( 15) “Methods of Radiochemical Analy-
sis,” World Health Organization, Geneva, 1966. (16) Lange, R. C., in “Progress in Nuclear Energy, Ser. IX,” Vol. 6, H. A. Elion and D. C. Stewart, Eds., p. 213, Pergamon Press, Oxford, N. Y . , 1966. (17) Kirby, H. W., Ibid., Vol. 8, p. 89, 1967. (18) Brown, D., hladdock, A. G., Ibid., p. 1. (19) Burney, G. A., Dukes, E. K., Groh, H. J., Zbid., Vol. 6, p. 181, 1966. (20) Chenouard, J., Lucas, M., in “Non-
Destructive Testine in Nuclear Technology, V O ~ . 11,” -Roc. Symposium, Bucharest, hlay 17-21, 1965, IAEA,
Vienna. __ ....- 196.5. (21) Krieger, H. L., Velten, R. J., Burmann, F. J., Rept. NP-16235 (1966). (22) Lai, hl. G., Goya, H. A., Rept. USNRDL-RT-912 ( 1965). (23) Passy, N., Rept.‘ IA-849 (1964). (24) Rothschild, S., “Advances in Tracer Methodology. Volume 3,” Plenum Press, New York, 1966. (25) hlchfillan, J. W., Analyst 92, 539 (1967). (26) Isotopes Radiation Technol. 4( 1) (1966) (27) Broda, E., Schoenfield, T., “The Technical Applications of RadioacI
tivity, Volume I,” Pergamon Press, Oxford, 1966. (28) Gardner, R. P., Ely, R. L., Jr,, “Radioisotope Measurement Applications in Engineering,” Reinhold, New York, 1967. (29) Stalder,,,J. R., “Radioisotopes for Aerospace, J. C. Dempsey, P. Polishuk, Eds., pp. 23-30, Plenum Press, New York, 1966.
(30) Thomson, S. J., Webb, G., Platinum Metals Rev. 11( 2), 46 ( 1967). (31) Niewodniczanski, J., in “Radio-
isotope Instruments in Industry and Geophysics,” Proc. Symposium, Warsaw, October 18-22, 1965, IAEA, Vienna, 1966. (32) Braun, T., Toelgyessy, J., “Radiometric Titrations,” Pergamon Press, New York, 1967. (33) “Radioisotope Instruments in Industry and Geophysics,” Rept. STIPUB-21/20, International Atomic Energy Agency, Vienna (1966). (34) “Radioisotope Sample hleasurement Techniques in Medicine and Biology,” Proc. Symposium, Vienna, May 24-28, 1965, Intern. At. Energy Agency, Vienna, 1966. (35) Washtell, C. C. H., Hewitt, S. G., “Nucleonic Instrumentation,” George Newnes Ltd., London, 1965. (36) Chatters, R. hl., Olson, E. A., “Radiocarbon and Tritium Dating,” R e d . CONF 650652. Proc. Sixth Intern. Cdnf., Washington’ State University, Pullman, Wash., June 7-11, 1965. (37) Tamers, 11.A,, Acta Cient. Venezol. 16, 156 (1965). (38) Horrocks, D. L., in “Progress in Nuclear Energy, Ser. IX,” Vol. 7,
H. A. Elion, D. C. Stewart, Eds.,
p. 1, Pergamon Press, Oxford, N. Y., 1966. (39) Albert, P., “L’Analyze par Radioactivation,” Gauthier-Villars & Cie., Paris, 1964. (40) Coleman, R. F., Pierce, T. B., The Analyst 92( logo), 1 (1967). (41) Girardi, F., Talanta 12, 1017 (1965). V. P., Lukens, H. R., Jr., (42) ?:inn,
in
Trace Analysis-Physical
lleth-
ods,” G. H. Morrison, Ed., Interscience New York, 1965. (43) Lenihan, J. hf. A., Thomson, S. J., Eds., “Activation Analysis, Principles and Applications,” Proc. XATO Advanced Study Institute, Glasgow, Academic Press, New York, 1965. (44) Albert, P., Chimia (Aarau) 21, 116 (1967). (45) Tilbury, R. S., Rept. NAS-NS-3110 (1966). (46) Stiain, J. E., iy, “Progress in Nuclear
Energy, Ser. IX, H. A. Elion, D. C. Stewart, Eds., Vol. 4, p. 137, Pergamon Press, Oxford, N. Y., 1965. (47) Adams, F., Hoste, J., At. Energy Rev. 4(2), 113 (1966). (48) Borg, D. C., Rept. BNL-10130 (1966). \ - - - - ,
(49) Ashworth, 31. J., Abeles, T. P., hature 2 10, 9 (1966). (50) Drake, A I . K.. Nucleonics 24(8). . ,, 108 (1966) (51) Girardi, F., Guzzi, G., Pauly, J., Rmt. EUR-2408.e (196.5). (52) ‘Girardi, -F.,- Guzzi, G., Pauly, J., Rept. EUR-1898.e. 1 (1966). (53) Lederer. C. 11.. Hollander. J. lf.. Perlman, ’ I., “Table of Isotopes,” 6th Ed., Wiley, Kew York, 1967. (54) Lukens, H. R., Jr., Anal. Chim. Acta 34, 9 (1966). (55) Oria, >I., Sorriaux, A., Rept. CEA-R2779 (1965). (56) Proc. I1 Symposium on Health Physics, Pecs, Hungary, Sept. 26-30, 1966, Vol. I, Budapest, 1966. (57) Hamilton, J. H., Ed., “Internal \ - -
- I
Conversion Processes,” incl. Proc. Intern. Conf. Internal Conversion Process, May 10-13,. 1965, Vanderbilt University, Nashville, Tenn., Academic Press, Sew York, 1966.
Polarographic Theory, Instrumentation, and Methodology David
N. Hume,
Massachusetts Institute of Technology, Cambridge, Mass.
T
HE PRESENT REVIEW continues in
the pattern of its immediate predecessor (85) summarizing in much the same categories the literature of the past two years to December 1967. It is concerned with classical polarography and the newer electroanalytical techniques of a basically polarographic nature, while papers on the theory and measurement of electrode processes, electrode kinetics and other subjects less directly applicable to analytical exploitation are treated in the review, “Relaxation Methods,” by W. H. Reinmuth (see page 185 R). -4s before, no attempt is made to discuss papers on applications unless they embody new developments in theory, instrumentation, or methodology, or unless they suggest a novel approach. For complete coverage of polarographic literature it is suggested that the reader consult Electroanalytical Abstracts, and the continuing bibliographies founded by Heyrovskj. (77) and Semerano (88,89). 174 R
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02 1 39
Several books have appeared in the period since the last review. Kalvoda’s (92) authoritative work on oscillographic polarography is now available in an English edition, and Kambara has published a volume consisting of a collection of 32 papers on a wide variety of polarographic topics written by outstanding authorities (96). A German translation of the principal Hungarian textbook on polarography has also appeared (182). illthough not strictly within the scope of this review, the authoritative text by Delahay (40) on double-layer and electrode kinetics should not be allowed to pass without mention. A notable feature of the polarographic literature is the growing importance of books of the “Advances in” type, series published annually or biennially under the direction of a continuing editor and consisting of authoritative monographs on specialized topics by experts in their respective fields. The series “Elec-
troanalytical Chemistry,” edited by A. J. Bard ( 7 ) has started with two volumes, each containing four articles, and the series, “Advances in Electrochemistry and Electrochemical Engineering,” the electrochemical volumes of which are edited by Delahay has reached Volume 6 (41). I n a somewhat similar category is the collection of papers given a t the International Congresses of Polarography, edited by G. J. Hills, the most recent volume of which is based on the Southhampton meeting in 1964 (79). Anyone interested in the historical development of polarography will find it worthwhile to read the article by Zuman (247) written on the occasion of the death of Jaroslav Heyrovskf. I n it is presented an unusually clear and concise account of the development of the ideas of polarography, the significant advances, and the importance of polarography in the development of modern electrochemistry. Few things progress more slowly than
standardization of nomenclature. The CITCE committee on symbols and definitions has recommended clarification of some points in the IUPAC Stockholm terminology (190). CLASSICAL POLAROGRAPHY
Instruments and Apparatus. T h e E. H . Sargent Co. is now offering its Model X V I , a n automatically scanning and recording direct current polarograph with a conventional motor-driven voltage divider and pen recorder. No batteries are used, t h e dc operating voltage coming from a Zener-diode regulated source. Circuitry for obtaining the first derivative of the current-voltage curve is built in. The Electro-Chem. Co. of Olympia, Wash., now offers a slightly more expensive version of their low-cost dc voltagescanning unit which, with a dropping mercury electrode and recorder, provides a simple polarograph a t minimum cost. The new Auto-Scan unit has a Zenerdiode stabilized power supply and improved accuracy of voltage measurement. The Bendix Corp., Cincinnati Division, is marketing an improved version of the Davis Differential Cathode Ray Polarograph “Polaro-Trace 1600” as well as the Southern-Harffell “Pulse Polaro-Trace.” This large (over 5 f t high and 4 ft wide) and very elaborate pulse polarograph has a price tag of over $17,000, a fact which is liable to work against its wide adoption. Jlelabs, of Palo Alto, offers a much smaller and simpler pulse polarographic analyzer in its model CPA-3 which can be used for differential pulse, integral pulse, dc, and single sweep voltammetry. It is all solid-state design and very compact. Chemtrix of Beaverton, Ore., offers a three-electrode, single-sweep oscillographic polarograph consisting of an amplifier unit and a time-base unit built into a storage oscilloscope of high quality. The apparatus can be used for conventional single-sweep scanning with blanketed return or triangular sweep cyclic voltammetry with triggering to start a t a specified time after the beginning of growth of the drop. The whole apparatus costs less than $3000. Kane (100) has described a simple apparatus for Tast Polarography: a cam activating a spring-loaded d.m.e. knocks off the drops and a second cam operates a microswitch which governs the time that the recorder is in operation. I t may be used with an ordinary polarograph without modification, or with an ac polarograph. Luck and Mills (123) describe a simple solid-state oscillator circuit for triggering electromechanical drop dislodgement devices and suggest its use for matched dropping electr?des in differential polarography. Nygard, Johansson, and Olofsson (158) have developed an automatic device for precise drop-time measurements on single drops,
and for recording complete electrocapillary curves. They claim a standard deviation of less than one microsecond for single drop-times in the range of three to five seconds. Kemula (103) has described a cell with a sluice to permit the addition of standard solutions to a sample without need for additional deaeration. The apparatus was designed primarily for anodic stripping voltammetry but should be equally applicable to conventional polarography. Hakl (72) has devised a cell for the polarographic determination of reducible substances in thin-layer chromatography spots. The repeatability of the potential of the saturated calomel electrode is ordinarily taken for granted, but Shams el Din and Kame1 (199) pointed out that the potential of an electrode made with mercury contaminated with even trace amounts of zinc, cadmium, tin, or other active metals exhibits large deviations from the theoretical potential and can therefore be a major source of error. A similar effect is observed with other mercury-based reference electrodes (200). Tungsten has been suggested for use as a reference electrode in polarography ( 6 ) but the data given suggest that the practice is apt to be hazardous and should not be recommended. Barradas and French (11) describe suitable techniques for sealing typical high-lead polarographic capillaries directly to soft glass or to borosilicate glasses through a graded seal. Priem (181) discussed techniques for insertion of fine wire electrodes into narrow-bore capillary tubes. By means of air-driven diamond drills and ultrasonic drills, holes may be cut through into capillaries with diameters as low as 0.1 mm. Half-Wave Potentials. Gleicher and Gleicher (68) have developed a new theoretical correlation between polarographic oxidation potentials and structure for organic compounds, and conclude that the self-consistent-field approach offers a small but real improvement on the simpler Hiickel molecular orbital method. Zuman (248) has discussed the effects which two interacting electronegative substituents have on the half-wave potentials and wave-shapes of aromatic compounds. Israel (84) has suggested use of matrix algebra for resolution of overlapping polarographic waves and derivative polarographic curves in multicomponent systems. Reasonably satisfactory results were obtained for the concentrations of the constituents of two-component mixtures, but a typical three-component mixture such as ortho, meta, and paranitro benzoic acids, for which the halfwave potentials lay within an 80 mV range, did not give satisfactory resolution. Momoki, Sato, and Ogawa (141) have studied the calculation of successive stability constants of complex ions
from polarographic data using highspeed digital computers. They have shown very clearly the importance of including proper weighting in the program. Treatment of the same data by various least-squares approaches revealed that direct application of least squares without weighting gave significant errors as compared to the more rigorous modified Gauss method. Of interest was the fact that the original graphical method of DeFord and Hume gave a much closer approach to the results of the most sophisticated computer calculations than did the computer using unweighted least squares. Currents. The expression of current as a function of time, capillary characteristics, and diffusion coefficients a t the dropping mercury electrode continues to be a problem of considerable interest. Smoler (208) has made very extensive studies of individual-drop current-time curves and has attempted to determine which of the various theoretical equations gives the most accurate description of the behavior of real capillaries. On the basis of his work, he concluded that the Kouteck? extension of the IlkoviE equation gives the best fit for thin-walled capillaries, while the Matsuda version of the IlkoviE equation is superior for thickwalled capillaries. H e also reported that the use of a thick capillary inclined a t a 45” angle eliminates depletion effects, an observation which does not agree with recent work in the reviewer’s laboratory. The use of a thin-walled capillary does not eliminate depletion effects but does minimize the shielding effect. Duffey, Rahilly, and Kidman (61) have re-derived Matsuda’s equation with refinements which give a more exact solution. Smith, McCord, and Hung (207) have subjected to mathematical examination a simplified approximatipn of the expressions for the Koutecky and Matsuda functions based on a generalization of the ,Meites and Israel treatment of irreversible waves. Delmastro and Smith (44) discussed two approximate theoretical approaches to extension of the Lingane-Loveridge modification of the IlkoviE equation and show that they gave satisfactory allowance for the effect of drop growth and curvature without involving the complexity of the rigorous approaches of Koutecki, Levich, and Matsuda. Newman (153)has also used a modification of Levich’s treatment to obtain a simpler means of calculating the exact values of the coefficients of the extended IlkoviE equation without using the complicated approach of Koutecki. Flemming and Berg (62)have made an extensive study of current-time curves of individual drops using thin-walled, pointed capillaries. They observed that most of the current-time curve for first VOL. 40,
NO. 5, APRIL 1968
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drops fit the Kouteckj. equation quite well. Screening does not seem to be important for the deviations a t the beginning of drop life, and thin-walled pointed capillaries reduce but cj? not elimijlate the depletion effect. Stasnj. and Strdfelda (211) examined the properties of very finely pointed, upturned dropping electrodes which they claimed to give average diffusion currents within 1% of the theoretical. Depletion and streaming effects were essentially absent. The shielding effect of the capillary point was found to be very small and could be easily corrected for. I n later work (217),they examined similar finely-pointed electrodes for convection, depletion, shielding, and tendency to produce maxima. They claimed close correspondence to theoretical behavior and used the electrodes t o measure the diffusion coefficients of a number of ions in various supporting electrolytes. Chapman and Xewman (33) studied the reduction of electroactive ions in the absence of supporting electrolytes. I n the early stages of drop life, the current was found to vary as tliaJ gradually approaching a '/e power dependence as the drop became larger. The current was always found to be proportional to concentration. Nicholson, Wilson, and Olmstead (156) derived theoretically expressions for current at the dropping electrode in the situation where two successive electron transfer steps are separated by a chemical reaction. Although the problem had previously been treated for chronopotentiometry, cyclic voltammetry, and cyclic chronopotentiometry it had not been developed in a rigorous manner for the dropping mercury electrode. Guidelli and Cozzi (70) derived a very general theory from which expressions for the polarographic current can be deduced with any number of homogeneous chemical equilibria associated with a rapid electron transfer. Adsorption. T h e study of adsorption phenomena has been a particularly active field during the last two years, both because of increased realization of t h e importance of adsorption at electrodes and development of techniques which permit its measurement. Phillips (1'74) studied diffusion limited adsorption at spherical electrodes and showed that both the diffusion coefficients and maximum surface concentrations of surfactants can be calculated from potentiostatic currenttime curves. K i t a and Smoler (114) observed the influence of temperature on the rates of reactions inhibited by adsorption of surface active agents by means of current-time curves at the dropping electrode. As would be expected, the time for covering the surface decreases with increasing temperature. The same workers (113) examined the influence of strongly adsorbed surface
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ANALYTICAL CHEMISTRY
active agents on the anodic dissolution curves of cadmium and zinc at dropping amalgam electrodes. Surface active quaternary ammonium ions result in inhibition and produce mirror images of the typical current-time curves seen with cathodic reactions. The authors attributed the effect partially to simple coverage of the surface and partially to slowing of the electron transfer process a t the occupied surface. Barradas and Kimmerle (12) from a study of the adsorption of nonionic surface active agents on the dropping mercury electrode developed the concept of the adsorbed layer as a two-dimensional micellar film. The same workers (13) by oscilloscopic current-time studies at constant potential using similar nonionic surface active agents observed that streaming a t the dropping electrode stops when a monolayer has been formed. Armstrong, Fleischmann, and Koryta (5) studied anodic polarographic waves involving the formation of insoluble mercury salts. Many of these show a characteristic prewave and measurements of current-time curves and differential capacities led the authors to conclude that adsorption of the anions was involved. They suggested that a distinction should be made between adsorption and monolayer formation, in that monolayer formation requires nucleation whereas adsorption does not. Phillips (173) has also studied the effect of surface active agents on currenttime curves and developed an equation from hydrodynamic considerations which relates surface coverage, diffusion coefficient, bulk surfactant concentration, and time. H e has used the effect on the streaming during the oxygen maximum as the basis of a trace-level determination of cysteine a t micromolar levels, which is ten-fold lower than is readily achievable by diff usion-limited electrode reactions. The same investigator (1'72) has also used adsorption of surface active tetra-alkyl ammonium ions a t the stationary mercury drop as a means of determining them at trace levels. Using a convection-forced hanging drop electrode he plots current against time, thereby observing the inhibition of the cadmium reduction by the surface active agents. He found the method applicable in the range of 4 X to 8 X 10-6Ji. Butler and Meehan (28,29) have used current-time curves on single drops to separate charging and faradaic current components as have Lauer and Osteryoung (115) who developed a computer technique for analyzing the data and pointed out the usefulness of the approach for irace analysis. They were able to obtain results on cadmium solutions in the range of 1 - 10 X 10-6X. Very extensive progress has been made on the determination of the extent of adsorption of reactive species on both solid
and mercury electrodes. The method of choice now appears to be chronocoulometry, which turns out to have many advantages compared with chronopotentiometry, differential capacity measurements, and similar conventional approaches. Discussion of this work is beyond the scope of the present review but the papers by Anson (4), and Magenheimer and Boggio (125) should be mentioned. The selective inhibition of electrode reactions by adsorption of surface active substances has been exploited by several investigators. Khta (112)has discussed the theory of using readily adsorbable materials for masking undesired waves and has given examples of its application. Vinogradova et al. (236) have explored the use of surface-active agents for the polarographic determination of mixtures of elements with similar halfwave potentials. Good separation, for example, of thallium, cadmium, and indium in a trihydroxyglutaric acid medium is achieved by the addition of betanaphthol, thymol, and diphenylamine. Thallium in slightly acid chloride medium is determinable in the presence of a 250-fold excess of cadmium and indium if camphor is present. Murray and McNeely (146) have studied the inhibition of waves by the adsorption of metal halide complexes and their effect on single drop currenttime curves. Maxima. Slow progress continues t o be made on t h e understanding of the ever puzzling phenomenon of t h e polarographic maximum. Urbach (231) interprets maxima of the first kind in terms of dipole repulsion forces. These are the product of the dipole moment and the tangential derivative of the electric-intensity field. When the current distribution is asymmetric, as i t is in polarography, there results a nonuniform surface concentration and dipole repulsion forces cause migration at the surface. The theory seems to fit his experimental observations. Mairanovskii (126) likewise studying maxima of the first kind observed that short drop times and low concentrations tended to diminish maximum formation and suggested that nonuniform distribution of surface-active products at the electrode surface may set up streaming. Rao, Suryanarayana, and Doss (183) observed the effect of ultrasonic irradiation on polarographic maxima, and suggested that the suppressing effect of ultra-sound is actually due to a shortening of the drop time which circumvents the maximum by knocking the drop off before streaming gets underway. Barker and Bolzan (9) in a long paper on the effects attributable to specific adsorption of depolarizers suggested that such adsorption may lead to maxima in dc polarography when the depolarizer concentration is low and the solution has a high
conductivity. Adsorption may cause maxima not associated with streaming in pulse polarography and may increase the apparent Sensitivity in alternating current polarography. IR Drop Compensation. Pouli, Huff , and Pearson (178) have suggested a device for continuous compensation of ohmic polarization based on a new principle using operational amplifier circuits. Compensation is achieved by inserting in series with the cell an effective resistance, the value of which is determined by the iR drop as measured between reference and working electrodes when a square ivave pulse is applied across the cell. The voltage developed across the effective resistance is sensed by a feedback circuit and compensation applied. The results agree well with the potentials measured with the aid of a Luggin capillary. Brown et al. (24) achieved compensation of ohmic potential loss with the aid of a positive feedback loop added to a conventional operational amplifier potentiostat. They also described techniques for compensating for the double layer charging current, so that readout of the faradaic component was possible with a n apparent high degree of accuracy for a t least moderately demanding conditions. Application was made to cyclic voltammetry, ac polarography (both normal and higher harmonic), as well as ordinary polarography. Ilamokos and Juhhsz (39) proposed the use of an asymmetrically polarized dropping mercury electrode to alleviate ohmic drop errors, but from the description of their experimental setup i t would appear that they were simply using a Luggin capillary and the alleged asymmetry of polarization was irrelevant. Did erential Polarography. Shults a n d his collaborators have published a series of papers emphasizing the opportunities made available b y controlled potential differential dc polarography through the use of modern electronic instrumentation. Apparatus is described which filters electronically the perturbations due to drop oscillations and makes the difficult task of synchronizing dropping electrodes unnecessary (201). Another paper (205) treats in detail the principles of what they call 4E-differential polarography, the technique in which a constant voltage difference is applied between two dropping electrodes in the same solution, resulting in a pseudo-derivative curve. Shults, Fisher, and Schaap (202) have also discussed in some detail the theory and present potentialities of comparative polarography, the differential technique which involves the measurement of the difference between two diffusion currents, one due to the species of interest in the unknown solution, and the other to the same electroactive species present in accurately known concentra-
tion in a similar reference solution. Using either the conventional dual cell comparative technique or a new single cell comparative technique, these n-orkers have obtained analytical results with accuracy and precision of 0.1%. Shults and Schaap (204)have made a statistical study of determinate and random errors in comparative polarography comparing the theoretical with experimental results. Chronoamperometry. This technique, also called single-sweep linear voltage scan polarography a n d cathode-ray polarography, has continued to develop in interest a n d practicality. Rooney (188) has revien-ed some of the newer developments. Polcyn and Shain (175) have studied the theory of linear potential s ~ v chronoamperometry ~ ~ p applied to stationary electrodes for uncomplicated multistep reversible or irreversible charge-transfer reactions. I n a subsequent paper (176),the case of reactions involving catalytic regeneration of reactant was also considered. Saveant and Vianello (191) developed a general treatment of potential-sweep current curves involving a chemical reaction of any order before, after or simultaneous with the electron transfer step, and gave diagnostic criteria by which the nature of the reactions involved might be recognized. Gorokhovskaya (69) studied the effect of base electrolyte on the peak currents of reversible and irreversi_ble reactions and found the Randles-SevEik equation to hold. Powell and Reynolds (179) have reported on the effects of uncompensated iR drop on the shape of AE-differential curves using a cathode-ray polarograph for differential chronoamperometry. Vyaselev (257), investigating chronoamperometry with a staircase voltage sweep, calculated theoretical oscillopolarograms from the Randles-SevEik equation using a digital computer and found good agreement between theory and practice for thallium and cadmium. He cited high resolving power (45 mV) and high sensitivity as advantages shown by fractionally differentiated oscillopolarograms over the conventional chronoaniperometric scan. Laviron (116) was able t o explain the anomalous shapes of oscillograms of anodic processes leading t o formation of surface films-e.g., calomel-in terms oi the inhibiting effect of the film on the current flow. Equations were given whereby the effects of concentration and rate of scan on current and peak potential could be predicted. Wopschall and Shain (244) in an important paper developed an extension of the theory of chronoamperometry and triangular wave cyclic voltammetry to systems in which the reactant, the product, or both are adsorbed at the electrode interface according to a Langmuir isotherm. Diagnostic criteria were developed by
which to characterize unknown systems by the variation of peak shape and currents as a function of scan rate and concentration. Francini, Martini, and llonfrini (64) showed that a sodium chloride-aluminum chloride melt between 150' and 230" C wab a suitable medium for chronoamperometry of a number of elements a t the d.m.e. Ruby and Tremmel (189) described cells suitable for chronoamperometry and cyclic voltammetry. Inverse Polarography. This technique, also known commonly as stripping voltammetry, has had a great rise in popularity because of i t s simplicity and sensitivity. Barendrecht has twice reviewed recent developments in the field (7, 8 ) . Neeb and Saur (161) have automated many of the operations involved and described a device directed by a programming unit which forms a hanging mercury drop, purges oxygen, plates and strips the element being determined, and registers the peaks automatically. Lovasi and Zomborg (122) suggested improvements in cells and electrodes with particular reference to the determination of heavy metals in light metal alloys. Duyckaerts and Cosemans (52) have described suitable apparatus and studied the factors involved in the accuracy of the method. Kamenev and Vinogradova (99) have made extensive studies of the variables and conditions involved. For the determination of copper and lead, they recommend silver as a better base metal then gold or platinum for holding a mercury drop, and make use of a special procedure involving pre-electrolysis of the base electrolyte before adding the test solution. Kamenev et al. (98) have done factorial experiments to identify the important characteristics of the supporting electrolyte as they pertain to the formation of satisfactory peaks in stripping voltammetry. They compare, for example, the effective radius of supporting electrolyte cation, valence of supporting electrolyte cation, and anion characteristics in three-factor, two-level experiments using thallium, lead, and cadmium. I n another study on thallium (97)an eight-factor, two-level factorial was carried out but without any startling results; the factors which they found to be important were those which one would predict a priori to affect the results. Stromberg (219) has concerned himself with estimating the limit of sensitivity of inverse polarography and suggested it should be of the order of 10-llM, but recognized that higher limits are set by the impurities in reagents and desorption of impurities from the containing vessels. Stromberg and Kaplin (120) examined the effects of electrode surface area and volume of solution on the sensitivity of inverse polarography and estimated that they could determine 10-Q70impurities VOL. 40, NO. 5, APRIL 1968
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under optimal conditions. Zakharov and Stromberg (245) have deduced a complicated equation involving six coefficients by which, if the coefficients are kiiown, peak currents can be predicted. Because of the number of coefficients involved and their sensitivity to experimental conditions, the approach appears to be limited in value. Zakharov and Trushina (246) gave an equation relating peak current, the area of the electrode, the time of the electrolysis, and volume of solution for calculating sensitivity of the determination of trace impurities by inverse polarography using mercury-film electrodes. DeVries and Van Dalen (48) have taken into consideration the effect of ohmic drop distortion in inverse polarography with mercury thin-film electrodes and have concluded that the decrease in peak currents may be very significant. Yeeb and Kiehnast (150) studied the influence of various salts on inverse polarographic peaks and report that inert salts may decrease the peak height of cadmium by as much as 80%. DeJ’ries and Van Dalen (49) using planar mercury-film electrodes for inverse polarography concluded that stirring during the stripping stage was ambiguously advantageous inasmuch as the increased sensitivity was balanced by corresponding rise in noise in background current. Kaplan and Rezakova (101) used cathodic pulse inverse polarography for the determination of tellurite and selenosulfate. The deposition products were found to form a surface film with the result that the stripping peak potential was proportional to concentration. Stevens and Shain (215) have used inverse polarographic techniques as a means of measuring diffusion coefficients of metals dissolved in mercury. The advantages of mercury-coated graphite electrodes have attracted considerable attention. Perone and Stapelfeldt (170) to avoid the tedium and uncertainty of measuring areas under stripping curves have used a derivative technique with waxed graphite electrodes. The theory is based on Xcholson’s earlier work. The chief drawback is that a different type of calibration curve is needed for each metal, the micro-deposition of solid electrodes being notoriously unpredictable, but the method does work with good sensitivity. Perone and Brumfield (168) have used mercury-plated graphite electrodes with chronopotentiometric stripping and found a better correlation between theory and experimental results than with ordinary linearly varying potential stripping techniques. Matson and Roe (133) applied the mercury film on graphite electrode to trace metal determination in natural water systems. Vassos and Mark have studied the multiple peaks sometimes observed in 178 R
ANALYTICAL CHEMISTRY
the inverse polarography of various metals (235). With copper on pyrolytic graphite they conclude that the anomalous peaks are due to mono-layers, which conclusion is supported but not proved by electron microscopy. Hovsepian and Shain (82) studied the intermetallic compound formation between cobalt, zinc, and mercury by means of inverse polarographic techniques and confirmed previous work which suggested equilibrium between a zinc-cobalt dimer, Copand Zn2. Various alternative approaches to conventional linear voltage scan stripping are being studied. Lyalikov (124) has done stripping by pulse polarography and reports good stripping peaks for cadmium between and 10-9Jf in 0.1 ml of solution. Eisner et al. (53) claim a ten-fold increase in sensitivity for inverse polarography of metals when an ac signal superimposed on the stripping wave is examined with a phase-sensitive detector. DeVries (47) has discussed the theory of potentialstep electrolysis followed by linearsweep voltammetry and pointed out the various analytical implications. Vincent and Rise (284) suggest the use of constant current stripping with a potential vs. time readout (chronopotentiometry in reverse) and have studied cadmium and zinc. The exchange current is important and must be greater than low9A/sq em to give a measureable transition time. Keniula and Strojek (104) studied the controlled chronopotentiometric stripping of metals into hydrochloric acid and claimed that it is better for analytical purposes than integrating the usual current-voltage peaks. Brainina (22) has used organic precipitants to extend the capability of inverse polarography to metals which do not amalgamate or plate readily. A metal ion is oxidized a t an electrode and precipitated on the surface with a reagent present in the solution. The polarity is reversed and the reduction current is then measured. This has been applied with graphite electrodes using crystal violet or rhodamine C as precipitants for chloroantimonate ion, and 1-nitroso, 2-naphthol for cobalt. Good results were reported for concentrations of cobaltous ion as low as 4 x io-7~. Alternating Current Polarography. T h e use of lock-in amplifiers to make a n alternating current polarograph with a simplified phase-selective detector has been intraduced by Evilia and Diefenderfer (56) using a commercially available lock-in amplifier with an operational amplifier, alternating current polarograph of conventional design. These workers were able to develop a simple phase-sensitive ac polarograph with specifications equal to or better than those of earlier instruments. Beckman and Gauer (15) have given
details of a simple conversion unit to allow a Sargent Model XV dc polarograph to act as a low frequency alternating current polarograph. Peover and Powell (166) have pointed out the importance of migration current in disturbing alternating current processes in solutions of high specific resistance or high concentration of electroactive species. They described an auxiliary unit for compensating for voltage loss in ac polarography with special reference to organic systems in both aqueous and nonaqueous solvents. Kambara and Hasebe (96) suggested the use of an inductor in series with the cell to eliminate capacitative currents in ac polarography. Bauer and Foo (14 ) in studying second harmonic alternating current polarography have discussed the theory, the scope, and the experimental requirements, and checked experimentally the method of measuring 1%-ave-heights. McCord, Brown, and Smith (186) have shown that spherical diffusion is an influential factor in second harmonic alternating current polarography and related techniques, and suggested that i t may be an explanation for anomalous results which are sometimes obtained. Timmer, Sluyters-Rehbach, and Sluyters (229) have treated the potential dependence of faradaic impedance for an irreversible reaction theoretically and discussed the consequences for alternating current polarography. Breyer and Bauer (23) have given a somewhat philosophic discussion of the alternating current behavior of electrochemical cells as circuit elements. Cells involving faradaic and tensammetric processes are not representable as simple combinations of ordinary elements. They mention the concept of a negative impedance element and point out the work of Tamamushi (225, 226) who showed that coupling a negativeimpedance faradaic element with an inductance would give an electrochemical oscillator. McLean and Timnick (1%’) have explored the possibility of using larger amplitude alternating current potentials than has been customary. Previous theoretical treatment has implied a limitation of 16/n mV peak to peak amplitude and in the present work theoretical study has been done for amplitudes of 10,60, and 100 mV. The work hac shown that alternating current polarography is satisfactory for measuring heterogeneous rate constants of cadmium up to 70 mV at least. Large amplitude ac polarography was later used to measure charge transfer coefficients of cadmium (158). Delmastro and Smith (45)tested the “stationary sphere’’ theory of dependence of alternating current waves on h. Good qualitative agreement was achieved but the quantitative fit was poor. The same workers (42) examined the influence of
spherical diffusion on ac polarographic curves and found evidence supporting the previous suggestion of Biegler and Laitenen that this effect might be significant. Jacobsen (85) has observed that although surfactants usually suppress alternating current polarographic waves of complexes, some do not and others may even increase the wave height if the surfactant is of opposite charge to the depolarizer. Jacobsen has developed the concept of a penetration current with electron transfer taking place through a film of adsorbed material. Fujinaga, Izutsu, and Inoue (65) have applied electrochemical masking with surface active agents to inhibit the ac curves of some metals, especially in the presence of E D T A in trace analysis. Gupta and Sharma (71) studied the nature of the effect of temperature on tensammetric peaks. Kot unexpectedly they found that while faradaic peaks showed essentially constant potential and increasing height rj-ith temperature, tensammetric peaks were lowered with temperature and peak potentials became more negative. Neeb (148) has devised a new technique which he calls Doppeltonpolarographie in which two sinusoidal ac potentials of the same amplitude and small frequency spacing are superimposed on the polarographic cell. The corrbination tones formed by nonlinearity of faradaic impedance are measured. Measuring techniques are said to be simpler than harmonic polarography in that a n elementary filter circuit can be used to select the frequency. 11 simple apparatus for Doppeltonpolarographie was later described in connection with the study of the ac polarography of tin in chloride medium (149). Wolff and Surnberg (242) give a good description of the technique of radiofrequency polarography and its application to the trace determination of metals. Pulse Polarography. Peker, Herlem and Badoz-Lambling (166) have reported the characteristics and performance of the Barker pulse polarograph in their hands. Fonds, Brinkman, and Los (63) derived a complete diffusion equation for instantaneous currents in pulse polarography following the treatment of blntsuda which allows for the shielding effect. Experimental varification is given together with diffusion coefficients measured using this treatment. Oldham and Parry (163') have considered the depletion effect in pulse polarography with stationary electrodes. The normal pulse technique, involving as it does a series of short pulses in the cathodic direction applied to an electrode which is held most of the time at a potential positive enough to regenerate or strip the reduced material, does not have a depletion effect. If, however, the reactions are irreversible, depletion can occur with
resulting decrease in the limiting current region. The problem was studied theoretically and experimentally and it was shown that the proper choice of resting potential and stirring are effective in eliminating the difficulty. Christie, Parry, and Osteryoung (35)have applied normal pulse polarography to the study of electrode kinetics. The determination of kinetic parameters of electrode reactions from the current voltage curve is quite similar to the analysis of irreversible polarographic waves developed by Kouteck?. Wolff and Xurnberg (241) studying application of pulse polarography in organic analysis recommended the use of derivative pulse methods and concluded that the detection limits-e.g., of nitro compounds may go below 1 ppb. Square Wave. Niki, Shirai, a n d Kyoya (157) report improvements in the alternating current square wave bridge polarograph using the same circuit for gating and synchronous rectification. Buchanan and Bacon (26) have shown that a square mave polarograph is useful for continuous monitoring of effluents in ion exchange chromatography. The high sensitivity and lack of response to oxygen make its use particularly advantageous. -4 cell of very small volume which minimizes hold-up and mixing problems is described. Kinard, Philp, and Propst (107) have revived interest in the Kalousek technique, a large amplitude square wave method which, although known for 20 years, has been little used outside Czechoslovakia. These workers used operational amplifier circuitry M hich removes some of the inconvenience of the traditional Kalousek device. The authors point out that it has a number of advantages in that it has potentiality for resolving reversible and irreversible processes, kinetic coniplications are immediately apparent from the polarographs, and it has specific usefulnesses such as in determining metals which are ordinarily masked by hydrogen evolution. -4 mechanical square wave polarograph apparently similar in principle to that of Kalousek has recently been developed in China (60). The theory for the waves of reversible systems as obtained on this instrument has been given (61). Oscilloscopic Polarography. Kalvoda and Z$ka (94) reviened the application of oscilloscopic polarography to organic compounds. Kalvoda has recommended the use of rectangular alternating current for measurement of differential double-la3 er capacity n i t h his oscilloscopic polarograph (93). The effects of variations in current and presence of evtraneous reducible ions on oscillopolarograms has been discussed (46). &an (38) has demonstrated the usefulness of oscillopolarography in anhydrous molten ammonium formate as a solvent. Afairanovskii (127) used
a vibrating dropping mercury electrode with a period of 0.02 see for continuous sweep oscillopolarography. With a sweep of 0.5 to 4 V,/sec and a filter for 50 cycle frequency, he got good polarograms, the oscillations produced by falling drops being removed by the filter. Chronopotentiometry. This widely used technique is properly classified among t h e relaxation methods b u t it has enough analytical aspects closely related t o polarography t o justify some discussion here. The theory, instrumentatioii and applications have been revien ed recently by Paunovic (I6 4 ) . J. J Lingane (120) has subjected the analytical aspect5 of chronopotentionietry to a critical examination and evaluation. P. J. Lingane (121) has I\ ritten a very inqtructive critique of chronopotentionietry a\ a tool for the study of ad.orption, one of its most lcidely w e d applications. He points out that there is a bevere limitation due to the difficulty in accurately determining transition timei, and that different methods may give varying by 10%. There i b like problem in interpietiiig transition times of several charge-consuming processes operating in parallel. H e concludes after a statistical eyamination of chronopotentiometric data that the method is poorly adapted to the study of adhoiption and that chronocoulonietry is superior in that the arialjsi\ is simpler, readily automated and efficient Christie and Osteryoung (34) ha! e noted the evistence of desorption from chronopotentiometiic electrode, in the doublelayer charging region and clte this as additional evidence of unreliability as a method for the stiidy of adsorption. Ileron and Laitinen (45) have examined the techniquc of chioiiopotentioinetry n i t h current reLersa1. The principle of additivity of perturhations is developed and illustrated by studies on the constant current ieduction of mercurous ions formed by constant anodic current generation a t a hanging mercury drop. The influence of spherical diffusion and distortion by double-layer charging was considered. These worker\ showed that by correcting for sphei ical diffusion, migiation, double-layer charging, electrode geometry and edge effects it should be possible to obtain chronopotentiometric data good to a few tenths of a per cent. Murray and Gross (143) studied spherical diffusion, shielding and streaming effects in current reversal chronopotentiometry and concluded that the effects exist and may be quite significant. Xlexander and Barclay (3) suggested application of current-cessation chronopotentionietry, or decay chronopotentiometry as they call it, to quantitative analysis. Derivative chronopotentiometry has been studied by a number of workers. VOL. 40, NO. 5, APRIL 1968
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Peters and Burden (171) have pointed out several unique advantages compared to conventional chronopotentiometry. The minimum dE/dt is quantitatively related to transition time and provides therefore an absolute method for measuring transition times. The shape of the derivative chronopotentiograph is diagnostico f the kind of electrode processes involved and for irreversible electrode reactions, kinetic parameters can be calculated from the minimum value of d E / d t . This quantity is relatively little affected by double-layer charging and its measurement does not necessitate recording a complete chronopotentiogram. These authors used an operational amplifier differentiator with filtering and got reasonably good agreement with conventionally measured transition time values. They pointed out that the technique puts some extra demands on the differentiator and the constant current source. Sturrock et al. (221) has shown that derivative chronopotentiometry is applicable to solutions in the sub-micromolar concentration range, particularly if suitable corrections are made in the measurement of very short transition times. Shults and Mueller (208) derived theoretical equations for programmed-current derivative chronopotentiometry. Martin and Vogt (182) described a simplified all transistor chronopotentiostat with desirable characteristics and Tanaka and Yaniada (227) have shown the applicability of chronopotentiometry to the determination of stability constants of complexes. These workers, using a long period dropping electrode obtained results agreeing with literature values measured by other methods. DIVERSE ELECTRODES
Mercury Electrodes. Kuempel and Schaap (111) have devised a means of getting strictly planar mercury pool electrodes, thereby avoiding the complications due to curvature of the electrode when it contacts a glass surface, or caused by electrolysis in thin films of solution between the glass and the electrode. A platinum ring set in glass or Teflon is wet by the mercury and with careful adjustment forms a planar electrode making a right angle with the cell wall. The application is chiefly in chronopotentiometry and it was shown that the product of iT'/2/AC was constant over a very wide range of T with these electrodes. Harrar and Shain (76) made an extensive study of electrode potential gradients and cell design in constant potential electrode experiments. The article, which is worth study, has important implications for cell design in analytical methods using mercury pools. Adamse ( I ) described a mercury pool electrode periodically cleaned by a stirrer which was useful for 180 R
ANALYTICAL CHEMISTRY
recording dissolved oxygen in water and activated sludge. Clausen, Moss, and Jordan (36) have introduced a new mercury frit electrode for peak voltammetry techniques such as chronoamperometry. The contact between solution and mercury electrode is made in a glass frit and the resulting immobilization of solution cuts convection to negligible levels and gives peak voltammograms even with very slow scan speeds. Such an electrode immersed in a solution is highly insensitive to flow in the solution and should therefore have practical application. Carr and Meites (30) have examined the properties of the vibrating mercury electrode in some detail. They found various desirable characteristics such as the half-wave and slope being the same as in conventional polarograms within 2 or 3 mV. As would be expected results were unaffected by vigorous stirring of the solution. With an electrode having a drop time of about 0.1 see the limiting current was approsimately 0.6 that which it would have been if it had been operated normally. The residual current increased only four-fold and the noise was less than 5% of the limiting current. ii vibrating mercury solution interface obtained by electromechanical vibration at a frequency of 20 Hz has been recommended for determination of electrocapillary maxima (110). Usi, Yamasaki, and Shimoiizaka (232) used an interesting twin horizontal dropping mercury electrode system to study interaction hetween dissimilarly charged particles at controlled potential. A sensitive indicator of critical charge was the spontaneous coalescence of two drops on contact. Thin-film mercury electrodes have attracted attention particularly because of their superiority to a hanging drop configurations through larger effective areas and thinner diffusion layers in inverse polarography. Perone and Davenport (169) studied characteristics of mercury plated in thin films on graphite electrodes. DeVries and Van Dalen (49)undertook a theoretical study of linear potential sweep voltammetry a t planar mercury-film electrodes, refining the peak method of Roe and Toni with which they agreed in the limiting case of zero thickness. Vinogradova and Kamenev (235) have recommended Stromberg's mercury-on-silver electrode, finding unusual sensitivities with bismuth. Marsh and Bruckenstein (131) have worked with a continuously-deposited, mercury-coated, rotating platinum electrode. I n this technique, mercury and the metals being determined are deposited together on the electrode which is subsequently stripped by a chemical reagent while the potential-time curve is recorded. If activity coefficient corrections are applied to the metals in the amalgams where their concentrations are
appreciable, the predicted curves of these chemical chronopotentiograms agree within experimental error with the theory. Stevens and Shain (612) have made an experimental check on the theory of diffusion currents at hanging drop electrodes undergoing amalgam formation, and the effect of the sphericity of the electrode was as predicted. Birch and Manahan (18) give an account of a dropping amalgam electrode of simple design which they applied to the study of copper complex formation. Solid Electrodes. The lack of inertness of platinum electrodes continues t o be a source of interest. Hoare (81) discussed the question of why platinum electrodes slightly oxidized by preanodization tend to be more reversible than those cleaned by cathodization. He concluded that the enhanced reversibility is due to oxygen dissolved in platinum metal and not to oxygen adsorbed on the platinum surface. James (86) has reviewed various mechanisms proposed for the electrochemical activation of platinum electrodes. He concluded short anodic or cathodic activation pretreatment to result in improved reversibility and reproducibility because of desorption of impurities. H e suggested that long anodization gives activation with greater stability and that this is probably associated with oxide formation and different from the short activation effects. Trasatti (230) determined the surface area of platinum micro electrodes by anodic and cathodic charging curve? and reported that by using proper current densities and correcting for double-layer charging he was able to get agreement between anodic and cathodic charging areas within 0.6%. Blaedel and Klatt (19) have studied the characteristics of the tubular platinum electrode, a device particularly useful for flowing systems. Using hydrodynamic principles, they derived the theory of current-voltage curves for reversible charge transfer. Half-lvave potentials a t the tubular platinum electrode differ slightly, but definitely, from those of the dropping mercury electrode. These workers described a useful cell for applying the tubular platinum electrode. one feature of which is an ion-exchange membrane serving as a salt bridge. Klatt and Dlaedel (108) later developed the theory of the tubular electrode for quasi-reversible and irreversible charge transfer reactions. They verified the theory experimentally by measuring transfer coefficients and rate constants. Although good agreement was obtained between rate constants determined by this electrode and by the dropping mercury electrode, the authors pointed out that nonuniform potential distribution at the tubular electrode is a disadvantage which may limit its usefulness in kinetic studies.
Llatsuda (134) developed theoretically the shape of current-voltage curves for reactions with and without preceding homogeneous chemical reactions for tubular and channel electrodes. Oesterliiig and Olson (161) recommended the use of a mercury-coated tubular platilium electrode for estending the potential range. The same workers have shonn that chronoamperometric results with the tubular mercury film electrode agree ne11 Kith the theory (162). K m l a and StrafeIda (106) have used a platinum electrode in the form of a segment of a cylinder set into a glass rotating cylinder Equations for the current characteristics were developed and shonn to fit with relatively slow (less than 30 rpni) rates of rotation, irreproducibility being observed at higher speeds. The limiting current is proportional to the cube root of the rate of rotation, to the length of the electrodc and the concentration of the electro-active substance. atrifeIda, Vondrhk, and Kinila (218) have also studied electrodes consisting of platinum rings in nonconducting cylinders rotating about their axes. The theory was developed and verified experimentally. Roethlein and Maget (186) devised a novel platinum disc electrode covered with a thin hydrophobic film such as Teflon which allon s electrolytic contact but prevents wetting of the electrode surface. The disc used for the determination of oxygen is partially immersed in the supporting electrolyte and rotates continuously to get fresh surface and a new diffusion layer. Laivrance and Chambers (117) have employed a wheel in which is mounted a series of carbon paste electrodes. The wheel is slowly rotated so that fresh electrodes are continuously exposed t o the solution. The device is a niechanical analog of the dropping electrode and has the corresponding advantages that polarogranis can be obtained with a renening electrode surface and the electrolyzed solution is carried away as the electrode is shifted. A period of 8 drops ’min was employed and the significance for studies in nhich adsorption of reactantq or product: occurs was pointed out. Chambers and Lee (32) by use of radioactive traces and electrochemical analysis studied the extraction of organic molecules into carbon paste electrodes. The substances are first adsorbed on t h r electrode and then transferred into the electrode by diffusion. Farsang (67) used carbon paste electrodes made with powdered graphite and silicone oil which were satisfactory for anodic polarography but depolarized a t about -0.3 V. Farsang and Tomc s h y i (68)59) used these electrodes for cyclic voltammetry and recommended them for voltammetric titrations. Swofford and Carman (224) devised a new type of carbon electrode made from
-
carbon black suspended in an epoxy resin matrix. These electrodes containing up to 25% carbon by weight are easily fabricated to convenient shapes and have relatively low resistance. Schmidt and Gygax (192) have made extensive studies using a planar metallic electrode in a small chamber connected w-ith the bulk of the electrolyte by a thin passage. This Kammerelektrode as they call it tends to eliminate convection difficulties and give a situation corresponding closely to linear diffusion. The electrodes are about 10 m m in diameter and the electrolyte film in contact with the electrode is less than 0.3 mm in thickness. Chronoamperometry and inverse polarography give characteristic curves. Using a variety of metallic electrodes and various electroactive ions they sometimes obtained single peaks and sometimes double n hich they attributed to deposition of monolayers of activity lower than the standard state (194). The influence of supporting electrolytes on these waves was examined (196) and the mutual effects of monolayers of different metals on the same electrode observed (193). With the deposition of lead on gold, a particularly complex situation was found with as many as five peaks being visible (195). Rill (638) studying electrolysis of horizontal electrodes with somewhat thicker films (2 min) found evidence of convective effects. The versatility of truly thin-layer electrolysis, particularly between twin electrodes, is great and the characteristics of theqe techniques are considered in the review of relaxation processes. McClure and RIaricle (136) have developed a dip-type thin-layer electrolysis cell useful for in situ n value determinations for chronoamperometry, especially in nonaqueous niedia such as dimethylformamide, propylene carbonate, and acetonitrile. The rotating disc electrode has been found to be particularly attractive because its voltammetric characteristics can be derived rigorously from hydrodynamic theory and the equations often hold better than to within 1%. Marcoux and Adams (128) gave details for construction of rotating disc electrodes by modification of commonly available commercial platinum set-in-glass button electrodes. Wojtowicz and Conway (240) described the construction and operation of a rotating disc electrode for elevated temperatures. Blurton and Riddiford (20) studied various shapes for practical rotating disk electrodes and examined the considerations involved in making electrodes which would fit theoretical behavior. Prater and Adams (180) undertook a critical evaluation of practical rotating disc electrode designs. On considering the various types recommended in the literature and the practicality of making rotating carbon paste disc electrodes,
they experimented with both the bellshaped and the cylindrical types which they found to agree except a t very slow speeds of rotation. They found depth of immersion to be relatively unimportant and the solution volume and cell size to have little effect. Experiments showed satisfactory behavior in cells ranging from 5-em diameter up to those with a 9-liter volume. Sewman (164) treated the problem of ohmic drop from disc to reference electrode and found the current distribution on a rotating disc electrode below the limiting current values was not uniform, although it was a t the limiting current. The current a t the center of the disc might vary between 50 and 100% of the average current (165). 0’13rien (160) used laser interferometry to study hydrodynamic boundary and diffusion layers with a rotated disc electrode and Carrozza et al. (31) studied ionic mass transfer a t both disc and conical electrodes. Kapp, Johnson, and Bruckenstein (147) evaluated the performance of the integral rotating ring and disc electrode using simultaneous independent potentiostatic control of the two indicator electrodes. Related to Bruckenstein’s technique is Gerischer’s (67) use of closely spaced electrodes in a stream in which the reaction product of the first electrode is analyzed a t the second. Miscellaneous Electrode Systems. T h e behavior of the so-called bubbling platinum electrode in which bubbling of a n inert gas is used to renew the diffusion layer around a solid electrode has been improved by Subcasky (222) who has employed electronically pulsed bubbles to give uniformly timed renewal and reproducible concentration gradients. Cozzi, Raspi, Succi (37) achieved a similar effect by using a movable piston to displace the solution around the electrode a t periodic intervals. Raspi, Pergola, and Cozzi (184) applied this type of electrode arrangement for anodic waves of halides a t platinum electrodes. Roffia and Vianello (187) used a sudden 45’ rotation of a J-shaped electrode at 1-10 see intervals for periodic renewal of the diffusion layer a t a platinum-ball electrode. I t n a s observed that electrode could be treated as a plane with linear semi-infinite diffusion for electrol1 sis time. greater than a few tenths of a second. Bod6, Xndersen, and Eyring (21) used a periodic scraping technique with solid electrodes particularly for measurement of zero charge potentials. Mountcastle (142) studied the properties of a boron carbide electrode, previously introduced by lledler and Adanis. The hydrogen overvoltage was found to be high but the electrode did riot work especially 1% ell and showed deterioration in some solutions. TF70lkenberg (243) proposed a physical interpretation of anodic and cathodic VOL. 40, NO. 5, APRIL 1968
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polarization processes of germanium and silicon semiconductor electrodes by assuming polarization of a p-n junction existing a t the electrolyte-semiconductor interface. Escue, Tidaell, and Dickie (55) used a dropping gallium electrode in fused lithium, potassium nitrate eutectic and 180’ C. The electrode is not to be recommended in this medium because the oxidizing properties of the melt result in interference by S O z waves and the thallium coats readily a i t h oxide. Ducret and Corner (50) hare proposed the use of a thermal convection electrode in M hich a heated wire is used to set up a constant and uniform convection. The limiting currents were reported to be reproducible to mithin 0.3%, The possibility of studying electrode reactions and their products by simultaneous spectrophotometric or other optical approaches and electrolysis has always been attractive, and considerable progre.ss has been made recently on the necessary instrumentation. Mark and Pons (129) described an in situ method for observing infrared spectra of species a t an electrode surface during electrolysis. A frustrated multiple internal reflectance plate made of germanium \+as used a t the electrode and set in an ordinary infrared spectrophotometer nith a slightly modified double beam internal reflectance attachment. Marked changes in the frustrated multiple internal reflectance infrared spectra of saturated 8-quinolinol in diniethylfornianiide solutions containing lithium perchlorate nere observed on electrolysis. Hansen, Osteryoung and Iiuu ana (75) shon ed the feasibility of using internal reflectance spectroscopy for monitoring ymtrophotometrically electrocheniical reactions taking place at the surfaces oi doped tin o d e crystal electrodes. The same workers (74) derived the theory for measurement by internal reflectance spectrophotometry of concentiatioiis a t conducting tin oxide electrodes coated on glass. The result was an analog in Beer’s law which n as verified experimentally for ouidation of o-tolidine. Potential sweep, poteiitial step chronopotentiometric, and potentiometric techniques mere used to shon that the quantity observed spectrophotometrically is the surface concentration of oxidation product. Koch a i d Scaife (109) used reflectance a t a platiiimi electrode to show the presence of ctrongly adsorbed species such a. o\ygeii. Poll< el al. (177) shoned that it naq possible to produce on gla- thin conducting metal surfaces sufficiently traii-parent for internal reflectance qpectroscopy. By means of such cells it v a s pos.il)le t o study electrode reaction mechanism? without changes in the electrode or in the absorbance bacmkgi ouiid due to electrolybis. The relative inertiiesi: of the platinum 182 R
ANALYTICAL CHEMISTRY
on glass electrodes is a great advantage compared to tin oxide, germanium, and similar semiconductors. The base line was found not to change from run to run. Analogous results were obtained by Murray, Heineman, and O’Dom (144) who devised an optically transparent thin layer electrochemical cell of fixed thickness using a gold miiiigrid screen having a thousand wires/inch a$ the optically transparent electrode. O’Brien (159) used motion picture photography to follow changes in concentration gradient a t horizontal electrodes thereby allowing an independent check on electrochemical theory. Leja and O’Brien (118) made laser interferometric measurements to study transport mechanisms around the dropping mercury electrode and O’Brien (160) applied the same technique to the rotating disc electrode. Strtifelda and Kimla have continued their extensive studies of polarography in flowing systems. The theory for currents a t mercury drop electrodes (215 ) streaming mercury electrodes, (214 ) , and cylindrical electrodes (105) has been derived. The necessity of very stable electrodes of high capacity to use as reference electrodes in long term polarographic monitoring of flowing systems was emphasized and the advantages of electrodes of the first kind such as cadmium amalgam/saturated cadmium sulphate were cited (216). Janata and N a r k (87) described a flowthrough cell of very small volume which could be used for simultaneous polarographic, coulometric, and spectrophotometric measurements. The tubular platinum electrode aiid mercury coated tubular electrode mentioned earlier is designed particularly for flowiiig sy;* t ems. Photopolarography. Berg (16) has given a n extensive review of photopolarography, discussing photo residual currents, photo depolarization currents, phototransfer reactions, the influence of photochemical reactions on other faradaic processes, photokinetic currents a n d photosorption currents. The three principal hypotheses concerning the generation of photoresidual currents are discussed aiid compared: The electron cloud concept of Barker, Gardner, and Samnioii (1O ) , the charge transfer layer concept of Michael Heyrovskj (78) and Berg’s own “hot electrode” theory. Berg and Golliiiick ( 1 7 ) dudied the half-wave potentials of excited molecules aiid discuswd an empirical relationship between quantum yield and half-wave potential. Gerischer (66) described the electrochemical behavior of wniconducting electrodes under strong illumination. Silvestroni arid Carclli (206) ascribed the anomalous oxygen n ave observed in oscillopolarography, both with a dropping electrode and with
stationary electrodes of various metals irradiated by a high intensity lamp, to convection caused by heating. Perone and Birk (167) described the techniques of application of electrochemical methods to the study of products of flash photolysis reactions. At the hanging drop electrode, various potentiostatic techniques were studied with systems already well characterized both photochemically and electrochemically. A suitable cell was described. Nonaqueous Solvents. T h e use of nonaqueous solvents for polarography and electrochemistry in general has become quite commonplace. Schneider and Strelow (191) measured the half-wave potentials of thallous and cadmium ions with respect to cobaltocynium ion in 14 solvents and various mixtures of them with water. The results are discussed with reference to the rubidium standard proposed by Pleskov for comparing different solvents. RIcMasters et al. (139) measured the half-wave potentials of eleven species in dimethylsulfoxide and dimethylformamide, obtaining relative half-xave potentials. Schwabe and Geisler (198) examined liquid junction potentials between aqueous and nonaqueous electrolyte solutions noting that they might be as high as 100 mT’ or more but under most circumstances were of the order of a few mV. Murray and Hiller (145) studied supporting electrolyte effects in nonaqueous systems, particularly acetonitrile. The half-wave potential of ferric acetonyl acetate was shifted in the positive direction by the addition of lithium to a tetramethyl ammonium perchlorate supporting electrolyte, an effect attributed to the coordinative relaxation of the ferrous actonyl acetate produced. Butler (21) has given an extensive review of electrochemistry in dimethylsulfoxide which is, at present, an extremely popular solvent. Johnson, Pool, and Hamm (90) noted anonialous cathodic waves in oxygen solutions containing various metals, which they attributed to the formation of peroxides and superoxides of thc metals in DMSO. Jones and Fritsche (91) discussed voltages-weep chroiioaiii1)erometry in DJISO with hanging drop electrodes. Smyrl and Tobias (209) recommended a thallium amalgam-thallium chloride reference electrode for use in DMSO and IIarple (130) desciibcd a stable and reversible cadmium amalgam-cadmium chloride-sodium chloridr reference electrode for u c i n anhydrous DJIF. Nelson and .idanis (162) have explored the u.%e of ~~roi~~-lenecarbonate as an electrochemical solvent. I t is a good ionizing solvent (dielectric constant 69) with iavorahle miscibility characteristics, low vapor pressure and toxicity, and high chemical stability. A potential range from +0.4 to - 2 . 5 V is obtainable with mercury electrodes.
Bull and Stonestreet (26) used 1,1,3,3tetramethylurea as a solvent for polarography. The potential range is good but conductivities tend to be low. Hall, Sakuma, and Elving (7‘5) have done cyclic voltammetry of various dyes at platinum electrodes in liquid sulfur dioxide. The use of a sodium-aluminum chloride melt in chronoamperometry has already been mentioned (64). Eluard and Tremillion (54) have used molten potassium thiocyanate as an electrolyte. Below 275” C, a t which temperature it begins to decompose with evolution of sulfur, it is reported to be a good medium for electrochemical work. Swain et al. (225) have found tetrahexyl ammonium benzoate to be an interesting solvent inasmuch as it is a liquid salt a t 25’ C and useful for electrochemical purposes. MISCELLANEOUS
Katsanos, Tassios, and Zeliotis (102) have been investigating the uses of radiopolarography in the determination of trace levels of the various metals. They have developed an improved cell for radiopolarography and used an exchange technique to detect a wave of inactive manganese by exchange with radioactive zinc. Micka and Kadlec (140) have continued to study polarography of suspensions of insoluble salts. They claim that silver carbonate has crystal forms which differ in concentration of defects detectable not only by therniogravimetric techniques but also because the form which has the higher concentration of defects gives a secondary maximum on polarographic curves. The sensitivity of polarography can be improved, obviously, by a prior concentration step of liquid-liquid extraction. Afghan and Dagnall(2) have shown that a back extraction into a n aqueous electrolyte is not always necessary and have developed methods for the determination of six metals by extraction from a magnesium nitrate, citric acid, potassium thiocyanate, pyridine mixture using toluene as the solvent. If methanol and lithium chloride are added to the toluene extract, polarograms can be run directly. Sohr and Liebetrau (210) have shown that certain organic liquids added to the polarograph cell are adsorbed on the mercury electrode where they act as extracting agents for traces of heavy metal ions. For example, tri-alkyl phosphates or tri-alkyl phosphine oxides cause heavy metal ions to be extracted and concentrated in the double layer of the mercury drop with considerable increase in sensitivity of the subsequent polarographic measurement. Woggon and Schnaak (259) have discussed use of standard addition and the possibility of errors due to the blank. They recommend the use of a n equation
which takes the blank into consideration. Tenygl (228) has made use of an oxygen stripper involving zinc amalgam moistened with 10% sodium hydroxide, which is said to remove oxygen very efficiently from nitrogen. Reynolds (185) has given a general discussion of chromatopolarography and reviewed the apparatus techniques and applications to inorganic and organic separations and determinations. Hladik (EO), in a theoretical study of polarographic curves in thin-layer solid electrolytes, used a nonequilibrium thermodynamic formulation for describing mass transfer. Lightfoot (119) studied electrochemical phenomena, particularly convection and concentration polarization, in thin electrolyte films a t the edge of menisci. The results have bearing on the behavior of partially immersed electrodes with an air or gas iriterface, and also porous electrode behavior. LITERATURE CITED
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WORK supported in part by the U. S. Atomic Energy Commission under Contract AT(30-1)-905.
~
Sluyters, J. H., J . Electroanal. Chem.,
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Electrochemical Relaxation Techniques W . H . Reinmufh, Department o f Chemistry, Columbia University, N e w York, N. Y.
T
follows the line of the previous one in the series, confining attention to the literature published from January 1966 to January 1968 which was available to the author a t the time of writing. S o attempt is made to completely survey the literature, but rather attention is concentrated in those areas in which the reviewer felt competent to offer critical commentary. The reader seeking broader coverage is urged to scan the pages of Electroanalytical Abstracts. HE PRCSEXT REVIEW
SMALL AMPLITUDE RELAXATIONS
Possibly the most important and certainly the most widely discussed theoretical contribution in the area of small amplitude relaxation techniques in recent years has been the concept introduced by Delahay (50) of the a priori inseparability of faradaic and nonfaradaic currents. I n early treatments of the theory of small amplitude electrochemical relaxation processes it was assumed that faradaic and nonfaradaic processes proceed by parallel but independent paths, and further that the double layer charging process could be represented as a simple capacitance
which takes the same value in the presence and absence of the faradaic process. This was the model adopted by Randles (166) in his pioneering work and applied successfully by him and by later workers to a variety of systems. A second level of sophistication allows the possibility that the double layer charging process may be influenced by the presence of the faradaic reactants, but still assumes it to be representable as a pure capacitance in parallel with the faradaic impedence. A major contribution of Sluyters and coworkers (176) was to develop methods of data analysis which allowed treatment of experimental data when the second assumption was justified, and, equally importantly, which allowed recognition of its failure when that occurred. Prior to their work the methods of data analysis in common use not only relied on the validity of the first, less general, assumption but also often failed to give adequate indication of its failure. However, substantial evidence has aecumulated over the years that there exist experimental systems for which neither the first nor the second assumption is adequate-cf. (178). Attention has focussed on explana-
10027 tiors having to do with mechanistically complicated faradaic processes, and on the effects of experimental artifact on observed results. This work unquestionably constituted (and continues t o constitute) a valuable contribution to our knowledge, but still fails to eliminate the difficulty in the majority of cases where the source of complication lies in adsorption of the electroactive species. Most of the attempts to describe the effect of adsorption on small amplitude relaxation have focussed attent; on the faradaic branch of the equivalent circuit, and, by neglect, have implicitly assumed the applicability of assumption one or two above the nonfaradaic branch. Baticle and Perdu (19) have recently reviewed theoretical efforts in this area. Barker (12) was possibly the first to suggest the representation of the cell impedence in terms of a model in which the double layer charging process did not take the form of a simple frequency-independent capacitance. Incidentally, Barker’s classic paper seems worth reading once a year on general principle-it apparently contains enough ideas t o keep the rest of us busy for a long time to come. Barker’s suggestion was overlooked, probably in part beVOL. 40, NO. 5, APRIL 1968
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