Analysis of Industrial Wastes Rapid Methods of Analysis Analytical Problems in Biological Systems
Papers presented at the Ninth Annual Snmmer Symposium sponsored by the Division of Analytical Chemistry and -4naCgticaC Chemietrg. Los Angeles, Calif., June 14 to 18,1956
Coulometry ERNEST H. SWIFT Department o f Chemistry, California h t i t u t e o f Techno/ogy, Pasadena, Calif.
A
S HAS been true of ot’her fields, coulometrj- has collected a
certain amount of nomenclature. Therefore, in consideration of those n-ho may be resorting to tlie literature for the first time, an attempt to define certain of t,hese terms will be made. With full cognizance that definitions are likely to be quitmesubjective in nature, it is proposed that coulometry include those electrolytic processes in which a determination of the quantity of a constituent is made by a measurement of bhe quantity of electricity-that is, the coulombs-required to react, directly or indirectly, Tvith that constituent. Three terms in this definition should be emphasized: First, the process is electrolyt,ic in nature. Secondly, a measurement must be made of the quantity of electricity involved. Thirdly, the constituent being determined can be involved directly or indirectly in an electrode reaction. As will be shown later, t,his third factor is of import’ance in the classification of coulometric processes. Obviously, if an electrolytic process is t o be used for the quantitative determination of a constituent, t.hen one of the electrode reactions must be capable of yielding low0 current efficiency; and as many significant figures as are required can be used in writing the 100%. The work which was done in t,he latter part of the last century in connection with various types of coulometers, or voltammeters as they r e r e then called, demonstrated that there were a considerable number of electrode reactions which could be made to proceed xit’h 100% current efficiency. Although this n-ork had as its primary objective the determination of the value of the faraday, the principles underlying coulometry \$-ereclearly established. For this reason, and in spite of the advances which have been made recently, me analytical chemists are entitled to a certain amount of humility arising from the fact that the analytical implications of this work were not exploited to a significant extent unt’il after almost 40 years of this century had passed. It is true that in 1917 Grower ( 5 ) est,imated the thickness of a plating of tin on a copper wire by making the wire the anode in an electrolytic process and measuring the quantity of electricity required t,o dissolve the tin anodically. Here was a clear demonstration of the principles of coulometry. But, except for a f e x similar applications, over 20 years elapsed before this pioneering xork n-as generally exploited. Then in 1942 Hickling (3)devised an electronic instrument v-hich he called a potentiostat, whereby the potential applied to t,he working electrode could be controlled. By use of this device he demonstrated t’hat 100% current efficiency could be obtained in the reduction of cupric copper t o the metal at a platinum cathode and in t,he oxidation of iodide t o iodine a t a platinum anode. I n 1945 Lingane made
use of a mercury cathode for the determination of copper (4). He also used n silver anode at n-hich the various halides were precipitated as the corresponding silver salts (5). There are certain characteristic features of these procedures which are used as a means of classifying coulonietric processes. First, in all cases t,he substance to be determined is involved in the half-cell reaction taking place a t one of the electrodes-that is, the cupric copper is reduced a t the cathode and the halide is either oxidized or precipitated a t the anode. h s :iresult such procedures have become knorrn as primarj- or direct coulometric methods. I n order t,o obtain 100% current efficiency in such primary processes, the potential applied t,o the n-orking elect,rode must be controlled and, accordingly, such procedures are also designated as controlled potential processes. Choice 3s to which of the above terms is used is dictated by R-hether the predominant interest is in the external circuitry or in the reactions taking place within the electrolytic cell. The limits within which the potential control must be maintained will depend, first, upon t,he accuracy of the determination being made, and, secondly, upon the presence of other electrode reactive substances in the solution. il chronological deviation of a few years has lieen made in order to present the above examples of primary processee, hecause in 1938 a series of seven articles, titled “Coulometric .lnalysis as a Precision llet,hod,” was published by Sze’ielledy and Somogyi (Y), tvio Hungarian chemists. This t,it,le appears to have been the first use of the term coulometric anal the opinion of this author, these papers initiated t,he extensive developments in coulometry ivhich have taken place in the past 10 years. Rather than describe this work in general terms, a specific example can be used to illustrate the principles involved. These workers vere interested in the determination of thiocyanate. They either knew or learned that the anodic oxidation of thiocyanate to cyanide and sulfate with 100% current efficiency is difficult, if not impossible, of attainment. Horvever, it was known that thiocyanate could be quantit,at,ivel>-,stoichiometrically, and rapidly oxidized by bromine in acid solutions, and they knew or learned that bromide could he anodically oxidized to bromine in acid solut,ions TVith 1 0 0 ~ ocurrent efficiency. Therefore, they added a relatively high concentration of a soluble bromide to anacid solution containing the thiocyanate, anodically produced bromine, and allowed this bromine to diffuse into the solution and to oxidize the thiocyanate. By working with relatively large samples of thiocyanate and by measuring the quantity of electricity involved by means of a chemical coulometer, they demonstrated that an accuracy of
1804
V O L U M E 28, NO. 1 2 , D E C E M B E R 1 9 5 6 within 1 p.p.t. could be attained. Szebelledy and Somogyi used chemical indicat,ors for determining the end point, of such titrations. Such a process as this has subsequently been termed a coulometric titration. Two fundamental differences esist between such a process and those which have been described previously. First, in the previous ones, the substance being determined v a s directly involved in the electrode reaction; in this latter process an int'erniediate half-cell reaction is used to generat'e a n active intermediate compound n-hich t,hen reacts Tvith the substance being determined. For this reason such processes are called secondary or indirect coulometric methods. Secondly, for reasons n-hich are discussed later, such processes can be caused to take place a t a constant current value and in such cases are called constant current methods. Each of these t\vo types of processes has certain advantages and limitations. The primary process, when used with controlled potential, enables one to attain considerable specificity as to the half-cell reaction t,nking place. The reduction of one metal in the presenc-e of another and the selective determination of various niistnres of the halides has been demonstrated by Lingane ( 4 , 6 ) . The estent t o which this fundamental property of the controlled potential method can be esploited to minimize preliminary separations n-ill determine the ultimat,e analytical value of such methods. Ailsothe commercial :iv:dahility of reasonably priced potentiostats n-ill be a factor in the development' of such methods. I n a contmlled potential process the current decreases esponentiallg and Ltpproaches zero as>-mpt,otically. For this reason the time required for the process is fixed by the accuracy desiredessentiallj- the ratio of the initial to the final concentration of the const,ituent i jeing determined-and such a process is not adaptable to rapid determinations. I n addition, since the current is varying continiioiisl!; an accurate nipnwrenient of the quantitj. of electricity involved has required the use of chemical coulometers. It is to he hoped that accurate electronic coulometers v-ill be avuilahle in t,he future, becaiise chemical coulometers do not leiid tlienizelves to read>- w e for operations on a micro scale. The moyt fiind:iiiientd limitation to the development of primary coiilonietiic methods arises from the fact that an astonishing number or' substances are not capable of being oxidized or reduced a t conventional electrodes with 100% curreiit efficiency. Systematic esamiiution of a comprehensive list of half-cell reactions shows a surprising number that exhibit some degree of irreversibility. \J7hen the relative advantages of the secondary process are considered, it is apparent that, by taking advant.age of those intermediate half-cell reactions which can be made to proceed with 100% current efficiency, the coulometric principle can be applied to many substances n-hich are not reversibly oxidized or reduced at conventional electrodes. A s a result much of the research work in the p w t 10 years has been direct,ed toward studies of such intermediates. Among the osidants which have been investigated are bromine, iodine, chlorine, hypobromit,e, ceric cerium, ferric iron, argentic silver, and manganic manganese. A partial list of the reduct'ants includes cuprous copper in chloride and bromide solutions, ferrous iron, and titanous titanium. The possibility of coulometrically plating an excess of a metal, allowing the metal to react with an oxidant, and t'hen stripping the escess by a reversal of polarity has been studied and has promise. Precipitants have included ferrocyanide and unipositive silver and niercurj-. Both acids and bases have been coulometrically generated. The secondary process also alloivs a constant current t o be maintained throughout the process, thus permitting the attainment of several desirable features. The first of these is rapidity; titrations of 100 seconds or less are conventional. Secondly, by control of t,he constancy of the current, to within the desired accuracy of the titration, one can substitute for the chemical
1805 couloniet,er the constant current-time method of nieasuring the quantity of electricity involved in the titration. Finally, b3reducing the current value, the times required for the titration of micro quantities of constituents can be estended to accurately measured intervals. As a result the titration of microgram quantities is readily made with accuracies as good as or better than those attained by the best volumetric methods with milligram quantities. Application of the secondary method of coulometric analysis to t,he field of quantitative microanalysis presents a fruitful field for further research and development. Finally, because an electric current is EO much more amenable to automat.ic control than is a stopcock or other mechanical device, coulometry off ers obvious advantages in the ever developing field of automation. hIet,hods and instruments for automatic control of titrations have already been developed, as have methods for obtaining a continuous record of the concentration of a constituent in solutions or in a gas phase. Because of the interest of this conference in atmospheric constituent,~,it seems appropriate to describe briefly two examples of coulometric methods for Obtaining a continuous analysis for two gaseous constituents. The first of the9e instruments \vas developed initially t o obtain a cont,inuous record of the mustard gas concentration in the air (6). It has since been produced as a commercial inst,rumentthe Titralog of Consolidated Electrodynamics Corp.-for obtaining a continuous record oE rediicing gases such as hydrogen sulfide or sulfur dioxide in the air. I n the original instrument the air n-as dran-n through a sulfuric acid solution cont'aining a soluble bromide. A platinum sensing electrode coupled xvith a standard hslf-cell was used in conjunction with an amplifier to control a generating circuit. Bromine was produced a t a platinum anode and hydrogen a t a platinum cathode. With no mustard gas or other reducing constituent present in the incoming air, the sensing electrode controlled t,he generating current a t a value which maintained a very low concentration of bromine in the electrolytic solution. K h e n mustard gas \vas present' in t,he air, it cauped a decrease in the bromine concentration. The resultant change in potential of the sensing electrode caused the generating current to increase until the initial bromine concentration was restored. The generating current was sent through a recorder and a continuous record t,hus obtained. The second instrument has been developed by Regener for obtaining a cont,inuous record of the atmospheric ozone concentration. Only a preliminarJ- model has been described (1). I n his present instrument air is drnlyn a t a known rate through a chamber in which a neutral solution of potassium iodide containing a known concentration of thiosulfate is also passing a t a predetermined rate. The sensing system in the case consists of two similar platinum electrodes with a potential difference impressed across them. These elect,rodes control the current through the generating circuit, which produces iodine a t the anode. Kit'h no ozone or other agents capable of oxidizing iodide in a neutral solution, in the influent air the sensing electrodes maintain the generating current at a value which will produce iodine just sufficient to oxidize the thiosulfate and produce a minimal iodine concentrat,ion in the solution passing from the reaction chamber. If ozone is present, it produces iodine and less current is required to maintain the minimal iodine concentration. I n order to provide for the effect of other osidants such as nitrogen dioxide, which could be present, Regener's latest instrument makes provision for dividing the influent air stream equally and passing the separate streams through identical absorbing and titrating systems. The air in one stream is heated to a temperature which will decompose ozone. Therefore, one titrating system measures ozone plus other osidants, while the second system measures only the other oxidants. The difference between the values of the two generating currents is recorded as a measure of the ozone concentration. Although the description of t,hese tTvo methods is brief, if
1806
ANALYTICAL CHEMISTRY
illustrat.es the flexibility with which the coulometric principle can be applied to such problems, and indicates the tremendous possibilities for future developments in this relatively new field of chemical analysis.
(3) Hickling, d.,Trans. Faraday SOC.38, 27 (1942). 679 1916 (1945)* (4) L1nganei J. J. Am. (5) Lingane, J. J., Small, L. A., ANAL. CHEY. 21, 1119 (1949). , Jr.. Brockman, J. A , , Jr., Ibid., 20, (6) Shaffer, L. A., Jr., ~ r i g l i oA., J.i
1008 (1948).
(7) Szebelledy, L., Somogyi, Z., 2. Anal. Chem. 112, 313, 323, 332,
LITERATURE CITED
385, 391, 395, 400 (1938).
(1) Bowen, I. G., Regener, V. H., J . Geophgs. Research 5 6 , 307 (1951). (2) Grower, G. G., Am. SOC.Testing Materials, Proc., Pt. 11, 17, 129 (1917).
RECEIVED for review July 21, 1956. Accepted September 26, 1966. Contribution No. 2193 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena 4, Calif.
Ninth Annual Summer Symposium-Analysis of Industrial Wastes
Spectrometric Investigations of Atmospheric Pollution R. A. FRIEDEL Central Experiment Station, Bureau of Mines,
U. S.
Department o f the Interior, Bruceton, Pa.
Applications of spectrometry to studies of organic atmospheric pollutants are reviewed briefly. Recent work of this laboratory includes a mass spectral method for determining nitrogen dioxide and other components in complex mixtures, an infrared method for measuring specific hydrocarbons in coal-mine gases, and pyrolytic oxidation of aliphatic oxygenated compounds. Early results of a combination of methods had shown the presence of nitro- and possibly other nitrogen-oxygen groups in a Los Angeles air sample. Rlore combinations of spectral methods and of spectral plus separations methods could be utilized in atmospheric pollution w-ork. Suggestions for new approaches to stud>-ing atmospheric pollution include solvent extraction of organic constituents in the ice from freeze-out samples to reduce decomposition and other reactions, highresolution infrared and mass spectrometry, and possibly magnetic resonance methods.
S
PECTRAL investigations of atmospheric pollutants are being carried on a t an increasing rate. The earliest such applications of mass spectra, described by Washburn (34), were followed by others (7, 8, 16, 29, 55). The principal information gained from investigation of metropolitan atmospheres has been the carbon number distribution of hydrocarbons in the air and an estimate of the total number of organic compounds present. The method does not permit discrimination between oxygenated compounds and hydrocarbons of the same molecular weight, nor between naphthenes and olefins. Mass spectrometry has also been used repeatedly to study automotive exhausts (6, 7 , 8, 18, 5s). Recently the mass spectrometer has been used for measurements of C12/C13ratios in atmospheric carbon dioxide (9, 19). These investigations hold promise for establishing the principal sources of carbonaceous materials in air. Until recent years infrared analysis has been applied very little to air pollution studies (10, 24, 26). Hanst, Stephens, and coworkers are utilizing very long cells in infrared studies of synthetic smogs (16). Gates has investigated Los Angeles smog with the infrared portion of the solar spectrum (12). Twiss and con-orkers have used infrared for investigating automobile exhausts (32). Ultraviolet spectrometry has been used extensively for analysis of industrial plant atmospheres (31). The absorption sensitivity of most compounds in the ultraviolet region is not sufficient for widespread use in air pollution analysis. With a spectroradiometer, Stair has obtained maximum values for atmospheric
ozone and nitrogen oxides (30). The absorption spectrum of ozone in the ultraviolet region has been used recently for the quantitative determination of ozone in the atmosphere (22, $7). PROBLEMS RELATED TO AIR POLLUTION
A mass spectral method was developed for determining oxides of nitrogen (11) and other components in complex mixtures. The principal difficulty in determining oxides of nitrogen was the reactivity of nitrogen dioxide in the mass spectrometer. Prior conditioning of the instrument with nitrogen dioxide made analysis possible. Analyses of various mixtures are given in Table I. These blends nere synthesized directly in the mass spectrometer a t micron pressures, in order essentially to eliminate reaction among the components. Sampling of gas from high-temperature exhaust systems containing water, oxides of nitrogen, and oxygen is difficult because of condensation of water, solution of nitrogen dioxide in water, reaction between nitric oxide and oxygen, formation of nitrogen trioxide, and other problems. These difficulties may be circumvented by collecting samples a t micron rather than atmospheric pressure (11). Although the sensitivity for nitrogen dioxide was no better than 0.5%, those for nitrogen dioxide and nitric oxide were of the order of 0.01%. Nitrogen dioxide is unstable in the mass spectrometer a t pressures corresponding to 0.01%, but the nitric oxide from its decomposition is conveniently determinable. Although separate determinations of low percentages of nitric oxide and for nitrogen dioxide may not be accurate, their sum from the mass 30 peak is a reliable indication of total nitric oxide plus nitrogen dioxide. Methane in Coal-Mine Air. Determination of methane in coal-mine air by infrared and mass spectrometry has been found feasible. The principal purpose of this analysis was provision of a specific test for traces of hydrocarbons found in coal-mine atmospheres, where the most plentiful hydrocarbon is methane. The customary Haldane gas analysis, although sufficiently accurate, is not specific enough. Mass spectrometry was applied Rith some success for determining less than 0.6 mole % methane, as shown in Table I1 by comparison with Haldane analyses. The mass spectral method requires only very small samples. The spectrum of methane is characteristic, but the chief atmospheric components (large percentages of nitrogen and oxygen, and small amounts of carbon dioxide and water) interfere x i t h all the mass peaks of methane. These interferences can be corrected, as shown by the results in Table 11; however, the presence of traces of methane in mine atmospheres is difficult to prove by mass spectra. Oxides of Nitrogen.
