interfere, and pretreatment of samples may be required. Titrimetric methods for the analysis of some common anions in environmental samples are highlighted in Table 21.10.

21.6.3 Capillary Ion Electrophoresis Many common inorganic anions can be determined in a single analysis by capillary ion electrophoresis. The method is rapid and similar to ion chromatography, giving a “fingerprint” of anions present in the sample matrix. Additionally, it can provide information on organic acids that may not be available from isocratic ion chromatography. The presence of cations and neutral organics does not interfere in the analysis of anions. No specific sample preparation step is necessary, although aqueous samples or their aqueous extracts may be diluted with reagent water before analysis. Total suspended solids in the sample may be removed by simple filtration, while oil and grease may be removed using solid-phase extraction cartridges. Unlike many colorimetric and titrimetric methods, capillary ion electrophoresis can detect halide ions or nitrate and nitrite ions in the presence of each other in the sample matrix. The anions are detected by indirect UV detection, as they displace the UV-absorbing electrolyte anion (chromate) charge for charge, in a silica capillary, thus decreasing the UV absorbance of the analyte anion compared to the background electrolyte. The anions are identified from their migration time on an electropherogram of the sample in a manner similar to retention time in chromatography. The analyte anions are quantified from their peak areas relative to standards. This capillary ion electrophoresis technique using indirect UV detection should be able to measure inorganic anions and organic acid anions at a minimum detectable concentration of about 0.1 mg/L.

21.6.4 Ion Chromatography Ion chromatography, like capillary ion electrophoresis, is a single-instrument technique that may be applied to determine several anions sequentially in a single analysis. The method distinguishes anions in different oxidation states, such as SO 42− and SO 32− or NO3− and NO −2 , as well as halides,

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TABLE 21.7 Most Intense IR Peak for Compound Identification and Quantitation Compound Base/neutral-extractables Acenaphthene Acenaphthylene Anthracene Benzo(a)anthracene Benzo(a)pyrene

nmax, cm−1

799 799 874 745 756

bis(2-Chloroethyl)ether bis(2-Chloroethoxy)methane bis(2-Chloroisopropyl)ether Butyl benzyl phthalate 1-Bromophenyl phenyl ether

1115 1084 1088 1748 1238

2-Chloronaphthalene 2-Chloroaniline 4-Chlorophenyl phenyl ether Chrysene Di-n-butyl phthalate

851 1543 1242 757 1748

Dibenzofuran Diethyl phthalate Dimethyl phthalate Di-n-octyl phthalate Di-n-propyl phthalate

1192 1748 1751 1748 1748

1,2-Dichlorobenzene 1,3-Dichlorobenzene 1,4-Dichlorobenzene 2,4-Dinitrotoluene 2,6-Dinitrotoluene

1458 779 1474 1547 1551

bis(2-Ethylhexyl) phthalate Fluoranthene Fluorene Hexachlorobenzene Hexachlorocyclopentadiene

1748 773 737 1346 814

Hexachloroethane 1,3-Hexachlorobutadiene Isophorone 2-Methyl naphthalene Naphthalene

783 853 1690 3069 779

Nitrobenzene N-Nitrosodimethylamine N-Nitrosodi-n-propylamine N-Nitrosodiphenylamine 2-Nitroaniline

1539 1483 1485 1501 1564

3-Nitroaniline 4-Nitroaniline Phenanthrene Pyrene 1,2,4-Trichlorobenzene

1583 1362 729 820 750

Acid-extractables Benzoic acid 2-Chlorophenol

1751 1485 (Continued)

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21.19

ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.20

SECTION TWENTY-ONE

TABLE 21.7 Most Intense IR Peak for Compound Identification and Quantitation (Continued) Compound Acid-extractables 4-Chlorophenol 4-Chloro-3-methylphenol 2-Methylphenol

nmax, cm−1

1500 1177 748

4-Methylphenol 2,4-Dichlorophenol 2,4-Dinitrophenol 4,6-Dinitro-2-methylphenol 2-Nitrophenol

1177 1481 1346 1346 1335

4-Nitrophenol Pentachlorophenol Phenol 2,4,6-Trichlorophenol 2,4,5-Trichlorophenol

1350 1381 1184 1470 1458

which often interfere with each other in certain colorimetric and titrimetric analyses. Oxyhalides and many carboxylate anions may also be determined by ion chromatography. In environmental analysis the technique is usually applied to determine common anions in potable and nonpotable waters, air impinger solutions, and air particulate extracts, and in aqueous extracts of soils, sediments, and solid wastes. Some common eluants used in ion chromatography for the analysis of anions include solutions of sodium hydroxide, sodium carbonate–bicarbonate, and sodium tetraborate–boric acid. Ion-exchange columns are commercially available from many suppliers. The stationery phases usually are composed of microporous polymeric resins that may contain styrene or ethyl vinyl benzene cross-linked with divinylbenzene, a chemically stable and porous resin core onto which a layer of ion-exchange coating is bonded to produce a reactive surface. A guard column is used to protect the ion-exchange column from contamination by organic and particulate matter in the samples.

21.7 DETERMINATION OF GENERAL AND AGGREGATE PROPERTIES OF SAMPLES Determination of certain general or aggregate properties of samples is often far more important for understanding and evaluating the properties of samples than measuring any individual analytes in the samples. This may also be essential for regulatory requirements. For example, the measurement of pH, acidity, alkalinity, hardness, conductivity, chemical and biochemical oxygen demand, and total dissolved solids in a sample aliquot may provide very useful information about the nature and composition of a sample and may predict its harmful effects on the environment and the ecosystem. Similarly, the characteristics of a hazardous waste may be determined by testing its corrosivity, ignitability, and toxicity. Testing of such common general and aggregate properties of aqueous and nonaqueous samples is briefly outlined in this section. 21.7.1 pH The pH of a sample measures its hydrogen-ion concentration and indicates whether a sample is acidic, neutral, or basic. The pH may be measured accurately using a pH meter. Meters are commercially available from a number of suppliers. The pH meter must be calibrated before making pH measurements or calibrated daily using three standard buffer solutions, usually at pH 4.00, 7.00, and

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.21

TABLE 21.8 Measurement of Inorganic Anions at Trace Concentrations by Various Analytical Techniques Anion

Analytical techniques

Comments

Bromide, Br−

Simple colorimetric Flow injection, colorimetric Ion chromatography

Many substances interfere Use a FIA bromide manifold Rapid; presence of other halides does not interfere

Chloride, Cl−

Titrimetry, silver nitrate (argentometric titration) Titrimetry, mercuric nitrate

Many substances interfere; detection limits not as low as other methods can provide Many substances interfere; the endpoint in the titration is sharper and easier to detect than in argentometric titration Iodide, bromide, chromate, and dichromate interfere; colored and turbid samples may also be analyzed An automated method; very little chemical interference Automated colorimetric method; large number of samples may be analyzed Rapid, accurate, and interference from other substances in the sample minimal

Chloride electrode Colorimetric, automated ferricyanide Colorimetric, flow injection Ion chromatography Fluoride, F−

Colorimetric, zirconium dye Ion-selective electrode Colorimetric, automated complexone

Ion chromatography Capillary ion electrophoresis Cyanide, CN−

Titrimetry, silver nitrate

Colorimetry, chloramine– pyridine–barbituric acid

Cyanide-selective electrode

Presence of several substances may interfere; preliminary distillation of sample may be required Sample distillation may be required to minimize interference effect An automated method; interference normally removed by distillation; has a lower detection limit than the electrode method Use weaker eluents to separate fluoride from interfering peaks Rapid; a detection limit of 0.1 mg/L can be achieved; formate may interfere Sample should be distilled into an alkaline solution for the removal of interfering substances; detection limit is higher than for colorimetric and electrode methods Sample distillation is recommended to remove interfering substances; the method is tedius; the formation of red-blue color upon addition of pyridine–barbituric acid after treating the sample with chloramine-T is sharp; avoid inhalation of toxic cyanogen chloride Sample distillation eliminates interference; the method measures cyanide in the concentration range 0.05–10 mg/L

Cyanate, CNO−

Cyanate hydrolysis

Cyanate hydrolyzes to ammonia when heated with acid; ammonia produced may be analyzed by colorimetry, titrimetry, or ammonia-selective electrode; oxidizing substances in the sample may oxidize cyanate to CO2 and N2; this interference effect may be reduced by treating the sample with sodium thiosulfate

Iodide, I−

Voltammetry

Most suitable and sensitive method; measures iodide concentration in the range 0.10–10 mg/L; presence of iodate, iodine, and organic iodine does not affect the test; sulfide may interfere but which can (Continued)

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21.22

SECTION TWENTY-ONE

TABLE 21.8 Measurement of Inorganic Anions at Trace Concentrations by Various Analytical Techniques (Continued) Anion

Analytical techniques

Iodide, I−

Leuco crystal violet, colorimetry

Catalytic reduction method, colorimetry

Ion chromatography

Comments be removed as H2S by acidification and purging and then readjusting the pH back to 8 before analysis; the voltammetry analyzer system consists of a potentiostat, static mercury drop electrode, a saturated calomel electrode as the reference electrode, a stirrer, and a plotter This colorimetry method can measure iodide concentration in water in the range 50–5000 mg/L; chloride at concentrations over 200 mg/L may interfere Iodide ion catalyzes the reduction of ceric ions by arsenious acid; after a specific time interval the reaction is stopped by addition of ferrous ammonium sulfate; the resulting ferric ions, proportional to the remaining ceric ions, form a colored complex with potassium thiocyanate; intensity of color proportional to iodide concentration; plot a calibration curve for quantitation; iodate, elemental iodine, and hypoiodite ion interfere; this procedure measures iodide concentrations below 100 mg/L; sensitivity of this test falls between that of leuco crystal violet and voltammetric methods Iodide ion may be measured simultaneously with other anions in the sample; other halide ions do not interfere

Iodate, IO 3−

Polarography

Iodate is reduced to iodide under mild basic conditions; measured by a polarographic analyzer system consisting of a static mercury drop electrode, a saturated calomel electrode, a potentiostat, and a plotter; the method is highly sensitive and can measure iodate at 5 mg/L in the presence of iodide and other iodine species; dissolved oxygen and zinc interfere and must be removed before analysis

Nitrate, NO 3−

Colorimetry, cadmium reduction, and hydrazine reduction

Nitrite ion, NO2−, interferes; samples must be measured separately for nitrite ion before carrying out any cadmium or hydrazine reduction Usually applied for screening samples to monitor their nitrate levels; UV absorption is measured at two different wavelengths, 220 and 275 nm; dissolved organic matter in samples also may absorb UV light at 220 nm Nitrate electrode can measure NO3− concentrations in samples in a wide range between 0.5 and 5000 mg/L; samples must be buffered over the pH range 3–9 Rapid and probably the most confirmatory technique; the interference effects from other ions are easy to overcome by changing the conditions to alter the migration time of co-eluting peaks; the minimum detection limit, however, is about

Ultraviolet spectrophotometric method

Nitrate ion-selective electrode method

Capillary ion electrophoresis with indirect UV detection

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21.23

TABLE 21.8 Measurement of Inorganic Anions at Trace Concentrations by Various Analytical Techniques (Continued) Anion

Analytical techniques

Ion chromatography

Nitrite, NO2−

Colorimetric, diazotization coupling

Capillary ion electrophoresis

Ion chromatography

Orthophosphate, PO43−

Colorimetric, vanadomolybdophosphoric acid

Colorimetric, stannous chloride

Colorimetric, ascorbic acid

Capillary ion electrophoresis with indirect UV detection

Ion chromatography

Sulfate, SO42−

Gravimetry

Turbidimetric method

Colorimetry, methylthymol blue

Comments 0.1 mg/L, about 10 times higher than that of the cadmium-reduction colorimetric method The test is rapid and readily distinguishes nitrate from nitrite in the sample; the limit of detection is about 0.10 mg NO3−/L The method is very sensitive; the reddish-purple color of the azo dye produced can be detected at 543 nm at a lowest concentration of 10 mg/L Not as sensitive as the colorimetric method; the analysis, however, is fast and free from interference Rapid; interference effects are minimal; the detection level is not as low as what may be achieved by colorimetric techniques This test is not as sensitive as the other two colorimetric tests; yellow color of the product may be easily masked in dirty or colored samples; if the sample is dirty, shake it with activated carbon and then filter the carbon to remove color before analysis; the detection limit is in the range of 0.5 mg PO43−/L The molybdophosphoric acid obtained from treating phosphate with ammonium molybdate may be reduced by stannous chloride to give an intensely colored molybdenum blue, which may be extracted into benzene–isobutanol solvent; the test is more sensitive than the above method; a detection level of 10 mg/L may be achieved Instead of stannous chloride, ascorbic acid may be used to produce molybdenum blue; somewhat less sensitive than the stannous chloride reduction test Although the minimum detection limit of phosphate obtained by this technique is well above those from colorimetric tests, the analysis is faster, simpler, and almost free from interference The test is rapid, and the presence of interfering substances does not affect the test; the detection limit, however, is higher than that obtained with the molybdenum blue colorimetric test Sulfate is precipitated in HCl solution as BaSO4 by addition of BaCl2; the gravimetric method is susceptible to many errors; also, the analytical procedure is very tedious Sulfate is precipitated as BaSO4 of uniform size in acetic acid medium and the light absorbance of the suspension is measured; the lower detection limit is in the range 2–5 mg/L The method is less sensitive than the turbidimetric procedure; the use of continuous-flow automated analytical equipment is ideal to analyze a large number of samples (Continued)

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21.24

SECTION TWENTY-ONE

TABLE 21.8 Measurement of Inorganic Anions at Trace Concentrations by Various Analytical Techniques (Continued) Anion

Analytical techniques

Sulfate, SO42−

Capillary ion electrophoresis with indirect UV detection

Ion chromatography

Sulfide, S2−

Titration, iodometric

Colorimetric, methylene blue and gas dialysis, automated methylene blue

Ion-selective electrode

Sulfite SO32−

Colorimetry, phenanthroline

Titration, iodometric

Thiocyanate, SCN−

Colorimetry, ferric nitrate

Comments A detection level in the range of 2 mg/L may be achieved, which is lower than with the turbidimetric method; the analysis time is very short, less than 5 min Analysis time is short; other anions do not interfere; detection limit may be lower than with the turbidimetric method The method is accurate for measuring S2− at concentrations above 1 mg/L; oxidizing substances in the sample may interfere The methylene blue method is based on the reaction of sulfide in the presence of FeCl3 with dimethyl-p-phenylenediamine to produce methylene blue; this test can measure sulfide at concentrations ranging from 0.1 to 20.0 mg/L; also, continuous flow automated analytical equipment based on the above reaction may be used; a gas dialysis technique is used to separate sulfide from the sample matrix and most interfering substances; the automated methylene blue method is suitable for analyzing a large number of samples and can measure sulfide at concentrations in the range 0.002–0.10 mg/L The electrode method can measure sulfide in the sample in a much wider range of concentrations, between 0.05 and 100 mg/L This method is applied to measure sulfite at low concentrations; the minimum detectable concentration is 0.01 mg/L This method can analyze sulfite only at much higher concentrations, the detection limit may be in the range of 2 mg/L; also, the presence of other oxidizable substances in the sample can give erroneous results The test measures thiocyanate in concentrations between 0.1 and 2.0 mg/L; several substances such as reducing agents or hexavalent chromium interfere

10.00, to cover more or less the entire pH range. Solid samples may be crushed and treated with an equal mass of water, shaken, and the pH of the supernatant aqueous phase measured by a pH meter. A more accurate method of determining the pH of a solid sample may involve its extraction with an aqueous solution of calcium chloride instead of reagent water, where calcium ions exchange with the hydrogen ions bound to the pores and release the hydrogen ions from the solid matrix into the solution. The pH of the supernatant solution is then measured.

21.7.2

Acidity Acidity of water measures its quantitative capacity to react with a strong base to a designated pH, that is, any pH of interest. Acids influence the rates of chemical reactions, biological processes, and

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21.25

TABLE 21.9 Determination of Inorganic Anions by Colorimetric Methods Anion

Method

A brief outline of the method

Bromide, Br−

Phenol red

Sample treated with a dilute solution of chloramine-T in the presence of phenol red at pH 4.5–4.7 turns the solution reddish to violet, depending on the concentration of bromide ion; presence of free chlorine, higher concentrations of Cl− and HCO3− can interfere; measured at 590 nm

Chloride, Cl−

Ferricyanide (automated)

Sample treated with mercuric thiocyanate; chloride liberates thiocyanate ion, which in the presence of ferric ion forms red ferric thiocyanate; absorbance or transmittance measured at 480 nm

Fluoride, F−

Zirconium-dye lake

Fluoride reacts with a zirconium-dye lake, a mixture of sodium 2-(parasulfophenylazo)-1,8-dihydroxy-3,6-naphthalene disulfonate and zirconyl chloride octahydrate solutions in concentrated HCl; fluoride dissociates the dye lake, forming the colorless complex anion, ZrF62−; color becomes lighter with increase in the amount of F− in the sample; measured at 570 nm; samples may be distilled before analyses for removal of interfering substances

Iodide, I−

Leuco crystal violet

Iodide is selectively oxidized to iodine by potassium peroxymonosulfate, KHSO5, to produce iodine; the latter reacts instantaneously with the colorless indicator reagent containing 4,4′,4″-methylidynetris (N,N-dimethylaniline), also known as leucocrystal violet, to produce a highly colored crystal violet dye; absorbance or transmittance measured at 592 nm within the pH range 3.5–4.0 in the presence of a buffer (citric acid–ammonium hydroxide–ammonium dihydrogen phosphate)

Nitrate, NO3−

Cadmium reduction

Nitrate is reduced to nitrite, NO2−, in the presence of cadmium; the NO 2− produced is diazotized with sulfanilamide and coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride to form a highly colored reddish-purple azo dye that may be measured at 543 nm; certain metals, such as iron or copper at high concentrations, may lower the reduction efficiency of cadmium; EDTA may be added to samples to eliminate such interference; the test measures the concentration of both nitrate and nitrite in the sample; measure the concentration of nitrite in the sample on another sample aliquot without cadmium reduction to determine the amount of nitrate The method is similar to cadmium reduction; however, instead of cadmium granules, hydrazine hydrate is employed to reduce nitrate to nitrite

Hydrazine reduction

Nitrite, NO2−

Diazotization and coupling

The procedure is the same as above except that no reducing agent, such as cadmium or hydrazine hydrate, is added to the sample

Orthophosphate, PO43−

Ammonium molybdate

Ammonium molybdate reacts with phosphate under acid conditions to form a heteropolyacid, molybdophosphoric acid, which in the presence of vanadium gives yellow vanadomolybdophosphoric acid; the color is measured at 400–490 nm (usually at 470 nm); certain anions at high concentrations may interfere in the test Molybdophosphoric acid as formed above may be reduced by stannous chloride or ascorbic acid to produce an intensely colored molybdenum blue; the method is more sensitive than the one above; the absorbance or transmittance of the solution may be measured at 650 nm; alternatively, the molybdenum blue in

Molybdenum blue

(Continued)

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21.26

SECTION TWENTY-ONE

TABLE 21.9 Determination of Inorganic Anions by Colorimetric Methods (Continued) Anion

Method

Orthophosphate, PO43−

A brief outline of the method aqueous solution is extracted with benzene–isobutanol and measured at 625 nm; for ascorbic acid reduction, use potassium antimonyl tartrate along with ammonium molybdate in acid medium to produce phosphomolybdic acid; arsenate present in the sample may interfere In acetic acid medium, barium chloride precipitates barium sulfate crystals of uniform size; white suspension of BaSO4 is measured at 420 nm; silica in excess (above 500 mg/L) may interfere

Sulfate, SO42−

Barium chloride (turbidimetry)

Sulfide, S2−

Methylene blue

Sample is treated with ferric chloride solution, followed by diammonium hydrogen phosphate and methylene blue reagent; an intense blue color develops; absorbance or transmittance is measured at 664 nm

Sulfite, SO32−

Phenanthroline

Sample is acidified and purged with nitrogen gas; SO2 gas liberated is trapped in an absorbing solution containing ferric ion and 1,10-phenanthroline; SO2 reduces Fe3+ to Fe2+, which complexes with 1,10-phenanthroline to produce the orange complex tris(1,10-phenanthroline)iron(II); the intensity of the color, proportional to SO32− concentration, is measured at 510 nm

Thiocyanate, SCN−

Ferric nitrate

In acid medium, thiocyanate reacts with ferric ion, forming an intense red color; the concentration of SCN− in the sample is determined from the intensity of color measured at 664 nm

Cyanide, CN−

Chloramine T-pyridine– barbituric acid

Cyanate, CNO−

Acid hydrolysis

Reaction of cyanide in alkaline medium with chloramine-T reagent produces cyanogen chloride, which forms a red-blue color upon addition of pyridine–barbituric acid reagent; the intensity of color of the solution is measured between 575 and 582 nm; interference may be removed by distilling an acidified sample and purging the liberated hydrogen cyanide with nitrogen into an aqueous solution of caustic soda; calibration standards are prepared in NaOH solution Cyanate hydrolyzes to ammonia (ammonium) when heated at a low pH; ammonia produced may be measured by various methods

contribute to corrosiveness. The acidity of water is determined by titration against a standard solution of an alkali and depends on the endpoint pH or indicator used. Traditionally, standard acidity is measured by titration to endpoint of pH 3.7 or 8.3. The former is known as methyl orange acidity, and the latter as phenolphthalein acidity or the total acidity. While the former may be determined using a color indicator, such as methyl orange or bromophenol blue, the standard endpoint of titration for acidity may be measured by using an indicator such as phenolphthalein or metacresol purple. Acidity of a sample with respect to any pH of interest may be accurately determined from a titration curve, constructed by plotting the sample pH against the volume of titrant, added successively in small increments and recording the inflection point.

21.7.3

Alkalinity Alkalinity measures the acid-neutralizing capacity of water. Alkalinity, like acidity, is a measure of an aggregate property of water. It is attributed to the presence of hydroxide, carbonate, and bicarbonate ions in the sample. Weak bases, such as phosphates, silicates, and borates, may also contribute to alkalinity. Alkalinity is measured by titrating a measured volume of a sample aliquot against a standard acid solution to a designated pH endpoint, usually 8.3 (phenolphthalein alkalinity) or 4.5

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21.27

TABLE 21.10 Determination of Common Inorganic Anions by Titrimetric Methods Anion

Method

A brief outline of the method

Chloride, Cl−

Argentometric

Chloride reacts with silver nitrate to quantitatively precipitate silver chloride; in neutral or slightly alkaline solution, potassium chromate can indicate the endpoint, forming red silver chromate after all silver chloride is precipitated; bromide, iodide, and cyanide interfere; interfering effects of sulfide, sulfite, and thiosulfate ions can be removed by treatment with hydrogen peroxide; orthophosphate and iron may interfere at high concentrations, above 25 and 10 mg/L, respectively

Cyanide, CN−

Silver nitrate

Hydrogen cyanide distilled from an acidified sample into caustic soda solution is titrated with a standard solution of AgNO3, forming the soluble cyanide complex Ag(CN)−2 ; at the endpoint of titration, after all CN− ions complex, the excess Ag+ is detected by the silver-sensitive detector, p-dimethylaminobenzalrhodamine present in the solution; the color changes from yellow to salmon

Dichromate, Cr2O72−

Redox titration

3+ In acid medium, orange Cr2O2− 7 ion is reduced to green Cr ion; diphenylamine sulfonic acid is used as an indicator; at the endpoint the green color of the reduced form of the indicator changes to red-violet in its oxidized form

Sulfide, S2−

Iodometric titration

Sample is acidified and a measured excess amount of iodine solution is acid S + 2I−; the excess added; iodine oxidizes sulfide to sulfur: I2 + S2− ----→ unreacted iodine is back-titrated with a standard solution of sodium thiosulfate or phenylarsine oxide using starch indicator; at the endpoint the blue color of the solution disappears; presence of other oxidizable substances, such as sulfite, SO32−, and thiosulfate, S2O32−, may interfere; sulfide may be separated from the sample by precipitation with zinc acetate to give zinc sulfide; the acidified solution of the latter may be subjected to iodometric titration as described above

(total alkalinity). A color indicator such as phenolphthalein or metacresol purple may be used for the titration to pH 8.3, and bromocresol green or a mixed bromocresol green–methyl red indicator for titration to the pH endpoint 4.5. The acid titrant used for such titration is usually a standard solution of 0.02N or 0.1N sulfuric or hydrochloric acid. Alternatively, a pH meter may be used instead of a color indicator. When a pH meter is used, the standard solution of the acid titrant is added in measured amounts in successive small increments to the sample to attain the designated endpoint. Alkalinity of water is expressed as mg CaCO3 per liter of sample. If both the total and the phenolphthalein alkalinity of the water are known, then the alkalinity attributed to hydroxide, carbonate, and bicarbonate may be calculated from the relationship shown in Table 21.11.

TABLE 21.11 Alkalinity Relationships Result from titration

Hydroxide alkalinity (as CaCO3)

Carbonate alkalinity (as CaCO3)

Bicarbonate alkalinity (as CaCO3)

P=T P ≥ 12 T P = 12 T P ≤ 12 T P=0

T 2P − T 0 0 0

0 2(T − P) 2P 2P 0

0 0 0 T − 2P T

P = phenolphthalein alkalinity; T = total alkalinity.

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21.28

SECTION TWENTY-ONE

21.7.4

Conductivity Conductivity of water measures its ability to conduct electric current. It is another important aggregate property of water and depends on the presence of ions and their concentrations, their oxidation states, and the mobility and the temperature of the water. The purity of water is assessed from its conductivity. Reagent water of high quality, typically prepared by distillation, deionization, or reverse osmosis treatment of feedwater, should have a conductivity below 0.1 micromhos/cm (or resistivity above 10 megohm-cm) at 25°C, while the conductivity of potable water should be below 2 micromhos/cm. Conductivity is the reciprocal of resistivity and expressed in the unit micromhos per centimeter (µmhos/cm) or mhos per centimeter (mho/cm). In SI units, conductivity is reported as millisiemens per meter (mS/m) or microsiemens per centimeter ( µS/cm). The conversion of these units is as follows: 1 mS/m = 10 mmhos/cm 1 mS/m = 1 mmhos/cm Conductivity of aqueous samples in the laboratory or in the field may be measured directly using a conductivity meter. Conductivity meters equipped with temperature sensors that can measure and display a direct readout of conductivity are commercially available. Before taking a measurement, the instrument must be calibrated with conductivity standard solutions. Such standard solutions are also commercially available or may be prepared in the laboratory. A 0.0100M potassium chloride solution (made by dissolving 745.6 mg of anhydrous KCl in reagent water to 1 L at 25°C and stored in CO2-free atmosphere) may be used as a standard reference solution that has a conductivity of 1412 micromhos/cm at 25°C.

21.7.5

Solids The presence of solids in water, including total dissolved and suspended materials, may adversely affect the water quality. High concentrations of solids may affect the taste of drinking water or produce ill effects or make the water unsuitable for bathing or industrial applications. The total dissolved solids in a potable water is recommended not to exceed 500 mg/L. The total solids in water may be determined by simple gravimetry. A measured volume of wellmixed water is evaporated in a weighed dish and dried in an oven at 103–105°C to constant weight. The difference in weights between that of the empty dish and the weight taken after evaporating all water should be equal to the weight of the total solids in the sample aliquot. The total suspended solids in the water may similarly be determined by gravimetry at 103–105°C. The sample is well mixed and then filtered through a weighed standard glass-fiber filter. The residue collected on the filter is dried in an oven at 103–105°C to constant weight. The increase in weight in the filter is due to the total suspended solids in the sample. The total dissolved solids in water are also determined by a similar procedure. A measured volume of well-mixed sample is filtered through a standard glass-fiber filter. While the suspended particles remain on the filter, the filtrate containing dissolved solids is evaporated to dryness in a weighed dish and dried to a constant weight at 180°C. The total dissolved solids in the sample is equal to the increase in the weight of the dish. It may be noted that dissolved solids in water are usually determined at 180°C and not at 103–105°C.

21.7.6 Hardness Hardness measures the capacity of water to precipitate soap and is attributed primarily to the presence of calcium and magnesium ions in the water and to a small extent to the presence of other polyvalent metal ions. In general, the total hardness is defined as the sum of the concentrations of calcium and magnesium with both expressed as mg calcium carbonate per liter solution.

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21.29

Hardness can be measured by titrating a measured volume of an aliquot of the sample against a standard solution of ethylenediaminetetraacetic acid (EDTA) or its sodium salt using an indicator dye, such as Eriochrome Black T or Calmagite. The pH of the sample is adjusted and maintained at 10.0 with a solution of NH4Cl–NH4OH buffer. Upon addition of the indicator, the color of the solution turns wine red in the presence of calcium and magnesium ions in the sample. The solution is then titrated with a standard solution of EDTA. At the endpoint the color of the solution turns from wine red to blue. Usually a small amount of neutral magnesium salt of EDTA is added to the buffer before titration to yield a satisfactory endpoint. The hardness of water can also be calculated from the concentrations of Ca2+ and Mg2+ in the solution using the following relationship. Both metals can be analyzed by atomic absorption or atomic emission spectrophotometry. Hardness, as mg equivalent CaCO3/L = 2.497[Ca (mg/L)] + 4.118[Mg (mg/L)]

21.7.7 Chemical Oxygen Demand Chemical oxygen demands (COD) is a measure of the oxygen equivalent of organic matter in the sample that is susceptible to oxidation by a strong oxidizing agent. Certain inorganic oxidizable substances that may be present in the sample, such as nitrite or sulfite ion, may readily undergo oxidation under such conditions and thus may interfere in the test. The COD determination is usually carried out using a boiling mixture of potassium dichromate and H2SO4 that can oxidize practically all organics under refluxing condition. Other strong oxidizing agents, such as a KMnO4–H2SO4 mixture are also effective. A measured volume of an aliquot of a sample is refluxed with a known excess of K2Cr2O7 in a strong acid solution in the presence of Ag2SO4 and HgSO4. During oxidation the dichromate ion is reduced to Cr3+, that is, Cr6+ is reduced to Cr3+. The amount of Cr6+ consumed in the reaction is determined either by titration or by colorimetry. The Cr6+ is measured by titration against a standard solution of ferrous ammonium sulfate using ferroin indicator. At the endpoint of titration the color of the solution changes from greenish blue to reddish brown. The colorimetric analysis is faster if the sample is digested with K2Cr2O7–H2SO4 in an ampule or a vial under closed reflux conditions. The concentration of dichromate is measured by a spectrophotometer at 600 nm by comparing the absorbance of the solution against a standard calibration curve. COD vials are commercially available for measuring the COD of a sample in low, medium, and high ranges. Potassium hydrogen phthalate (KHP), the theoretical COD value of which is 1.175 mg for 1.00 mg KHP, is used as a reference standard in COD analysis.

21.7.8 Biochemical Oxygen Demand Biochemical oxygen demand (BOD) is an empirical test that measures the amount of oxygen required for microbial oxidation of organic compounds in water. Usually the BOD is measured for a 5-day incubation period, during which the amount of oxygen utilized by the microorganism to oxidize organics and the oxidizable inorganics in the sample are determined. Incubation is carried out in the dark at a temperature of 20 ± 1°C. BOD oxidation reactions follow first-order kinetics, and therefore the BOD for any incubation period may be calculated from the following relationship if the rate constant k, for the reaction is known: Bt = U(1 − 10−kt) where Bt = BOD for t-day incubation period U = ultimate BOD, which should be more or less equal to the COD K = rate constant for the reaction

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21.30

SECTION TWENTY-ONE

The BOD values of wastewaters are used to calculate the waste loading capacity in the design of wastewater treatment plants. Different volumes of sample aliquots are diluted with “seeded” dilution water to 300 mL in BOD incubation bottles filled to their full capacity, without leaving any headspace. The bottles are tightly closed and placed in an air incubator or water bath at 20 ± 1°C in the dark for 5 days. At the end of the incubation period the concentration of dissolved oxygen (DO) in the water is measured. The concentration of the DO in the dilution water before incubation is also measured. Thus, the amount of oxygen consumed by microbes in the oxidation of organics in the sample is determined from the difference between these two DO values. The DO in the sample may be measured either by using a DO probe (electrode method) or by titration (Winkler titration). The electrode method is simple and faster and gives a direct readout of the concentration of dissolved oxygen. Oxygen-sensitive membrane electrodes are commercially available. Alternatively, the DO may be measured by Winkler iodometric titration. In this technique, divalent manganese (MnSO4) is added to a measured volume of the sample. This is followed by addition of caustic soda solution, resulting in the precipitation of divalent Mn(OH)2. The dissolved oxygen in the sample rapidly oxidizes an equivalent amount of dispersed Mn2+ precipitate to highervalent Mn4+. In the presence of iodide ion and in acid medium, Mn4+ is reduced back to Mn2+ with liberation of iodine. The liberated iodine is titrated against a standard solution of sodium thiosulfate or phenyl arsine oxide using starch indicator to a colorless endpoint.

21.7.9 Organic Carbon Total organic carbon (TOC) is a measure of total organic content in the sample. Measurement of TOC, like that of COD or BOD, provides vital information on waste loading and operation of water and waste treatment plants. All analytical methods are based on breaking down organically bound carbon in organic molecules and converting the carbon to carbon dioxide utilizing high temperatures, catalysts, oxidizing agents, or UV radiation. The CO2 may be analyzed for quantitative measurement. Alternatively, the CO2 may be analyzed by coulometric titration or by a CO2−selective electrode. The inorganic carbon, such as CO32− and HCO3−, in the sample may be removed by treating the sample with an acid and sparging out the CO2 produced before measuring TOC. Treating the sample with persulfate under heating or in the presence of UV radiation may also convert the organic carbon to CO2. Many types of TOC analyzers are commercially available.

21.7.10 Oil and Grease Oil and grease in aqueous samples may be measured by two different methods: (1) partition gravimetry and (2) partition infrared. In the partition gravimetry method, a measured volume of sample is shaken with trichlorotrifluoroethane or, preferably, with a mixture of n-hexane and methyl-tert-butyl ether (80 : 20). The solvent(s) from the extract is distilled out in a water bath, the flask is cooled in a desiccator, and the residue is weighed. Trichlorotrifluoroethane is used as an extraction solvent in the partition infrared method. The absorbance of the carbon–hydrogen bond in the infrared is used to measure oil and grease. An infrared spectrophotometer with a cell of 1-cm path length may be used to measure the absorbance at 3200–2700 cm−1 with solvent in the reference beam.

21.7.11 Kjeldahl Nitrogen The total Kjeldahl nitrogen (TKN) is an indirect measure of most types of organic nitrogen in the water. It is the sum of organic nitrogen and the ammonia nitrogen in the sample, so the difference

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21.31

between TKN and ammonia nitrogen measured separately in two sample aliquots should give an estimation of organic nitrogen in the sample. TKN, however, does not measure certain nitroorganics, including oxime, semicarbazone, hydrazone, azide, nitrile, and azo, nitro, and nitroso derivatives. On the other hand, the method readily measures the amino nitrogen of many organic materials along with free ammonia. The method involves digesting on a heating device a measured volume of sample in a Kjeldahl flask in the presence of potassium sulfate, sulfuric acid, and cupric sulfate as catalyst. This converts amino organics and any free ammonia in the sample to ammonium ion. A base is then added. The ammonia is then distilled from the alkaline medium and absorbed in boric acid solution. Ammonia is then measured in the boric acid solution by titration with a standard mineral acid, or by colorimetric methods or by using an ammonia-selective electrode (see ammonia).

21.8

DISSOLVED GASES IN WATER Water may contain a variety of gaseous substances at trace but significant concentrations, which may affect aquatic life or may produce adverse health effects upon human consumption. These gases may include the toxic hydrogen sulfide, chlorine, and ammonia, or the common atmospheric gases such as oxygen, nitrogen, argon, and carbon dioxide, concentrations of which above or below the normal levels may affect the water quality for drinking purposes or for industrial processes. Various analytical methods for the determination of some of the dissolved gases in water are highlighted in Table 21.12.

21.9

METALS Metals in general can be analyzed by various techniques, including colorimetric methods, atomic absorption and atomic emission spectrometry, X-ray diffraction and fluorescence methods, neutron activation analysis, ion-selective electrode methods, ion chromatography, eletrophoresis, redox titration, and gravimetry. Although these techniques all have their own merits and limitations, atomic spectroscopy is the most widely used analytical tool for industrial analysis of metals and is currently applied exclusively in environmental analysis for the determination of metals in aqueous and nonaqueous matrices. Atomic absorption and emission spectroscopy are discussed fully in Sec. 8. Their applications in environmental analysis are briefly highlighted here. Both atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES) provide rapid, multielement determination of metals at low ppm and ppb levels as found in ground-, surface-, and wastewaters and in soils, sediments, and hazardous wastes. Some of these metals are toxic and are regulated by the U.S. Environmental Protection Agency. Some of them, known as priority pollutant metals and classified under various EPA programs include aluminum, arsenic, antimony, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, silver, selenium, thallium, vanadium, and zinc. Analysis of these metals by atomic spectroscopic methods is discussed below in brief.

21.9.1 Flame and Furnace Atomic Absorption Spectrophotometry The AAS methods are of two types; (1) flame and (2) furnace, differing only in atomization process. In flame AA spectrometry the heat source is a flame, air–acetylene or air–nitrous oxide. The sample is aspirated into the flame and atomized. The metal atoms absorb energy at their own characteristic wavelengths from a light beam directed through the flame. The energy absorbed is proportional to the concentration of the metal in the sample. In the furnace mode of AAS the heat source is a graphite furnace heated electrically, producing a temperature much higher than that obtained from flame. Thus, better sensitivity and a much lower detection limit for metals are obtained in furnace mode.

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.32

SECTION TWENTY-ONE

TABLE 21.12 Determination of Dissolved Gases in Water Dissolved gas

Analytical methods

Oxygen (DO)

(1) Winkler or iodometric titration: Divalent manganese solution is added to the sample, followed by a strong alkali in a glass-stoppered bottle. The divalent manganous hydroxide, obtained as a precipitate and dispersed in the sample mixture, is rapidly oxidized by the dissolved oxygen in the sample, giving an equivalent amount of manganese in higher valency states. In the presence of iodide and in acid medium, the higher-valent manganese reverts to Mn2+ with the liberation of iodine equivalent to the original dissolved oxygen in the sample. Iodine is titrated with a standard solution of sodium thiosulfate or phenyl arsine oxide using starch indicator. At the endpoint of titration the blue color of the solution decolorizes. Alternatively, the liberated iodine can be measured directly by an absorption spectrophotometer. The iodometric method has been modified to minimize the effect of interfering substances. In the presence of Fe3+ and NO2− ions, the sample should be treated with sodium azide and sodium hydroxide solution after adding Mn2+ solution and potassium fluoride before acidification. Interference from Fe2+ may be removed by addition of a small amount of KMnO4–H2SO4 and then potassium oxalate solution to remove the permanganate color completely before analysis as above. Interference from suspended solids may be removed by alum flocculation modification, in which a solution of aluminum potassium sulfate and ammonium hydroxide is added, the sample is allowed to settle, and the clear supernate is siphonned off for analysis as before. (2) Membrane electrode method: Unlike Winkler titration, membrane electrodes provide a simple, rapid, and excellent method for measuring dissolved oxygen in situ in all kinds of polluted waters, including highly colored waters and strong wastes. These oxygen-sensitive membrane electrodes are used to measure DO both in the field and in the laboratory. Portable DO meters equipped with direct display of the concentrations of DO in water are commercially available.

Ozone

Ozone (or residual ozone after water treatment) in water may be measured by the indigo colorimetric method. In acidic solution, ozone rapidly decolorizes the color of indigo (potassium indigo trisulfonate) solution. The decrease in absorbance is inversely proportional to increasing ozone concentration and is measured at 600 nm. A minimum detectable concentration of 10–20 mg ozone per liter may be measured by this method.

Chlorine

Chlorine (total or free chlorine) may be found in water in the form of molecular chlorine (Cl2), hypochlorous acid (HClO), and hypochlorite ion (ClO−), and the relative proportion of these are pHand temperature-dependent. HClO, the hydrolyzed form of Cl2 and the ClO−, however, predominate at the pH of most waters. (1) DPD ferrous titration: The sample is titrated with a standard solution of ferrous ammonium sulfate (FAS) using N,N-diethyl-p-phenylenediamine (DPD) as indicator. In the absence of iodide ion, free chlorine reacts with DPD, producing a red color. A phosphate buffer solution is prepared from Na2HPO4 (24 g), KH2PO4 (46 g), and 0.8 g of EDTA disodium salt dissolved in 1 L of water. To 5 mL of this phosphate buffer solution, 5 mL of DPD indicator solution are added. This is followed by addition of 100 mL of sample (or a suitable volume of diluted sample). The mixture is titrated with a standard solution of FAS (1.106 g/L of distilled water containing 1 mL of 1 + 3 H2SO4) until the red color disappears. 1 mL FAS standard solution = 1.00 mg Cl as Cl2/L. (2) DPD colorimetric test: This method is very similar to the above DPD titration, except that the mixture is not titrated against a standard solution of FAS. To a small volume of phosphate buffer and DPD reagent mixture as prepared in the above test, and in a test tube, add 10 mL of sample and mix well. The absorbance is read at 515 nm. The concentration of chlorine is determined from the chlorine standard calibration curve. Chlorine standard solutions are prepared in the range 0.05–4 mg/L and standardized by iodometric titration with a standard solution of Na2S2O3 using starch indicator. The DPD colorimetric test can measure chlorine as Cl2 at approximately a minimum detection level of 10 mg/L. (3) Syringaldazine colorimetric test: The free chlorine reacts with syringaldazine (3,5-dimethoxy-4hydroxybenzaldazine) in 2-propanol to produce a colored product. The intensity of color is measured at 530 nm. The optimum color in the solution is obtained at a pH between 6.5 and 6.8. This method measures free chlorine over the range 0.1–10 mg/L. (4) Amperometric titration: Free chlorine in the sample may be determined by titrating the sample at pH 6.5–7.5 against a standard solution of phenyl arsine oxide, observing current changes on a microammeter. Titrant is added progressively in smaller increments until all needle movement ceases, that is, until there is no needle response. Sluggish needle movements signal the approach of the endpoint.

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.33

TABLE 21.12 Determination of Dissolved Gases in Water (Continued) Dissolved gas

Analytical methods

Chlorine dioxide (ClO2)

DPD colorimetric or titrimetric test: Chlorine dioxide may be distinguished from chlorine, chlorite, or hypochlorite by DPD colorimetric or titrimetric test. To 100 mL of sample add 2 mL of glycine solution (10 g glycine/100 mL distilled water). In another container, mix 5 mL of phosphate buffer solution and 5 mL of DPD indicator solution (see Chlorine). Add sample–glycine mixture to buffer– indicator mixture. A red color forms immediately. The red solution is titrated with a standard solution of FAS until the color disappears. Alternatively, the absorbance of the red solution is measured at 515 nm and ClO2 content in the sample is determined from a standard calibration curve. The standard solutions of ClO2 are prepared from dissolving NaClO2 in water and adding 5N H2SO4; the ClO2 gas is passed through a saturated NaClO2 scrubber solution and then collected in distilled water. The FAS titration method measures ClO2 as chlorine (Cl). The results, therefore, have to be multiplied by 1.9.

Cyanogen chloride (CNCl)

Pyridine–barbituric acid colorimetric test: Cyanogen chloride is unstable and hydrolyzes to cyanate (CNO−) at a pH of 12 or more. The analysis should therefore be done immediately after the collection of sample. To 20 mL of sample in a 50-mL volumetric flask, add 1 mL of phosphate buffer (138 g NaH2PO4⋅H2O in 1 L of water), stopper, and mix by inversion one time. Allow the solution to stand for a minute. This is followed by addition of 5 mL pyridine–barbituric acid reagent. Stopper and mix the solution one time by inversion. Allow the color to develop over 3 min. Dilute the solution with reagent-grade water to 50 mL. Mix thoroughly and allow to stand for another 5 min. Measure absorbance of the red-blue solution at 578 nm in a 1-cm cell using distilled water as reference. Determine the concentration of CNCl in the sample from a cyanide calibration standard (CN− standard solution + NaOH dilution solution + chloramine − T reagent + phosphate buffer; mix and then add pyridine–barbituric acid reagent; dilute to the same volume with water and let stand for color development). [See Cyanide, under Colorimetric Tests.]

Ammonia (NH3)

Ammonia in water may be measured by several methods, including colorimetry, titration, and electrode techniques, and can be done by both manual and automated methods. A preliminary distillation of sample is required for titrimetric analysis. However, in other methods the distillation step may be omitted. It is recommended that the sample be distilled if it is dirty or interfering substances are present. If the test cannot be performed within 24 h after collection, then acidity the sample to a pH below 2 and store in a refrigerator at 4°C. In such a case, bring the sample to room temperature and neutralize before analysis. (1) Ammonia-selective electrode method: Ammonia-selective electrodes are commercially available to measure ammonia in water. A chloride-ion-selective electrode serves as the reference electrode. A pH meter having an expanded millivolt scale or a specific ion meter is used for potentiometric measurements. Dissolved ammonia and NH4+ ion are measured by this method in the range 0.03–1500 mg NH3 − N/L. Amines interfere in the test. (2) Phenate colorimetric test: The sample is treated with phenol solution followed by addition of sodium nitroprusside solution and an oxidizing solution (a mixture of sodium hypochlorite, trisodium citrate, and sodium hydroxide in deionized water). The solutions are thoroughly mixed and allowed to stand. An intense blue color of indophenol developes. The intensity of color is measured at 640 nm. The concentration of ammonia is determined from a calibration standard curve. (3) Titration: A suitable volume of sample, accurately measured, is distilled in a distillation flask. Ammonia distilled out is absorbed and collected in a solution of boric acid containing the mixed color indicators methyl red and methylene blue. The distillate is titrated against a standard solution of 0.02N H2SO4 to a pale lavender color endpoint. The lower detection limit of NH3 − N in the sample depends on the volume of the sample distilled and the normality of the standard titrant.

Carbon dioxide (CO2)

Titrimetric method: A measured volume of sample is titrated with a 0.01N standard solution of sodium bicarbonate (NaHCO3) containing phenolphthalein indicator to an endpoint where the solution turns pink. Alternatively, add a few drops of phenolphthalein indicator solution (in ethanol) to the sample and titrate against a standard solution of sodium hydroxide to the pink endpoint. The sample may also be titrated potentiometrically to a pH of 8.3. The titration should be carried out immediately after the sample is collected. Free CO2 in water may also be evaluated by a nomographic method if the pH and the temperature of the sample, as well as the bicarbonate alkalinity and the total dissolved solids content of the sample, are known.

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21.34

SECTION TWENTY-ONE

TABLE 21.13 Recommended Wavelength, Flame Type, and Technique for Flame Atomic Absorption Analysis Element

Wavelength (nm)

Flame

Technique

Aluminum Antimony Arsenic Barium Beryllium Bismuth Cadmium Chromium Cobalt Copper Iron Lithium Lead Magnesium Manganese Molybdenum Nickel Silver Selenium Silicon Strontium Tin Titanium Vanadium Zinc

309.3 217.6 193.7 553.6 234.9 223.1 228.8 357.9 240.7 324.7 248.3 670.8 283.3, 217.0 285.2 279.5 313.3 232.0 328.1 196.0 251.6 460.7 224.6 365.3 318.4 213.9

N2O–acetylene Air–acetylene Air–hydrogen N2O–acetylene N2O–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene Air–acetylene N2O–acetylene Air–acetylene Air–acetylene Air–hydrogen N2O–acetylene Air–acetylene Air–acetylene N2O–acetylene N2O–acetylene Air–acetylene

DA, CE DA H DA DA, CE DA DA, CE DA, CE DA, CE DA, CE DA, CE DA DA, CE DA DA, CE DA DA, CE DA, CE H DA DA DA DA DA DA, CE

Note: DA, direct aspiration; CE, chelation extraction; H, hydride generation. Source: P. Patnaik, Handbook of Environmental Analysis, CRC Press, Boca Raton, FL, 1997.

Another advantage of the graphite furnace technique over the conventional flame method is that the former requires a smaller volume of sample. On the other hand, a disadvantage of the furnace technique is that because of its high sensitively, interference from other substances present in the sample is often manifested. Such interference can be removed or reduced by adding a matrix modifier to the sample or by correcting for background absorbance. Analysis in flame mode is simple, and if the metals are present in relatively high concentrations—above 1 mg/L—the method gives satisfactory results. The wavelengths recommended for the metals of environmental interest, the flame type, and the techniques for sample introduction are presented in Table 21.13.

21.9.2 Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) ICP-AES offers certain advantages over furnace AAS. Although the latter has lower detection limits than the former, ICP-AES provides simultaneous or sequential multielement analysis in a simple analysis. Thus, several metals can be determined rapidly in a sample in a simple run. Also, a single point calibration suffices in IEC-AES analysis and, unlike furnace AA, chemical interference is very low. The principle of the ICP method is discussed in Sec. 8, Atomic Spectroscopy. The recommended wavelengths and the instrument detection levels for some selected metals are highlighted in Table 21.14.

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21.35

TABLE 21.14 Recommended Wavelength and Instrument Detection Level for ICP-AES

Element

Wavelength recommended (nm)

Alternative wavelength (nm)

Approximate detection limit (µg/L)

Aluminum Antimony Arsenic Barium Beryllium Boron Cadmium Calcium Chromium Cobalt Copper Iron Lead Lithium Magnesium Manganese Molybdenum Nickel Potassium Selenium Silica (SiO2) Silver Sodium Strontium Thallium Vanadium Zinc

308.22 206.83 193.70 455.40 313.40 249.77 226.50 317.93 267.72 228.62 324.75 259.94 220.35 670.78 279.08 257.61 202.03 231.60 766.49 196.03 212.41 328.07 589.00 407.77 190.86 292.40 213.86

237.32 217.58 189.04 493.41 234.86 249.68 214.44 315.89 206.15 230.79 219.96 238.20 217.00 — 279.55 294.92 203.84 221.65 769.90 203.99 251.61 338.29 589.59 421.55 377.57 — 206.20

50 30 50 2 0.5 5 5 10 10 10 5 10 50 5 30 2 10 15 100 75 20 10 25 0.5 50 10 2

Source: P. Patnaik, Handbook of Environmental Analysis, Boca Raton, FL, 1997.

21.9.3 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) ICP-MS provides a method for multielement determination of dissolved metals at ultratrace levels. ICP-MS combines inductively coupled plasma with a quadrupole mass spectrometer. ICP of high energy generates charged ions from the atoms of the elements present in the sample. The ions generated are directed onto a mass spectrometer, separated, and measured according to their massto-charge ratio. The method is highly sensitive, and the detection limits for some metals may be 100 times lower than that obtained by graphite furnace AA technique. Nonmetals and isotopes can also be measured by ICP-MS. However, ICP-MS has not yet found much application in routine environmental analysis of metals because of the high cost of the instrument and also because furnace AA and the ICP-AES can effectively achieve the detection levels for metals required for regulatory purpose. 21.9.4 Sample Digestion To measure metals by the instrumental methods discussed above, a nonaqueous sample must be brought into aqueous phase. The metals must be solubilized in water. Even for aqueous samples, such as wastewaters or aqueous sludges that may contain high suspended solids, such sample digestion for extraction of metals is essential. Additionally, by reducing the volume of the final sample extract to a

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21.36

SECTION TWENTY-ONE

TABLE 21.15 Acid Combinations for Sample Preparation Acid combination

Suggested use

HNO3–HCl HNO3–H2SO4 HNO3–HClO3 HNO3–HF

Sb, Sn, Ru, and readily oxidizable organic matter Ti and readily oxidizable organic matter For organic materials that are difficult to oxidize Siliceous meterials

Source: P. Patnaik, Handbook of Environmental Analysis, CRC Press, Boca Raton, FL, 1997.

small volume, the detection levels of metals present in the samples may be lowered accordingly. Normally, a sample, aqueous or nonaqueous, is digested with a strong acid, usually nitric acid or its combination with another acid or a strong oxidizing agent, to solubilize metals and bring the solution into aqueous phase. Various acid combinations for sample preparation are suggested in Table 21.15. 21.9.5 Chelation Extraction Method Many metals, such as Cd, Cr, Co, Pb, Ni, Mn, and Ag, at low concentrations may be extracted with a chelating agent, such as ammonium pyrrolidine dithiocarbamate (APDC). The metal chelate formed is then extracted with a suitable solvent, such as methyl isobutyl ketone (MIBK). The MIBK extract is then aspirated directly into the air–acetylene flame. Other chelating agents, such as 8-hydroxyquinoline, are often used. If an emulsion forms at the interface between the MIBK and water, use anhydrous Na2SO4 to break up the emulsion. 21.9.6 Hydride Generation Method Metals such as arsenic and selenium are converted into their hydrides in an HCl medium by treatment with sodium borohydride. The hydrides formed are purged into the atomizer with argon or nitrogen for conversion into gas-phase atoms. When the sample is digested with nitric acid these metals are oxidized to their higher valence states, As(V) and Se(VI). The digested sample should therefore be heated with concentrated HCl (and sodium iodide for As determination) to reduce these metals into their lower oxidation states, As(III) and Se(IV), for conversion into their hydrides. The calibration standards for these metals should also be converted into their hydrides in the same manner. 21.9.7 Cold Vapor Method for Measuring Mercury Mercury is analyzed by cold vapor AA technique. Nitric acid digestion of sample converts mercury into its nitrate. When the solution is treated with stannous chloride, mercury is reduced to its elemental form and volatilizes to vapor. Under aeration the vapor is carried into an absorption cell. The absorbance is measured at 253.7 nm. The calibration standards are subjected to similar oxidation, reduction, and vaporization. Interference from sulfide and chloride is removed prior to reduction by treatment with KMnO4. Free chlorine formed from chloride is removed by treating with hydroxylamine sulfate and sweeping the sample gently with air.

Bibliography American Public Health Association, American Water Works Association, and Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Association, Washington, DC, 1998.

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ANALYSIS OF TRACE POLLUTANTS IN THE ENVIRONMENT

21.37

American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 11.01, Water and Environmental Technology, American Society for Testing & Materials, Philadelphia, PA, 1989. Burrell, D. C., Atomic Spectrometric Analysis of Heavy-Metal Pollutants in Water, Ann Arbor Science Publishers, Ann Arbor, MI, 1975. Patnaik, P., Handbook of Environmental Analysis, CRC Press, Boca Raton, FL, 1997. U.S. Environmental Protection Agency, Methods for Organic Chemical Analysis of Municipal and Industrial Wastewater, Environmental Monitoring and Support Laboratory, Cincinnati, OH, 1982. U.S. Environmental Protection Agency, Methods for Chemical Analysis of Water and Wastes, Environmental Monitoring and Support Laboratory, Cincinnati, OH, 1983. U.S. Environmental Protection Agency, 40 CFR Part 136, Federal Register, 49, No. 209, 1984. U.S. Environmental Protection Agency, Methods for the Determination of Metals in Environmental Samples— Supplement I, EPA 600/R-94-111, 1994. U.S. Environmental Protection Agency, Test Methods for Evaluating Solid Waste: Physical/Chemical Methods, SW-846, National Technical Information Service, Washington, DC, 1997.

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 22

AIR ANALYSIS 22.1 22.2

INTRODUCTION AIR SAMPLING AND ANALYSIS Figure 22.1 Solvent Desorption Tube 22.2.1 Desorption of Pollutants from Sorbent Tubes Table 22.1 Air Sampling Techniques 22.2.2 Analysis Table 22.2 Analysis of Common Pollutants by NIOSH Methods Table 22.3 U.S. EPA Methods for Air Analysis Bibliography

22.1

22.1 22.1 22.2 22.2 22.3 22.4 22.5 22.22 22.22

INTRODUCTION Analysis of ambient air is critical to identify pollutants present in the air and to determine their concentrations. These analyses involve several steps that begin with extensive preplanning depending on the objective—the foremost being whether the air is indoor and confined or outdoor or atmospheric. Indoor air analysis is usually carried out to ascertain occupational safety in a workplace, to maintain industrial hygiene, or to address possible health-related problems with respect to a specific case history. The monitoring of atmospheric air, in contrast, is usually done year around, especially in urban areas, to measure the extent of air pollution. A comprehensive sampling plan, therefore, is the starting point for air analysis. The air sampling plan involves determination of proper sites for sampling, and the time, duration, and number of samples to be collected. For outdoor air, weather conditions, topography, humidity, and altitude are the critical factors in selecting sampling sites. A detailed discussion of sampling plans is beyond the scope of this text. Readers interested in further information may refer to the Annual Book of ASTM Standards and Patnaik’s Handbook of Environmental Analysis as listed in the Bibliography. Various techniques employed in air sampling, especially pertaining to indoor workplaces and the methods of analysis, are briefly highlighted in this section. The National Institute of Occupational Safety and Health (NIOSH) has developed and validated a number of methods for measuring organic and inorganic pollutants in indoor air. Some of these methods are cited below. Also, the U.S. Environmental Protection Agency (EPA) has developed a series of sampling and analytical methods for a small number of pollutants belonging to certain classes of organics. They too are discussed briefly below. Also presented below are some general guidelines that may be followed to analyze pollutants for which no method is known.

22.2

AIR SAMPLING AND ANALYSIS The term air sampling refers to collection of air or trapping of the air for analysis. Usually, it refers to trapping pollutants in the air by various techniques, identifying them, and measuring their concentrations in the air. Direct air sampling methods include collection of air from the sites in Tedlar bags, canisters, or any appropriate container following repeated evacuation of the containers to flush out the existing air, or collecting the air in a canister under pressure. Also, air may be liquified under low temperature and high pressure and collected in a canister and brought to the laboratory for analysis. In this case, the existing air in the container is first flushed out repeatedly 22.1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

AIR ANALYSIS

22.2

SECTION TWENTY-TWO

FIGURE 22.1

Solvent desorption tube. (Supelco Catalog 2003, Supelco Inc.,

Bellefonte, Pa.)

at the sampling site and then the sampled air in the container is placed in a liquid argon or oxygen bath. Such direct air sampling methods may only be applicable to determine organic pollutants present in the air or to measure pollutants that are gaseous at ambient temperature. The most common sampling technique, however, involves passing a measured volume of air either through a tube packed with adsorbent materials or through a filter cassette or through an impinger solution, depending on the nature of the analyte. Among the adsorbent materials, activated charcoal is most commonly used to trap many types of organic pollutants in air. Other adsorbents include carbon molecular sieves, Tenax (2,6-diphenylene oxide), many types of porous polymers under various trade names, and silica gel. This last is used to trap polar organic molecules such as lower aliphatic alcohols, aldehydes, and ketones. The adsorbent may alternatively constitute a derivatizing substance to convert the pollutants to derivatives for easier determination. For example, formaldehyde may be converted to 2-benzyloxazolidine by passing air through a solid sorbent tube containing 2-(benzylamino)ethanol on Chromosorb 102 or XAD-2 . Glass beads sometimes are packed in cryogenically cooled traps to increase surface area, providing inert and thermally stable adsorption sites. Often they are used as a filter at the inlet end of multibed adsorbent tube to condense large molecules from air. The adsorbent tubes usually have two sections, the front and the back sections, separated by a thin porous pad. The back portion is meant to trap any material that escapes from the front or when the latter is fully saturated with analyte molecules. A diagram of a typical sampling tube is shown in Fig. 22.1. The open ends of the adsorbent tube are packed with glass wood to prevent any mechanical loss of adsorbent during air sampling. A sampling pump, usually a small vacuum pump, is attached to the back end of the adsorbent tube. Air is sucked in by turning on the pump. Air is allowed to flow at the desired rate through the tube from the front to the back section, passing through the adsorbent packing. The pump is calibrated before use to regulate the flow rate of the air and to measure the total volume of air sampled. At the end of sampling, the sampling tube is disconnected from the pump and both ends of the tube are capped for storage or shipment. When air is sampled to measure particulate matter, metal dusts, or other solid particles, a membrane filter of appropriate pore size, such as 0.45 mm, is placed on a cassette and the latter is attached to a sampling pump. The solid particles are retained on the filter. The total volume of air sampled is calculated from the flow rate and the total time of sampling. Water-soluble pollutants present in air may be collected as aqueous solutions by bubbling the air through a measured volume of water in an impinger. Acid or alkaline vapors or gases may be absorbed in the solution by this technique rather than being adsorbed over a solid adsorbent. Also, organic solvents may be used instead of water in such impingers to collect organic pollutants that are readily soluble in such solvents. The vapor pressure, toxicity, and all safety factors, however, must be taken into consideration before using such organic solvents to absorb soluble pollutants from air. Solid adsorbents are usually preferred over organic solvents to trap pollutants from air. The general sampling techniques discussed above are summarized in Table 22.1. 22.2.1 Desorption of Pollutants from Sorbent Tubes Pollutants trapped over the adsorbents may be desorbed either by using an appropriate solvent or by thermal desorption. Solvent desorption methods are more common in air analysis, especially for

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AIR ANALYSIS

AIR ANALYSIS

22.3

TABLE 22.1 Air Sampling Techniques Sampling technique Direct sampling: At ambient temperature and pressure

Types of pollutants Many common gases, e.g., CH4, CO2, N2O

Under cooling

Organic vapors and gases

Under pressure

Organic vapors and gases, and many other common gases

Adsorbent tubes: Activated charcoal

Most organic pollutants

Tenax

Many organic substances

Silica gel

Acid vapors or acidic gases, such as HCl; polar organic molecules Many organics that can readily be derivatized, such as aldehydes and ketones

Adsorbent coated with derivatizing agents

Adsorbing solvents or solution in an impinger two-necked flask or other suitable apparatus

Water-soluble inorganic pollutants or organic substances that are soluble in water or nonvolatile organic solvents

Outline of the method Air is collected from sampling sites in Tedlar bags or glass flasks after repeated evacuation and flushing of the containers with air from the site; the air is injected directly onto a GC column for separation of pollutants and their determination by thermal conductivity detection (TCD) or mass spectrometry Air is collected in a canister and placed in a liquid argon or oxygen bath; organic vapors and many gaseous substances condense and collect in the canister; the pollutants are transported from the canisters and interfaced onto a GC injector port with heating under helium or nitrogen flow Canisters are pressurized with air from the sampling site using an air pump after flushing out its inside; the containers are brought to the laboratory and interfaced to a GC; the air in the canisters is transferred to the GC port under cryogenic cooling or by other techniques. Air is passed through the front and back sections of an adsorbent tube; adsorbed organics are desorbed out from activated carbon by desorption with a solvent such as CS2 or by heating under vacuum; desorbed organics are measured by GC, HPLC, or GC/MS Sampling procedure is the same as above; adsorbed compounds are desorbed using a suitable solvent and analyzed Desorption by a weak base, such as, Na2CO3 or NaHCO3; the solution is analyzed by an appropriate instrumental method Sampling procedures are the same as above; the organic derivatives in the sorbent tube are desorbed into an appropriate solvent using an ultrasonic bath; the solution is analyzed by GC, GC/MS, HPLC, or other instrumental technique Using an air sampling pump, a measured volume of air is bubbled slowly at a constant rate through an appropriate solvent or solution; the pollutants in the air dissolve in the solvents (or solutions) and the solutions may be analyzed by various wet methods or instrumental techniques (Continued)

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AIR ANALYSIS

22.4

SECTION TWENTY-TWO

TABLE 22.1 Air Sampling Techniques (Continued) Sampling technique Filter cassettes

Types of pollutants Particulate matter, dust particles, metal powder

Outline of the method A measured volume of air is passed through a membrane filter of appropriate pore size placed on a cassette; the particulate matter and dust collected on the filter is weighed to determine its concentration in air; the powder may be digested in nitric acid alone or in combination with another acid or oxidizing agent; the solution is diluted and analyzed for metals by atomic absorption (AA) or inductively coupled plasma (ICP) spectrophotometry

measuring pollutants in the workplace. The methods for analyzing a number of organic pollutants in the air developed by NIOSH using gas chromatography/flame ionization detection (GC/FID) or gas chromatography/mass spectrometry (GC/MS) are based on solvent desorption. An important criterion in solvent desorption is to know the desorption efficiency of the solvent for the pollutant. That is, one must know how much of the pollutants retained on the adsorbent material will desorb out into that solvent when transferred into a small measured volume of that solvent. Also, the conditions under which the desorption attained reaches maximum must be known. Such conditions may include contact time of adsorbent material with the solvent, swirling rate, the volume of solvent, and the temperature required to achieve best desorption. The desorption efficiency of a solvent for a compound may be determined by “spiking” a known amount of the compound to the adsorbent, placing the latter in contact with the solvent, and analyzing the resulting solution by GC to measure percent spike recovery. Thermal desorption is an alternative to solvent desorption. The adsorbent tube is placed in a thermal desorption device connected to the GC injector port. The tube is electrically heated and the adsorbed molecules are desorbed under vacuum or with an inert carrier gas to be transported onto the injector port of a temperature-programmed GC for analysis. 22.2.2 Analysis Organic pollutants are mostly analyzed by GC, GC/MS, or high-performance liquid chromatography (HPLC) techniques. GC methods commonly employ flame ionization detectors. Halogenated substances are detected by electron-capture detection or Hall electrolytic conductivity detection. GC detectors that may measure nitrogen-, phosphorus-, or sulfur-containing organics other than FID include the nitrogen–phosphorus detector and the flame photometric detector. Thermal conductivity detectors are employed to analyze common gases such as methane or nitrogen oxides. Compounds containing double bonds such as olefins or aromatics may conveniently be determined with a photoionization detector (PID). HPLC detectors in common use are UV and fluorescence types. Many U.S. EPA methods are based on GC/MS determination. Although its sensivity is lower than with most GC or HPLC detectors, an advantage is that it identifies unknown compounds from their mass spectra. Various portable instruments, mostly consisting of infrared detectors, are commercially available to measure in-situ volatile organic substances, solvent vapors, and organic and inorganic gases. Metal dusts and fumes are usually determined by atomic absorption and emission spectrometry. Particulate matter in the air is collected on filter cassettes and their weight determined by gravimetry. Fibrous materials and dusts may also be analyzed by electron microscopy and X-ray methods. Other general analytical techniques for many types of pollutants include colorimetry, ion-specific electrodes, and titrimetry. Badges or tubes are commercially available to routinely monitor specific pollutants in the workplace from color change in the material. Various air sampling techniques are outlined in Table 22.1. Air analysis of a number of pollutants by NIOSH and EPA methods are summarized in Table 22.2 and Table 22.3, respectively.

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AIR ANALYSIS

AIR ANALYSIS

22.5

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods Specific compound or class of pollutants

Examples

NIOSH method(s)

Outline of the method

—

1606

1–25 L of air at a flow rate of 0.01–0.2 L/min is passed through coconut shell charcoal (400 mg/200 mg); analyte is desorbed with 2 mL of methylene chloride–methanol mixture (85:15) in an ultrasonic bath for 45 min; the solution is analyzed by GC/FID using a fused-silica capillary column, cross-bonded polyethylene glycol (PEG), or equivalent

Acids, mineral

Hydrochloric, hydrobromic, sulfuric, nitric, phosphoric, hydrofluoric acids

7903

Between 3 and 100 L of air at a flow rate of 0.2–0.5 L/min are passed through a solid sorbent tube containing washed silica gel (400 mg/200 mg) with a glass-fiber filter plug; the analytes are desorbed from the solid sorbent with 10 mL of 0.0017M NaHCO3/0.0018M Na2CO3 solution; the acid anions are determined by ion chromatography using an appropriate anion separator column and NaHCO3/Na2CO3 eluent at the above concentrations

Alcohols

Ethanol, 2-propanol n-butanol

1400, 1401

Adsorbed over coconut charcoal; desorbed with a 1% solution of another alcohol in CS2; analyze by GC/FID (volume of air sampled is usually lower for lower alcohols)

Aldehydes

Formaldehyde, acetaldehyde, acrolein, furfural

2539, 2538

Adsorbed on 10% 2-(hydroxy methyl) piperidine on XAD-2; derivatized to oxazolidine derivative; desorbed into toluene under ultrasonication; analyzed by GC/FID or GC/MS

Alkaline dust

Caustic soda, caustic potash, lye

7401

Between 70 and 1000 L of air at a flow rate of 1–4 L/min are passed through a 1-mm PTFE membrane filter; after sampling, the filter placed in 5 mL of 0.01N HCl for 15 min under nitrogen with stirring; the alkalinity of the solution is measured by acid–base titration using a pH electrode

Amines, aliphatic

Dimethylamine, triethylamine

2010

Between 3 and 30 L of air at a flow rate of 0.01–1.0 L/min are passed through a solid sorbent tube containing silica gel (150 mg/ 75 mg); analytes are desorbed with 1 mL of dilute H2SO4 aqueous methanol during 3 h of ultrasonication; analytes are analyzed by GC/FID using a column containing 4% Carbowax 250M/0.8% KOH on Carbosieve B (60/80 mesh)

Amines, aromatic

Aniline, 2-amino toluene, N,N-dimethyl aniline

2002

Air is passed through a silica gel sorbent tube (150 mg/75 mg); analytes are desorbed into 1 mL of 95% ethanol in an ultrasonic bath over 1 h; the solution is analyzed by GC/FID using a packed column such as Chromosorb 103 or equivalent

Acetonitrile (cyanomethane, methyl cyanide)

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AIR ANALYSIS

22.6

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

Outline of the method

Amines, aromatic

Aniline, aminotoluene

2017

5–50 L of air at a flow rate of 0.2 L/min are passed through an assembly of a glassfiber filter and a sorbent tube packed with H2SO4-treated silica gel (520 mg/260 mg); the analytes are desorbed into 2 mL of ethanol; the solution is analyzed by GC/FID using an appropriate column

Asbestos and other fibers

Actinolite, amosite, crocidolite, tremolite, fibrous glass

7400, 7402

Airborne fibers are collected on a 0.45– 1.2-mm cellulose ester membrane placed on a filter cassette; fibers are counted by phasecontrast light microscopy to differentiate asbestos and nonasbestos fibers; air flow should be between 0.5 and 16 L/min, and the total volume of air to be sampled may be adjusted to give 100–1300 fiber/mm2

Benzoyl peroxide

—

5009

Pass 40–400 L of air at a flow rate of 1–3 L/min through a 0.8-mm cellulose ester membrane filter; analyte is desorbed with 10 mL of ethyl ether and analyzed by HPLC/UV at 254 nm using a pressure column and a 70 : 30 methanol/water mobile phase

Biphenyl

—

2530

Analyte is adsorbed on Tenax in a solid sorbent tube; desorbed with CCl4; analyzed by GC/FID

Boron carbide

—

7506

Particles are collected on a filter consisting of a10-mm Higgins-Dewell or nylon-type cyclone and a 5-mm PVC membrane; ashed in RF plasma asher; suspended in 2-propanol and measured by X-ray powder diffraction; between 100 and 1000 L of air at a flow rate of about 2 L/min may be sampled

1,3-Butadiene

—

1024

Between 3 and 25 L of air at a flow rate of 0.01–0.5 L/min are passed through a solid sorbent tube containing coconut charcoal; desorbed into methylene chloride over 30 min standing; analyzed by GC/FID using a fused-silica capillary column

Bromine and chlorine

—

6011

Air is passed through an assembly of a prefilter and a filter consisting of PTFE, 0.5 mm, and a silver membrane, 0.45 mm and 25 mm; halogens are converted to halide ions; extracted with 3 mL of 6 mM Na2S2O3 solution; analyzed by ion chromatography using a conductivity detector

5010

Analytes are collected on a 2-mm PTFE membrane filter; extracted with 3 mL of acetonitrile; analyzed by reverse-phase HPLC using a UV detector at 254 nm; the compounds may also be determined by GC

Bromoxynil and its octanoate

Dibromocyanophenol

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22.7

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

Outline of the method

Carbaryl (Sevin)

—

5006

Between 20 and 400 L of air at a flow rate of 1–3 L/min are passed through a glass fiber filter; analyte is extracted with a solution made up of 2 mL of 0.1M methanolic KOH, 17 mL of glacial acetic acid, and 1 mL of p-nitrobenzenediazonium tetrafluoroborate; the latter converting the analyte to a complex; absorbance of the solution measured by a spectrophotometer at 475 nm; calibration standard made from carbaryl in methylene chloride

Carbon dioxide

—

6603

Air is collected in a gas sampling bag at a flow rate of 0.02–0.1 L/min, the bag being filled to 80% or less capacity; the air is injected onto a portable GC equipped with a TCD and Porapak QS or equivalent column

Carbon disulfide

—

1600

Between 2 and 25 L of air at a flow rate of 0.01–0.2 L/min are passed through an assembly of a solid sorbent tube containing coconut shell charcoal (100 mg/50 mg) and a drying tube containing sodium sulfate (270 mg); analyte is desorbed with 1 mL toluene over 30 min standing; the solution is analyzed by GC/FPD in sulfur mode using GasChrom Q or equivalent GC column

Cyanuric acid

—

5030

A volume of 10–1000 L of air at a flow rate of 1–3 L/min is passed through a 5-mm PVC membrane filter; analyte is extracted with 3 mL of 0.005M Na2HPO4 at neutral pH for 10 min in an ultrasonic bath; the solution is analyzed by HPLC/UV at 225 nm using u-Bondapak C18 or equivalent column

1,3-Cyclopentadiene

—

2523

Between 1 and 5 L of air at a flow rate of 0.01–0.05 L/min are passed though a solid sorbent tube packed with maleic anhydride on Chromosorb 104 (100 mg/50 mg); maleic anhydride forms an adduct with 1,3-cyclopentadiene; the contents of the sorbent tube after sampling are placed in 10 mL of ethyl acetate and allowed to stand for 15 min; the adduct dissolves in the solvent; the solution is analyzed by GC-FID using 5% OV-17 on Chromosorb WHP or equivalent column

Diesel particulate (elemental carbon)

—

5040

An appropriate volume of air (about 140 L) at a flow rate of 2–4 L/min is passed through a 37-mm quartz-fiber filter; diesel particulate is determined with a thermal optical analyzer using evolved gas analysis (EGA technique); a cyclone should be used if (Continued)

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AIR ANALYSIS

22.8

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

Outline of the method heavy loadings of carbonate are anticipated; to minimize collection of coal dust (as in coal mines), an impactor with a submicrometer cut point must be used

Diesel particulate (elemental carbon)

1,1-Dimethylhydrazine

—

3515

An air volume of 2–100 L at a flow rate of 0.2–1 L/min is bubbled though 0.1M HCl; the solution is treated with phosphomolybdic acid, heated at 95°C for 60 min, and then cooled; analyte forms a complex; absorbance is measured at 730 nm with a spectrophotometer

Dimethyl sulfate

—

2524

An air volume of 0.25–12 L is passed through Porapak P (100 mg/50 mg); analyte is desorbed with 1 mL of diethyl ether over 30 min contact; the solution is analyzed by GC using an electrolytic conductivity detector in sulfur mode

Dioxane

—

1602

A volume of 0.5–15 L of air at a flow rate of 0.01–0.2 L/min is passed through coconut shell charcoal; analyte is desorbed into 1 mL of CS2 over 30 min contact; the solution analyzed by GC/FID

5013, 5509

Air is passed through a 5-mm PTFE membrane filter or a glass-fiber filter; analytes are desorbed with water under ultrasonic condition and reduced to free amine with sodium hydrosulfite; the solution is analyzed by HPLC using a UV detector at 280 nm and a C-18 column (method 5509 may be applied to measure benzidine only; a 13-mm glassfiber filter is used for sampling; the analyte is desorbed with triethylamine in methanol and measured by HPLC/UV at 254 nm)

1010

Between 2 and 30 L of air are passed over coconut shell charcoal (100 mg/50 mg) at a flow rate of 0.01–0.2 L/min; the analyte is desorbed into 1 mL of CS2 over 30 min contact of the charcoal with the solvent; the solution is analyzed by GC/FID

1450 (method 1454 describes a similar procedure for isopropyl acetate; method 1457, for ethyl acetate; and method 1458, for methyl acetate)

Between 1 and 10 L of air is sampled over coconut shell charcoal (100 mg/50 mg); esters are desorbed into 1 mL of CS2 by placing the adsorbent in CS2 for 30 min; the solution is analyzed by GC/FID using an appropriate column

Dyes, aminobiphenyl

Epichlorohydrin

Esters

Benzidine, o-tolidine, o-dianisidine

—

n-Propyl acetate, n-butyl acetate, isoamyl acetate, ethyl acrylate

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22.9

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants Ethers

Examples

NIOSH method(s)

Diethyl ether

1610

Diisopropyl ether

1618

Outline of the method An air volume of 0.25–3 L is sampled over coconut shell charcoal (100 mg/50 mg) at a flow rate of 0.01–0.2 L/min; analyte is desorbed with 1 mL of ethyl acetate over 30 min contact time; the solution analyzed by GC/FID Similar to above method; recommended volume and flow rate of air are 0.1–3 L and 0.01–0.05 L/min, respectively; analyte is desorbed into 1 mL of CS2 by placing charcoal in the solvent for 30 min; the solution is analyzed by GC/FID

Ethylene dibromide

—

1008

Between 0.1 and 25 L of air are sampled over coconut shell charcoal (100 mg/50 mg) at a flow rate of 0.02–0.2 L/min; analyte is desorbed with 10 mL of benzene– methanol (99:1) over 1 h standing; the solution analyzed by GC/ECD (63Ni)

Ethylenediamine

—

2540

Between 1 and 20 L of air are passed through a solid sorbent tube containing XAD-2 coated with 1-naphthylisothiocyanate (80 mg/40 mg) at a flow rate of 0.01–0.1 L/min; analyte is converted to its naphthylisothiourea derivative; it is desorbed with 2 mL of dimethylformamide under 30 min ultrasonication; the solution is analyzed by HPLC/UV

Ethylenimine

—

3514

1–48 L of air are bubbled through a solution of Follin’s reagent at a flow rate of 0.2 L/min; analyte is derivatized to 4-(1-aziridinyl)-1, 2-naphthoquinone; it is desorbed with 4 mL of chloroform; the derivative is analyzed by HPLC/UV at 254 nm

Ethylene oxide

—

1614

1–24 L of air are sampled at a flow rate of 0.05–0.15 L/min; analyte is adsorbed over HBr-coated petroleum charcoal (100 mg/ 50 mg), converted to 2-bromoethanol, desorbed with dimethylformamide, and analyzed by GC/ECD Ambient air, as is, or collected in a bag, is analyzed directly by a portable GC equipped with a PID

3702

Ethylene thiourea

—

5011

Between 200 and 800 L of air at a flow rate of 1–3 L/min is passed through a 5-mm PVC or cellulose ester membrane filter; analyte is extracted with distilled water at 60°C and complexed with pentacyanoaminoferrate; absorbance of the solution is measured at 590 nm by a spectrophotometer (Continued)

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AIR ANALYSIS

22.10

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

Outline of the method

Glycols

1,2-Ethanediol, 1,2-propanediol, 1,3-butanediol

5523

Between 5 and 60 L of air are passed through an XAD-7 OVS tube (13-mm glass-fiber filter and 200/100 mg XAD-7); analytes are desorbed into 2 mL of methanol under 30 min ultrasonication; the solution is analyzed by GC/FID using a fused-silica capillary column

Glycol ethers

Cellosolve, methyl cellosolve, butyl cellosolve

1403

Air is passed through a solid sorbent tube containing coconut shell charcoal (100 mg/ 50 mg); analytes are desorbed into 1 mL of 5% methanol in methylene chloride; the solution is analyzed by GC/FID

Herbicides, chlorophenoxy acid

2,4-D (2,4-dichlorophenoxy) acetic acid, 2,4,5-T (2,4,5trichlorophenoxy) acetic acid

5001

15–200 L of air at a flow rate 1–3 L/min is passed through a glass-fiber filter (binderless); after sampling, the filter is placed in 15 mL of methanol and allowed to stand for 30 min; the analytes are dissolved in methanol; the solution is analyzed for the chlorophenoxy acid anions by HPLC/UV at 284 nm using a Zipax SAX HPLC column or equivalent and NaClO4–Na2B4O7 eluent (0.001M mix); this method measures chlorophenoxy acids and their salts, but not their esters

Hydrocarbons, aliphatics, and aromatics (volatile, boiling between 36 and 126°C)

n-Pentane, n-hexane, cyclohexane, benzene, toluene

1500

Air is passed through a solid sorbent tube containing coconut shell charcoal (100 mg/50 mg); hydrocarbons trapped over the charcoal are desorbed with 1mL of CS2 over 30 min standing; the solution is analyzed by GC/FID

Hydrocarbons, aromatic

Benzene, toluene, xylene, cumene, styrene, naphthalene

1501

Sampling and analytical procedure similar to method 1500 above; the flow rate and the volume of air sampled may vary; the method involves adsorption of analytes over charcoal, desorption with CS2, and analysis by GC/FID

Hydrocarbons, polynuclear aromatic (PAH)

Anthracene, phenanthrene, chrysene, benzopyrene, fluoranthene, acenaphthene

5506

Between 200 and 1000 L of air at a flow rate of 2 L/min are passed through a 37-mm, 2-mm PTFE filter and a sorbent tube containing washed XAD-2 (100 mg/50 mg); the analytes are extracted with 5 mL of acetonitrile in an ultrasonic bath for 30–60 min; the solution is analyzed by HPLC using a fluorescence detector at 340 nm (excitation) or 425 nm (emission), or by UV detection at 254 nm; a reversedphase C15 (5-mm) column is suitable for HPLC analysis (the method is applicable to analyze pollutants that can be extracted with acetonitrile only)

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22.11

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s) 5515

5800

Hydrocarbons, halogenated

Hydrogen cyanide

Outline of the method Uses the same sampling technique as method 5506 but GC/FID measurement; a fused-silica capillary column such as DB-5 or equivalent is suitable for analysis Uses the same sampling technique and a HPLC flow injection method to measure the total PAH at two different sets of fluorescent wavelengths

Chloroform, bromoform, carbon tetrachloride, ethylene, dichloride, 1,1,1trichloroethane, benzyl chloride

1003

Air is passed through a solid sorbent tube containing coconut shell charcoal (100 mg/50 mg) at a flow rate of 0.01– 0.2 L/min; analytes are desorbed with CS2 over 30 min contact; the solution is analyzed by GC/FID; there are several other methods for one or more such compounds

—

6010

Between 2 and 90 L of air at a flow rate of 0.05–0.2 L/min are passed through a solid sorbent tube containing soda lime (600 mg/ 200 mg); the sample is desorbed into water over a period of 60 min contact; the solution is treated with succinimide and pyridine– barbituric acid; color is measured by a spectrophotometer at 580 nm Between 10 and 180 L of air at a flow rate of 0.5–1 L/min is passed through a 0.8-mm PVC membrane and 15 mL of 0.1N KOH solution; the combined extract and solution are analyzed for CN by a cyanide ion-specific electrode (the method cannot distinguish HCN from aerosol cyanide; also, there is interference from several other ions in cyanide measurement)

7904 (for aerosol cyanides and HCN gas)

Hydrogen sulfide

—

6013

Between 1 and 40 L of air are passed through an assembly of a 0.5-mm Zefluor filter and a solid sorbent tube packed with coconut shell charcoal (400 mg/200 mg) at a flow rate of about 0.2 L/min; H2S trapped from the air is desorbed from the sampler assembly into a solution containing 2 mL of 0.2M NH4OH and 5 mL of 30% H2O2; H2S oxidizes to sulfate (SO42−) ion; the latter is measured by ion chromatography using a conductivity detector, Ion-Pac AS4A separator or equivalent column, and 40mM NaOH eluent; SO2 is a positive interference this above test

Hydroquinone

—

5004

Between 30 and 180 L of air at a flow rate of 1–4 L/min are passed through a 0.8-mm cellulose ester membrane; analyte is extracted from the filter with 10 mL of 1% acetic acid; the solution is analyzed by HPLC-UV at 290 nm; a C18 m-Bondapak or equivalent column may be used (Continued)

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AIR ANALYSIS

22.12

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants Isocyanates

Isophorone

Ketones

Lead

Examples

NIOSH method(s)

Outline of the method Between 15 and 360 L of air are passed through 20 mL of tryptamine solution in dimethyl sulfoxide at a flow rate of 1–2 L/min; isocyanates are converted to their tryptamine derivatives; the solution is analyzed by HPLC using a fluorescence or an electrochemical detector and a m-Bondapak C18 or equivalent column and acetonitrile–sodium acetate buffer mobile phase Air is passed through an impinger solution containing 1-(2-methoxyphenyl)piperazine in toluene; isocyanates are converted to their urea derivatives; the solution is analyzed by HPLC using a UV or electrochemical detector (at 242 nm)

2,4- and 2,6-Toluene diisocyanate

5522

Monomeric isocyanates (most of the above isocyanates may also be analyzed by this method)

5521

—

2508

Between 2 and 25 L of air are passed through a solid sorbent tube packed with petroleumbased charcoal (100 mg/50 mg) at a flow rate between 0.01 and 1 L/min; isophorone is extracted into 1 mL of CS2 over 30 min contact time of the absorbent with the solvent; the CS2 solution is analyzed by GC/FID using an appropriate column

1300, 1301

An appropriate volume of air (depending on the analyte to be measured) at flow rates varying between 0.01 and 0.2 L/min is sampled over coconut shell charcoal (100 mg/50 mg); ketones are desorbed into 1 mL of CS2 (or CS2 containing a small amount of methanol to desorb out higher ketones); the solution is analyzed by GC/FID

7700 (a qualitative spot test)

Between 10 and 240 L of air at a flow rate of 2 L/min are passed through a 0.8-mm cellulose ester membrane; lead dusts collected on the membrane filter are tested qualitatively by chemical spot test using a rhodizonate test kit; lead forms an orange to pink-red complex under acid conditions; some other metals, e.g., Hg+, Tl+, Sn2+, Cd2+, and Ba2+, also form colored compounds, but only lead–rhodizonate complex gives a characteristic red color 20–1500 L of air at a flow rate of 1–4 L/min are passed through a 37-mm, 0.8-mm mixed cellulose ester membrane; lead dusts are extracted with 10 mL of 10% HNO3 under ultrasonication; the solution is diluted to 50 mL with 2% HNO3; lead is analyzed by a field-portable anodic stripping voltammeter

Acetone, cyclohexanone, methyl isobutyl ketone, camphor, mesityl oxide, ethyl butyl ketone

—

7701

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22.13

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

7702 (fieldportable XRF instrument)

7082 (Flame AA)

7105 (Furnace AA)

7300 (ICP-AES)

Lead tetraalkyls

Maleic anhydride

Tetramethyl lead, tetraethyl lead

—

Outline of the method using mercury film on glassy carbon as working electrode and a Ag/AgCl or calomel reference electrode; the method is quantitative; thallium may interfere in the test 570–1900 L of air at a flow rate of 1–4 L/min are passed through a 37-mm, 0.8-mm mixed cellulose ester membrane filter; lead is analyzed by a field-portable X-ray fluorescence instrument with a cadmium109 source; bromine may interfere in the test, giving a higher XRF reading; the method is quantitative, nondestructive, and applicable to all elemental forms of lead 200–1500 L of air at a flow rate of 1–4 L/min are passed through a 0.8–mm cellulose ester membrane filter; the metal dust collected on the filter is digested with a mixture of 6 mL conc. HNO3 and 1 mL 30% H2O2; the solution diluted to 10 mL with 10% HNO3; lead is determined by flame AA at 283.3 nm The procedure is similar to the above method; after sample digestion with HNO3–H2O2 at 140°C and dilution and cooling, lead is measured by furnace AA at 283.3 nm The procedure is similar to methods 7082 and 7105; the sample is extracted with an HNO3–HClO3 mixture (4 :1) at 130°C or, alternatively, by microwave heating; lead is analyzed by ICP/atomic emission spectroscopy at 220.4 nm

2534 (tetramethyl lead) 2533 (tetraethyl lead)

An appropriate volume of air between 15 and 200 L is passed through a solid sorbent tube packed with XAD-2 resin; after sampling, the adsorbent resin is transferred into 1–2 mL of pentane and allowed to stand for 30 min; lead tetraalkyls in the solution are analyzed by GC/PID using a Carbowax 20M on Chromosorb WHP column or equivalent

3512

An air volume of 40–500 L is bubbled through 15 mL of distilled water at a flow rate of 0.2–1.5 L/min; the solution is analyzed by HPLC/UV at 254 nm using a C-18 Bondapak or equivalent column; mobile phase, 0.5% dicyclohexylamine/ 0.5% formic acid/25% methanol/74% water, 1.7 mL/min (Continued)

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AIR ANALYSIS

22.14

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants Mercaptans

Examples Methyl-, ethyl-, and n-butyl mercaptans

NIOSH method(s) 2542

2525 (for n-butyl mercaptan)

Mercury

Metals

Methanol

Outline of the method Between 10 and 150 L of air are passed through a 37-mm glass-fiber filter impregnated with mercuric acetate at a flow rate of 0.1–0.2 L/min; analytes are extracted into 25 mL of a solution consisting of 20 mL HCl (25% v/v) and 5 mL of 1,2dichloroethane by shaking for 2 min; analysis is by GC in FPD mode using a narrow-bore fused-silica capillary column Between 1 and 4 L of air are passed over Chromosorb-104 (150 mg/75 mg) in a solid sorbent tube at a flow rate of 0.01–0.05 L/min; analyte is desorbed into 1 mL of acetone over 15 min contact with the solvent; the solution is analyzed by GC/FPD in sulfur mode using a Chromosorb-104 column

—

6009

Between 2 and 100 L of air at a flow rate of 0.15–0.25 L/min are passed through a Hopcalite sorbent tube; mercury is desorbed from the sorbent material into a mixture of conc. HNO3/HCl; the solution is diluted to 50 mL; mercury is determined by AA spectrophotometry using cold vapor technique

Aluminum, arsenic, calcium, copper, iron, lead, nickel, magnesium, manganese, sodium, silver, titanium, vanadium, zinc, and many other metals

7300

An appropriate volume of air depending on the metal to be determined is passed through a 0.8-mm cellulose ester membrane filter at a flow rate of 1–4 L/min; the metal deposited on the filter is extracted with 5 mL of HNO3–HClO3 (4 :1) mixture under heating; the extract solution is cooled, diluted, and analyzed by ICP/AES technique; metals may alternatively be measured by flame or furnace AA; furnace AA gives greater sensitivity over ICP/AES for many metals; the above method may also be applied to determine phosphorus, a nonmetal

—

2000

An air volume of 1–5 L at a flow of 0.02– 0.2 L/min is passed through silica gel (100 mg/50 mg) in a solid sorbent tube; methanol adsorbed onto the silica gel is desorbed with 1 mL of water–isopropanol mixture (95 : 5); 1 mL of solution is injected into GC a capillary column containing 35% diphenyl–65% dimethyl polysiloxane or equivalent and detected by FID

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AIR ANALYSIS

AIR ANALYSIS

22.15

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants Methylal (dimethoxymethane)

Naphthylamines

Examples —

a-Naphthyl amine, b-naphthylamine

—

Nicotine

NIOSH method(s)

Outline of the method

1611

Between 1 and 3 L of air at a flow of 0.01–0.2 L/min are passed through coconut shell charcoal (100 mg/50 mg); analyte is desorbed into 1 mL of hexane over a contact time of 30 min; the solution is analyzed by GC/FID

5518

Between 30 and 100 L of air at a flow rate of 0.2–0.8 L/min are passed through a glass-fiber filter and a solid sorbent tube containing silica gel (100 mg/50 mg); analytes are desorbed into a 0.5 mL solution of acetic acid (0.05% v/v) in 2-propanol; the solution is analyzed by GC/FID using an appropriate column

2544

Between 60 and 400 L of air are passed through XAD-2 (100 mg/50 mg) at a flow rate 1 L/min; the adsorbent is then transferred into 1 mL of ethyl acetate; the solution is allowed to stand for 30 min and then analyzed by GC/NPD The procedure is similar to the above method except that XAD-40 (80 mg/40 mg) is used as adsorbent; also, the method is applicable to measure air at a lower flow rate, between 0.1 and 1 L/min, and a wider volume range, 0.5–600 L; the analyte is desorbed into 1 mL of ethyl acetate containing 0.01% triethylamine; the solution is analyzed by GC/NPD

2551

Nitroaromatics

2-, 3-, and 4-nitrotoluenes, nitrobenzene, 4-chloronitrobenzene

2005

An appropriate volume of air, depending on the types of nitroaromatics (usually between 10 and 150 L for most compounds, but 1 to 30 L for nitrotoluenes) is passed through a solid sorbent tube packed with silica gel (150 mg/75 mg) at a flow rate of 0.01–0.02 L/min; analytes are desorbed out from the silica gel into 1 mL of methanol in an ultrasonic bath over 30 min; the methanol solution is analyzed for nitroaromatics by GC/FID; a fused-silica capillary column is suitable for the purpose

Nitrogen oxides

Nitrous oxide (N2O)

6600

Nitric oxide (NO) and nitrogen dioxide (NO2)

6014

Ambient air or bag samples may be measured for N2O by a field-readout, long-pathlength, portable infrared spectrophotometer at 4.48 mm; samples are usually stable for 2 h at ambient temperature Between 1.5 and 6 L of air are passed at a rate of 0.025 L/min (for NO) or 0.025– 0.20 L/min (for NO2) through an assembly of three sorbent tubes connected (Continued)

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AIR ANALYSIS

22.16

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

Nitrogen oxides

Nitrogen dioxide (NO2)

Nitroglycerine

Nitrosamines

Nuisance dusts (particulates not regulated, must not contain asbestos and quartz >1%)

—

N-nitrosodimethylamine, N-nitrosodiethylamine

—

6700

Outline of the method in series, the first and the third tubes packed with triethanolamine-coated molecular sieve (400 mg) and the second tube packed with 800 mg of chromate (oxidizer) to convert NO to NO2; contents of the tubes are extracted separately with 10 mL each of an absorbing solution containing 15 g of triethanolamine and 0.5 mL of n-butanol in 1 L of water; the extract is treated with 1 mL of 0.02% H2O2, 10 mL of sulfanilamide solution (2% strength in ~4% H3PO4 solution), and 1.5 mL of N-(1-naphthyl) ethylenediamine dihydrochloride solution (0.1% w/v); NO2 is converted to NO 2− ion; the solution is allowed to stand for 10 min to complete color development; absorbance is measured at 540 nm with a spectrophotometer; concentrations of NOx are determined from calibration standard solutions of nitrite ion (NaNO2) The procedure is similar to method 6014 above except that a Palmes tube with three triethanolamine-treated screens is used without any oxidizer to sample air

2507

Between 3 and 100 L of air at a flow rate of 0.2–1 L/min are passed through GC-grade Tenax (100 mg/50 mg); analyte is desorbed into 2 mL of ethanol over 30 min contact time; the solution is analyzed by GC/ECD using an appropriate detector

2522

Between 15 and 1000 L of air at a flow rate of 0.2–2 L/min are passed through the commercially available Thermosorb/N sorbent tube attached to a sampling pump; analytes are desorbed into 2 mL of methylene chloride/methanol mixture (3 : 1) over a sorbent-solvent contact time of 30 min; the solution is analyzed for nitrosamines by GC equipped with a thermal energy analyzer (TEA); 10% Carbowax 20M + 2% KOH on Chromosorb W-AW or equivalent column may be used

0500

Between 7 and 133 L of air at a flow rate of 1–2 L/min are passed through a 37-mm, 5-mm PVC filter; airborne particulate material collected on the filter is measured by gravimetry using an analytical balance; organic and volatile matter may be removed by dry ashing

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AIR ANALYSIS

AIR ANALYSIS

22.17

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants

Examples

NIOSH method(s)

Outline of the method

Organoarsenic acids

Methanearsonic acid, cacodylic acid, atoxylic acid

5022

Between 50 and 1000 L of air are passed through a 1-mm PTFE filter at a flow rate of 1–3 L/min; analytes are extracted from the filter with 25 mL of borate carbonate buffer; the solution is analyzed for respective organoarsenic anions by ion chromatography; alternatively, the solution can be analyzed by hydride atomic absorption spectroscopy at 193.7 nm

Pesticides, organochlorine

Endrin, chlordane, aldrin, methoxychlor

5519 (endrin) 5510 (chlordane)

Air is passed through an assembly of a 0.8-mm cellulose ester membrane and a solid sorbent tube packed with Chromosorb 102 (100 mg/50 mg) at a flow rate of 0.5–1 L/min; analytes are desorbed out from the adsorbent into 10 mL of toluene over 30 min contact time; the toluene solution of pesticide(s) is analyzed by GC using a Ni-63 ECD and an appropriate packed or capillary column (the same techniques may be applied to measure similar chlorinated pesticides in the air)

Pesticides, organophosphorus

Malathion, ronnel, mevinphos

5600

An air volume of 12–480 L at a flow of 0.2–1 L/min is passed through a 13-mm quartz filter and a solid sorbent tube packed with XAD-2 (270 mg/140 mg); pesticides are desorbed with 2 mL of tolune–acetone mixture (90 : 10); the solution is analyzed by GC/FPD in P mode using a fused-silica capillary column

1550

Between 1 and 20 L of air at a flow rate of 0.01–0.2 L/min are passed through a solid sorbent tube containing coconut shell charcoal (100 mg/50 mg); analytes are desorbed with 1 mL of CS2 over a solvent; contact time of 30 min; the naphtha hydrocarbons in the solution are measured by GC/FID using a fusedsilica capillary column

5020

Between 6 and 200 L of air are passed through a 0.8-mm cellulose ester membrane at a flow rate of 1–3 L/min; the analytes on the filter are desorbed into 2 mL of CS2 in an ultrasonic bath over 30 min; the solution is analyzed by GC/FID

6002

1–16 L of air at a flow rate of 0.01– 0.2 L/min are passed through a sorbent tube packed with Hg(CN)2-coated silica gel (300 mg/150 mg); analyte is extracted

Petroleum naphtha (mineral spirits, Stoddard solvent)

Phthalates

Phosphine

—

Dibutylphthalate; bis(2ethylhexyl) phthalate

—

(Continued)

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AIR ANALYSIS

22.18

SECTION TWENTY-TWO

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants Phosphine

Examples

NIOSH method(s)

—

OSHA Method 180

Outline of the method with 10 mL of hot acidic permanganate solution at about 70°C; the adsorbance of the solution is measured at 625 nm with a spectrometer; analyte is quantified from standard solutions of KH2PO4; PCl3 and PCl5 vapors and certain organic phosphorus compounds may interfere Employs KOH-impregnated carbon medium for air sampling

—

6402

10–100 L of air at a flow rate of 0.05– 0.2 L/min are bubbled through 15 mL of water in a bubbler; the aqueous solution of analyte is treated with 3 mL of Br2 water, 5 mL of sodium molybdate into molybdenum blue; the absorbance of the solution is measured at 830 nm by a spectrophotometer; analyte is quantified from calibration standard solutions of KH2PO4; phosphorus (V) compounds do not interfere in the test

Aroclor-1242, Aroclor-1254

5503

1–50 L of air at a flow rate of 0.05–0.2 L/min are passed through an assembly of a 13-mm glass-fiber filter and a solid sorbent tube packed with Florisil (100 mg/ 50 mg); PCBs are desorbed from the filter and front section with 5 mL of hexane and from the back section with 2 mL of hexane; the solution is analyzed by GC/ECD (Ni-63); chlorinated pesticides such as DDT and DDE and sulfurcontaining compounds in petroleum products may interfere in the test

Propylene oxide

—

1612

0.5–5 L of air at a flow rate of 0.01–0.2 L/min are passed through a solid sorbent tube packed with coconut shell charcoal (100 mg/50 mg); propylene oxide is desorbed into 1 mL of CS2 over 30 min contact time of adsorbent with CS2; the solution is analyzed by GC/FID using a fused-silica capillary column, such as DB-5 or equivalent

Pyridine

—

1613

20–150 L of air at a flow rate of 0.01–1 L/min are passed through a solid sorbent tube packed with coconut shell charcoal (100 mg/50 mg); pyridine is desorbed into 1 mL of methylene chloride over 30 min contact of charcoal with the solvent; the solution is analyzed by GC/FID using a packed column, 5% Carbowax 20M on acid-washed DMCS Chromosorb W

Phosphorus trichloride

Polychlorinated biphenyls (PCBs)

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AIR ANALYSIS

AIR ANALYSIS

22.19

TABLE 22.2 Analysis of Common Pollutants by NIOSH Methods (Continued) Specific compound or class of pollutants Refrigerants

Sulfur dioxide

Examples Trifluorobromomethane, chlorodifluoromethane, dichlorodifluoromethane

—

NIOSH method(s) 1017, 1018

An appropriate volume of air (less than 4 L) is passed at a flow rate of 0.01–0.05 L/min through two coconut shell charcoal tubes in series (400 mg/200 mg and 100 mg/ 50 mg); analytes are desorbed with methylene chloride and analyzed by GC/FID

6004

4–200 L of air are passed through a filter assembly of a 0.8-mm cellulose ester membrane and a Na2CO3-treated filter at a flow rate of 0.5–1.5 L/min; analyte is extracted with 10 mL of 0.00175M NaHCO3/0.002M Na2CO3 solution mixture; the solution is analyzed for sulfite and sulfate ions by ion chromatography using a conductivity detector; SO3, if present in the air, interferes in the test Alternatively, the air may be bubbled through 0.3N H2O2; the solution is titrated with NaOH or barium perchlorate Air is bubbled through tetrachloromercurate solution; color developed due to SO2 reaction with reagent is measured by visible spectrophotometry SO2 is trapped over a 5A molecular sieve, thermally desorped, and analyzed by mass spectrometry

5308

P&CAM 160

P&CAM 204 Sulfur hexafluoride

—

6602

5244

Sulfuryl fluoride

—

6012

5245 Stibine

—

Outline of the method

6008

Air is collected in a Tedlar or other gas sampling bag at a flow rate of 0.02–0.1 L/min until the bag is filled to 7) with NaOH + EtOH, or with H2S. Ignite in H2 and weigh as met. Pt. Final detn. may also be made colorimetrically with SnCl2.

Ppt. Pt with excess NH4Cl from dil. HCl soln. Ppt. Pd from filtrate with dimethylglyoxime, filt., and weigh. Pd may also be sepd. from Pt by “BrO2− hydrolysis.” Ppt. Ir, Pd, Pt, and Rh with H2S from soln. remaining after Os distn.; dissolve sulfides in aqua regia, ppt. Ir and Pt from dil. HCl soln. with excess NH4Cl, and ppt. Pd from filtrate with dimethylglyoxime as in preceding meth.

Same. Small amts. of Pb are best detd. polarographically, photometrically (with dithizone), or spectrographically. Same.

After expelling As and Sb, dissolve sol. sulfates in H2O, filt. off PbSO4, dissolve it in NH4OAc, and ppt. and weigh as PbCrO4. Dissolve the PbSO4 in NH4OAc soln. and ppt. and weigh PbCrO4.

Ppt. P as phosphomolybdate (see above), filt., wash free from acid with KNO3 soln., dissolve in exccess std. acid, and back-titr. with std. NaOH. Or det. colorimetrically as the heteropoly Mo blue complex.

After removal of H2S group and reoxdn. of the soln., ppt. with NH4 molybdate from dil. HNO3 soln. Filt. off phoephomolybdate ppt., dissolve in dil. NH3, and ppt. with MgCl2. Finally weigh as Mg2P2O7. Grav. as Mg2P2O7.

Procedure

MINERAL ANALYSIS

23.22

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Heating with H2SO4 KHSO4 fusion

30, 168, 185, 271

98, 100, 200, 208, 215, 272

Minerals contg. U

Ra

Rb

By mass spectroscopy or radiochemically. Evap. the HCl fraction contg. the Ru, filt., and ppt. the Ru with NaHCO3 at pH 7.0. Filt., ignite the hydrated oxide in H2, and weigh as met. Ru. Ru may also be detd. colorimetrically with thiourea.

See Os, Ru, Ir, Pt, Pd.

All elements

See Pd above.

See Os above.

Dil. HCl

182a

Air

182a

5, 12, 94, 110, 117, 120, 136, 139, 166, 201

Rh

Rn

Ru

S

Filtn.

Leach melt in H2O, acidify with H2SO4 + H3PO4, and dist. Re out as bromide. Det. Re colorimetrically in distillate by a SCN− meth. similar to that outlined above for Mo. Distn.

All elements

Na2O2 fusion

167

Re

Insol. matter

Flame-photometric comparison of H2SO4 soln. with stds. contg. known concns. of Rb and interfering elements. Larger amounts of Rb can be detd. grav. by pptn. of RbClO4, RbB(C6H5)4, or Rb2PtCl4. Sepn. of Rb from Cs and K is difficult, and mixts. of these elements are best analyzed by x-ray fluorescence.

None

HF + H2SO4

146, 151, 198

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(Continued)

To the clear HCl soln. of the samp. add excess BaCl2 soln., filt., and weigh as BaSO4.

In filtrate from Pd detn., recover Rh with H2S, ignite ppt. in H2, and weigh as met. Rh.

Radiochemically.

Various, finally copptn. with Ba

All elements

Acids

Heat samp. strongly with H2SO4, transfer soln. into ice H2O, filt., and ppt. R.E. from an aliquot with excess H2Ox. Leach KHSO4 melt in acid, remove sol. salts by pptn. with aq NH3, ignite ppt., and treat it repeatedly with HF. Filt. off insol. fluorides, convert to sol. salts by fuming with H2SO4 or HClO4, and finally ppt. with H2Ox.

Dissolve the NH3 ppt. in acid, ppt. R.E. with excess H2Ox, filt., and ignite the Ox ppt. to oxide.

Ignite the pptd. (NH4)2PtCl6 and weigh as metallic Pt.

NH3 pptn. of R.E. Treatment with HF

Dehydration and filtn. Pptn. of R.E. with NH3 Pptn. of R.E. with H2Ox Filtn. Ox pptn. of R.E.

Pptn. of Pt with NH4Cl

Salts, Ca, Mg Ta, Nb, Ti, Fe, U, Zr

Insol. matter PO3− 4

SiO2 Na, Ca, Mg R2O3

Na2CO8 fusion

3, 57, 103, 188, 249

R.E.

Pd

Aqua regia

183, 196

MINERAL ANALYSIS

23.23

Ti

S

Element determined

Pb

Ba, Sr SiO2 Cu, As, Sb, Ag, Bi, Pb, Hg, Te

Ag, Sb Fe

HCl

Fusion with Na2CO3

Br2 + HCl + HNO3

HNO3 + HCl Br2 + HCl + HNO3

KHSO4 fusion

Na2CO3 fusion

KHSO4 fusion KHSO4 fusion

11, 145

27, 55, 158, 159

20, 36, 40, 41, 42, 62, 64, 70, 73, 87, 90, 104, 107, 111, 134, 160, 161, 164, 174, 180, 199, 204, 211, 224, 232, 234, 235, 245, 246, 247 19, 206

156, 158a, 167, 207, 229

98, 200, 215

249

47, 131, 153, 214

34, 75, 98, 100,163, 200, 208, 215, 223, 243

None

None

Salts, Ca, Mg Ta, Nb, Ti, Fe, U, Zr F SiO2

Cu, As

Separations required from

Dil. HCl

Decomposition with

15, 44, 61, 63, 135

Mineral numbers

TABLE 23.4 Procedures for the Analysis of Minerals (Continued)

NH3 pptn. Treatment with HF (see R.E.) Pptn. as Ox (see R.E.) Dehydration in HCl soln. and filtn. NH3 pptn.

Treatment with met. Fe NH3 sepn.

Treatment with met. Fe in dil. HCl soln.

Filtn. of Na2CO3 soln. Dehydration in HCl soln.

Pptn. of Pb with (NH4)2CO3

Treatment with met. Fe

Type of separation

Leach melt in dil. H2SO4, pass soln. through a Jones reductor (→ Ti(III)], and titr. with std. Fe(NH4)(SO4)2 soln. Leach melt in a soln. contg. 1M H2SO4, 2% (w/v) succinic acid, and 0.5% H2O2, and meas. A400−420.

Dissolve the NH3 ppt. in dil. HCl, ppt. Th (+R.E.) with H2Ox, and ignite to ThO2.

As preceding meth., but treat with HNO3 before adding HCl. Make the very sl. acidic (HCl) soln. ammoniacal, filt., re-ppt. the Fe(OH)3, b. to expel excess NH3, acidify with HCl, and finish with BaCl3 as above. Same.

After removing As, Cu, and other elements with met. Fe, ppt. SO−4 with BaCl2 and finish as above. After removing Pb with (NH4)2CO3, b. filtrate until neut., add a little HCl, ppt. with BaCl2, and finish as above. Leach Na2CO3 melt with H2O, filt., acidify filtrate with HCl, evap. to dryness, filt. off SiO2, take up in dil. HCl, and finish with BaCl2 as above. Decomp. samp. as indicated, evap. several times on a steam bath with intermittent addn. of HCl, treat with met. Fe, filt., and finish with BaCl2 as above.

Procedure

MINERAL ANALYSIS

23.24

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SiO2

HNO3 + HCl + H2SO4 Na2CO3 fusion Na2CO3 fusion

31

126

188, 250

Ta, Nb, Ti, R.E., Th

None

Ti, Fe Cr

Fusion with KHSO4 + NaF

HNO3 + H2SO4

Na2CO3 + KNO3 fusion

See R.E. above.

23, 53a, 116, 194, 252, 258, 260, 261

34, 98, 200, 215, 249, 271, 272

53a, 81, 170, 258, 262

131, 214

Tm

U

V

H2S group Fe, Mo

HNO3 + HCl + H2SO4

207, 229

Tl

Ca

SiO2

Salts and Ni SiO2

KOH fusion in Ni crucible

26, 274

Filtn. Cupferron pptn. of V

(NH4)2CO3 sepn.

H2S sepn. Cupferron–CHCl3 sepn.

Dehydration in H2SO4 soln. and filtn. Dehydration in HCl soln. NH3 pptn.

NH3 pptn. Filtn. after dehydration

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(Continued)

To the H2SO4 soln. add FeSO4 [→ V(IV)], then a sl. excess of KMnO4 [→ V(V)], then NaNO2 (to red. excess MnO−4), then urea (to destroy HNO2), and titr. with std. FeSO4 soln. Filt. the CO−3 soln., acidify the filtrate, and ppt. V with cupferron. Ignite ppt., fuse it with KHSO4, leach in dil. H2SO4, and det. V colorimetrically with H2O2.

Ppt. H2S group elements from H2SO4 soln., filt., b. to expel H2S, and ox. soln. with KMnO4. Ppt. Fe, Mo, etc., with cupferron and ext. them into CHCl3. Evap. aq phase to SO4 fumes, destroy org. matter with HNO3, and evap. several times with intermittent addn. of H2O to remove N oxides. Dil., pass through a Jones reductor, aerate the red. soln. to ox. U(III) to U(IV), and titr. with std. K2Cr2O7, using diphenylamine sulfonate as indicator. Heat melt with H2SO4 to expel HF quant. In subsequent (NH4)2CO3 sepns., U remains in filtrate, which is evapd., acidified with H2SO4, red. in a Jones reductor, and titrd. as in preceding meth.

Spectrographically.

Leach melt in H2O, acidify with HCl, ppt. Ti with NH3, and ignite ppt. Fuse in KHSO4, leach in dil. H2SO4, pass through a Jones reductor, and titr. with std. Fe(III) soln.

Same.

Leach melt in H2O, acidify with HCl, ppt. ZrO2, TiO2, etc., with NH3, and filt. Fume ppt. with HNO3 + H2SO4, filt. off SiO2, and det. Ti colorimetrically with H2O2. Det. Ti colorimetrically in filtrate from SiO2 detn.

MINERAL ANALYSIS

23.25

Filtn. Anion exchange

Same Expulsion with HBr

Fe, Mn, Al

Pb Fe, Mn Cd

Pb, insol. matter Cu, V, Mn, Fe

Same + Sn

HF + H2SO4

HNO3 + H2SO4 HNO3 + H2SO4

HNO3 + H2SO4

Same

See R.E. above

102, 158a, 226, 229

49, 124, 266

104

115

81, 170, 212, 213

232

Yb

Zn

Filtn. of PbSO4 NH3 pptn. Anion exchange

NH3 pptn.

Pptn. with NH3 + (NH4)2S2O8

Fe, Mn, Al

HNO3 + HCl

See R.E. above

Y

Fractional distn. of liq. air

Anion exchange

Mo

HCl

202

Air

Filtn. of WO3 Extn. of WO3 with aq. NH3

Type of separation

Ca, Fe, Mn, Mo Insol. matter

Separations required from

HCl + HNO3(10:1)

Decomposition with

99, 127, 216, 268

Mineral numbers

Xe

W

Element determined

TABLE 23.4 Procedures for the Analysis of Minerals (Continued)

Evap. the soln. of samp. to dryness several times with intermittent addn. of HCl, add aq NH3 + (NH4)2S2O8 to ppt. Al, Fe, and Mn, filt., and b. to expel NH3. Remove Cu by treatment with met. Pb and titr. Zn with std. K4Feoc soln.; or titr. Zn with std. EDTA after addn. of CN− (to mask Cu) and HCHO (to demask Zn). Evap. the H2SO4 soln. of samp. to dryness, dissolve residue in dil. HCl, and continue with NH3 + (NH4)2S2O8, etc., as in preceding method. Evap. filtrate from the PbSO4 to dryness and continue as in preceding meth. Pass a dil. H2SO4-HI soln. of samp. through an anion-exchange column. If present, remove Fe, etc., from the eluate with aq NH3 after evapn. to dryness. Det. Zn in eluate by titrn. with EDTA. Evap. filtrate from the PbSO4 or insol. matter to dryness, dissolve the salts in 1M HCl, pass soln. through an anion-exchange column to remove the elements indicated, elute Zn with 3M HNO3, and det. it as above. Same.

Mass spectroscopy.

Decomp. samp. by heating with 100 mL HCl, add 10 mL HNO3, and evap. to 10 mL Dil., let stand, filt. off WO3, dissolve it in aq NH3, and filt. B. the soln. to expel NH3, ppt. W with cinchonine, ignite, and weigh as WO2. In a 50:10 HCl–HF soln., Mo is retained on anion-exchange resin. In the eluate, det. W either grav. with cinchonine as above, or photometrically with hydroquinone or dithiol.

Procedure

MINERAL ANALYSIS

23.26

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Filtn.

SiO2

KOH fusion (see treatment for minerals 26 and 274)

214

Filtn. of insol. ZrSiO4 (see R.E.)

R.E.

Heating with hot concd. H2SO4

168, 271

NH3 pptn. Dehydration and filtn. (NH4)2S pptn. in presence of H2Tart

Salts and Ni SiO2 Fe

KOH fusion in Ni crucible

26, 274

Source: L. Meites, ed., Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963.

Zr Leach melt in H2O, acidify with HCl, and ppt. Zr, etc., with aq NH3. Filt., evap. ppt. to fumes with HNO3 + H2SO4, dil., filt., add H2Tart, make soln. ammoniacal, and ppt. with H2S. Filt., acidify the filtrate, and ppt. Zr + Ti with cupferron. Ignite and weigh the mixed oxides; correct for TiO2 (detd. colorimetrically with H2O2). The zircon is not attacked by the H2SO4 treatment. After filtn., fuse residue with KOH as in preceding meth. and continue as described there. Filt. off SiO2. To the H2SO4 soln. add excess H2O2, then 5 g (NH4)2HPO4. Filt., ignite, and weigh as ZrP2O7. Or ppt. Zr with bromomandelic acid and ignite to ZrO2.

MINERAL ANALYSIS

23.27

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MINERAL ANALYSIS

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Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 24

METHODS FOR DETERMINATION OF WATER IN GASES, LIQUIDS AND SOLIDS Table 24.1 Table 24.2 Table 24.3

Methods for the Determination of Water in Gases Methods for the Determination of Water in Liquids Methods for the Determination of Water in Solids

24.2 24.4 24.7

There is no single technique applicable to measure water in solid, liquid, and gaseous substances although the Karl Fischer titrimetric procedure is widely used and the method to which all other methods are often compared. The analytical chemist must review need with respect to required precision and accuracy, water concentration, and skills available, as well as equipment on hand. Often speed is the most important criterion, particularly in production facilities. An excellent treatise on methods for the determination of water is the three-part monograph by Mitchell and Smith.1 There is also a much abbreviated treatment by Mitchell.2 Selected methods and techniques are outlined in Tables 24.1 through 24.3. Some comments on various methods are now given. The Karl Fischer method is perhaps the most widely used procedure for the determination of water. Although this method works well in many cases, the commercial reagents are rather costly, the visual titration end point is difficult to discern, and there are numerous interferences, including oxidizing agents, unsaturated compounds, and thio compounds. Liang3 discusses automatic on-line monitoring by flow-injection sampling. Infrared spectrometry is broadly useful for determining water in the gas, liquid, or solid phase. Several absorption bands can be used; the most useful are located in the near-infrared region at about 1.9 mm and in the fundamental region at about 2.7 and 6 mm. The reviews by Kaye4 and Wheeler5 and the report by Keyworth6 provide useful information on the near-infrared region. Procedures based on colorimetry usually employ CoCl2 (blue when anhydrous) or CoBr2 (green when anhydrous) that change to red for the fully hydrated salts. Cobalt chloride in ethanol gives an absorption maximum at 671 nm. Anhydrous ethanol can be used to extract water from the solid sample. Other substances that have found specific uses as colorimetric reagents have been methylene blue for traces of water in jet fuels, halides, ketones, and hydrocarbons; cobalt bromide–impregnated strips for testing halogenated refrigerants, gasoline, and oils; fuchsine for estimating water in granulated sugar and refinery pastes; and chloranilic acid for organic solvents except those containing amino-nitrogen.

1

John Mitchell, Jr., and D. M. Smith, Aquametry, 2d ed., Wiley, New York, 1977–1980, Parts I–III. J. Mitchell, Jr., in F. J. Welcher, ed., Standard Methods of Chemical Analysis, 6th ed., Van Nostrand, New York, 1966, Vol. 3, Part B, Chap. 64. 3 Y. Y. Liang, Anal. Chem. 62:2504 (1990). 4 W. Kaye, Spectrochim. Acta 6:257 (1954); 7:181 (1955). 5 O. H. Wheeler, Chem. Rev. 59:629 (1959). 6 D. A. Keyworth, Talanta 8:461 (1961). 2

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Other volatile bases or acids Colored substances ROH Other substances absorbed by desiccant ROH, RNH2 Other components of air have negligible effect Substances absorbing 105–150 nm, such as CH4, H2S Other compounds contributing to m/e = 18 Other compounds with same retention time Other compounds with same retention time

0.5% to few percent 0.5% to several percent 0.1%–1% ppm

ppm

Low ppm

0.5%–5% 0.5%–5%

Several volume percent ≥1 mg ppm High ppm

Pass through reagent at 100°C, absorb NH3 in standard H2SO4 [2]. NH3 ≡≡ 3H2O. Alcoholic extraction; measure absorbance at 671 nm [4]. Pass gas through CaC2 at 180 to 200°C; pass C2H2 released through ammoniacal Cu2SO4 and determine as red-colored CuC2 [2,16]. Pass through tared tube containing P2O5 or other suitable desiccant. Increase in weight is proportional to H2O [2]. Measure absorbance at suitable wavelength [2,5]. Absorbance of moisture in air at 27.97 mm using a water vapor laser [13]. Measure absorbance at 127 nm [6].

Measure m/e = 18 [2]. Separation through packed column of Carbowax 20M or Porapak Q at 150°C [2,7,12]. Reaction of H2O with CaC2; separation of resulting C2H2 by gas–liquid (OV-11 or DC 710) or gas–solid (silica gel at 80°C) chromatography [2]. Separation through packed column of Porapak N [17]. Measure heat transfer [8]. Pass gas sample through a cold trap containing CaC2 on which water is condensed; heat trap to 90°C and determine C2H2 produced [14]. Measure temperature before and after passage of gas through CaH2 [2]. Absorbed the released NH3 in H3BO3 and measure electrical conductivity [9].

Magnesium nitride (volumetric)

Cobalt(II) chloride (colorimetric)

Calcium carbide (colorimetric)

Absorption (gravimetric)

Infrared spectrophotometry

Vacuum ultraviolet

Mass spectrometry

Gas chromatography

Thermal conductivity

Mass spectrometry (indirect)

Calcium hydride (thermometric)

Magnesium nitride (conductometric)

2–100 ppm

24.2

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Other volatile bases or acids

ROH, RCOCH3, RCHO, NH3

Hydrocarbons

ROH, RNH2, R2NH, acids

0.1% to few percent

Pass gas through molten reagent at 60°C, absorb released HCl in water, and titrate with standard Na borate [3]. H2O ≡≡ 2HCl.

Succinyl chloride (volumetric)

ppm to few percent

RCHO, RSH

Interferences

ppm to several percent

Range

Condensation; alcohol, acid, or tert-amine extraction; titration to electrometric or visual end point [1]. H2O ≡≡ I2.

Procedure and references

Karl Fischer (volumetric)

Method and technique

TABLE 24.1 Methods for the Determination of Water in Gases

METHODS FOR DETERMINATION OF WATER IN GASES, LIQUIDS AND SOLIDS

Measure relative humidity with psychrometer or hygrometer [2,11].

Hygrometry

Measure pressure before and after removal of H2O [2].

Vapor pressure (manometric)

1. J. Mitchell, Jr., and D. M. Smith, Aquametry, 2d ed., Wiley-Interscience, New York, 1977–1980, three volumes. 2. J. Mitchell, Jr., in I. M. Kolthoff and P. J. Elving, eds, Treatise in Analytical Chemistry, Part II, Vol. 1, Interscience, New York, 1961. 3. C. B. Belcher, Thompson, and T. S. West, Anal. Chim. Acta, 19:148 (1958). 4. Singliar and Zubák, Chem. prumysl 6:426 (1956). 5. Curcio and Petty, J. Opt. Soc. Am. 41:302 (1951). 6. Garton, Webb, and Wildy, J. Sci. Instrum. 34:496 (1957). 7. S. Dal Nogare and Safranski, in J. Mitchell et al., eds., Organic Analysis, Interscience, New York, 1960, Vol. 4. 8. R. H. Cherry, Anal. Chem. 20:958 (1948). 9. Peck, Zedek, and Wittova, Chem. prumysl 5:219 (1955). 10. F. A. Keidel, Anal. Chem. 31:2043 (1959). 11. Weaver, Hughes, and Diniak, J. Res. Natl. Bur. Stand. (U.S.) 60:489 (1958). 12. V. M. Sakharov, G. S. Beskova, and A. I. Butusova, Zh. Anal. Khim. 31:250 (1976) (English, p. 214). 13. P. B. Lund and L. Kinnunen, J. Phys. E. Sci. Instrum. 9:528 (1976). 14. G. L. Carlson and W. R. Morgan, Appl. Spectrosc. 31:48 (1977). 15. H. Gerber, Res. Dev. 28:17 (Nov. 1977). 16. W. Boller, Chemiker-Ztg. 50:537 (1983). 17. F. F. Andrawes, Anal. Chem. 55:1869 (1983). 18. E. Flaherty, C. Herold, and D. Murray, Anal. Chem. 58:1903 (1986).

Pass over cooled polished metal surface; measure temperature at which dew forms, as observed photometrically or visually [2,18].

Dew-point measurement

Supersaturation hygrometer utilizing a thermo-optical system that senses growth of salt particles optically and controls their growth by heating the substrate with an infrared source. Output of heater is a measure of ambient relative humidity [15].

Pass through electrolyte cell containing P2O5 and measure current [10].

Electrolysis

Other compounds that condense Other substances that condense

Low percent

ROH, RCHO, NH3, CH3COCH3, HF

1–1000 ppm

Several volume percent Area of 100% relative humidity

1 ppm to 0.1%

METHODS FOR DETERMINATION OF WATER IN GASES, LIQUIDS AND SOLIDS

24.3

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0.1% to several percent ppm to few percent

Hydrolyze with strong acid and catalyst. Titrate sample and blank with standard NaOMe [3]. After hydrolysis, add known excess aniline, and titrate sample and blank with standard HClO4 in HOAc [4]. Titrate sample with acetic anhydride in HOAc, using strong acid as catalyst [2].

Measure absorbance in near-infrared region, 14 286 to 5000 cm−1 [2].

Acetic anhydride (volumetric)

Measure proton resonance. Chemical shift varies with water concentration and H bonding [2]. Measure intensity of blue color from water absorption in paper impregnated with FeSO4 and K ferricyanide [6].

Paper chromatography

Measure absorbance in fundamental region at or near 3590 cm−1. A drying technique was established employing vacuum distillation onto 4A molecular sieves [14].

Nuclear magnetic resonance spectroscopy

Infrared spectrophotometry

(spectrophotometric)

Hydrolyze at 110°C and determine excess acetic anhydride by measuring absorbance at 252 nm [2,5].

0.1% to several percent 0.1% to several percent 0.01% to few percent 0.01% to few percent

After reaction with reagent in Kjeldahl assembly, remove NH3 by steam distillation and determine acidimetrically [2].

Magnesium nitride (volumetric)

(conductometric)

0.02% to several percent

Reaction with reagent at room temperature, treatment of excess reagent with MeOH, and titration of sample and blank with standard base [1,2].

Acetyl chloride (volumetric)

0.1% to few percent

Several percent

0.05% to several percent

0.001%–0.1%

Water in organic solvents determined by flow injection and measurement at 546 nm [21].

Karl Fischer (flow injection)

ppm to 100%

Range

Titration in inert solvent (e.g., MeOH or pyridine) to electrometric or visual end point [1,2].

Procedure and references

Karl Fischer (volumetric)

Method and technique

TABLE 24.2 Methods for the Determination of Water in Liquids

ROH, RNH2

ROH, RNH2

ROH, RNH2, R2NH

Same as acetyl chloride method Same as acetyl chloride method ROH, RNH2, R2NH

Steam-distillable bases or acids; high concentrations of MeOH

HCOOH, RCHO, strong R3N, high concentrations of ROH, RNH2 or R2NH

Ketals

RCHO, RSH, NH2OH, (RCOO)2, quinone, ascorbic acid

Interferences

METHODS FOR DETERMINATION OF WATER IN GASES, LIQUIDS AND SOLIDS

24.4

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2.5 ppm up to 50% 0.04–0.2 mL

Columns using microparticulate normal phase, reversed phase, and ion exchange with NaCl in eluant and using a conductivity detector [22]. Water and alcohols in gasoline blends separated on HPLC ion exclusion columns (Ultrastyragel 100 and 500 Å) with toluene as mobile phase [20]. Thin-film perfluorosulfonate ionomer sensors overcoated with cellulose triacetate, polyvinyl alcohol–H3PO4 composite films operated in a pulsed voltammetric mode. Water in sample equilibrates with sensing film between pulses and is then electrolyzed by the pulse [17]. Molten salt (AlCl3–N-butylpyridinium chloride) and water generates HCl which is electrochemically reduced at a rotating Pt disk electrode [19]. Measure temperature rise on mixing sample with acetic anhydride and HClO4 catalyst [7]. Use a preliminary distillation step with a low-efficiency column, heating up to 135–150°C, then cooling the two-phase distillate at −10 to −20°C. The water will solidify and reject the dissolved hydrocarbons from the crystal matrix, which can then be decanted [12]. Measure dielectric constant directly in solution using high-frequency technique [8]. A microwave resonance method for water in oil emulsions uses the difference in dielectric properties between water and oil [15]. Measure current flowing through electrodes at fixed potential [9]. Titrant is LiH in dimethylsulfoxide for water in organic solvents [11]. Measure pressure rise when sample and CaC2 react in closed vessel [2]. Measure volume H2 evolved when sample and CaH2 react in gas-volumetric apparatus [9]. Water in dimethylformamide and dimethylsulfoxide determined by luminescence lifetime measurements of Eu(III) [18].

HPLC

Size exclusion and adsorption chromatography

Voltammetric sensor

Voltammetry

Acetic anhydride (thermometric)

Distillation

Dielectric constant

Conductivity

Conductometric titration

Calcium carbide (manometric)

Calcium hydride (gasometric)

Luminescence lifetime

0.05–5 mol %

Several percent

Several percent

0.3% to several percent

1% to high percent

0.5% to few percent

Linear up to 50 mM

0.03%–2% As low as 0.001% or 13.4 ppm As low as 0.001% or 13.4 ppm

Column packed with Porapak Q and kept at 150°C. Reaction with 2,2-dimethoxypropane and a solid acid catalyst (Nafion resin) for 5 min, followed by capillary column GC for acetone formed [16]. Instantaneous reaction with a 0.5 nM ortho ester (triethyl orthoformate) and catalyst (methanesulfonic acid) followed by capillary column GC for ethanol formed [17].

Gas chromatography

(Continued)

ROH, RCHO, NH3

ROH

Other conducting substances

Other compounds having high dielectric constant

ROH, RNH2, R2NH

METHODS FOR DETERMINATION OF WATER IN GASES, LIQUIDS AND SOLIDS

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24.5

Measure specific gravity directly in known system where water is only variable [2]. Measure refractive index of known system where water is only variable [2]. b -Rays passed through sample and measured with appropriate counter [10].

Density or specific gravity

Refractometry

Radiochemical (b -ray absorption)

Range

0.1%–1%

Variable

Variable

0.3% to few percent

1. J. Mitchell, Jr., and D. M. Smith, Aquametry, 2d ed., Wiley-Interscience, New York, 1977–1980, in three parts. 2. J. Mitchell, Jr., in I. M. Kolthoff and P. J. Elving, eds., Treatise on Analytical Chemistry, Interscience, New York, 1961, Part II, Vol. 1. 3. Toennies and Elliott, J. Am. Chem. Soc., 57:2136 (1935); 59:902 (1967). 4. Das, J. Indian Chem. Soc. 34:247 (1957). 5. S. Bruckenstein, Anal. Chem. 28:1920 (1956). 6. Stringer, Nature, 167:1071 (1951). 7. L. H. Greathouse, H. J. Janssen, and C. H. Haydel, Anal. Chem. 28:356 (1956). 8. Oehme, Angew. Chem. 68:457 (1956). 9. Perryman, Analyst 70:45 (1945). 10. Friedman, Zisman, and Sullivan, U.S. Patent No. 2,487,797 (1949). 11. C. Yoshimura, K. Miyamoto, and K. Tamura, Bunseki Kagaku 27:310 (1978); Chem. Abstr. 89: 16126 (1978). 12. T. H. Gouw, Anal. Chem. 49:1887 (1977). 13. J. Kovarik, Chem. Abstr. 86:56052 (1977). 14. A. Barbetta and W. Edgell, Appl. Spectrosc. 32:93 (1978). 15. D. A. Doherty, Anal. Chem. 49:690 (1977). 16. K. D. Dix, P. A. Sakkinen, and J. S. Fritz, Anal. Chem. 61:1325 (1989). 17. H. Huang and P. K. Dasgupta, Anal. Chem. 64:2406 (1992). 18. S. Lis and G. R. Choppin, Anal. Chem. 63:2542 (1991). 19. S. Sakami and R. A. Osteryoung, Anal. Chem. 55:1970 (1983). 20. M. Zinbo, Anal. Chem. 56:244 (1984). 21. I. Norden-Andersson and A. Edergren, Anal. Chem. 57:2571 (1985). 22. T. S. Stevens and K. M. Chritz, Anal. Chem. 59:1716 (1971).

Measure turbidity or cloud point on titration with water-immiscible liquid (e.g., xylene or mineral oil) [2].

Procedure and references

Turbidity

Method and technique

TABLE 24.2 Methods for the Determination of Water in Liquids (Continued)

ROH

Unknown constituents

Unknown constituents

Other compounds only slightly soluble in sample solution

Interferences

METHODS FOR DETERMINATION OF WATER IN GASES, LIQUIDS AND SOLIDS

24.6

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Several percent 0.1% to several percent