Polarographic Theory, Instrumentation, and Methodology David N. Hume, Massachusetts lnsfifute of Technology, Cambridge, Mass. 02 139
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not only of the literature but of new topics which might reasonably fall within the scope of this review has made it desirable that something be done to keep the size of the undertaking within manageable proportions. Accordingly, the subjects treated in previous reviews have been divided into two categories (it is hoped without significant overlap or gap), and the present review is restricted to topics which most people might agree fitted properly under the title. The recent literature of the nener electroanalytical techniques which may be described as polarographic only by adoption of a very elastic definition of the term, together with the papers on electrode processes, electrode kinetics, and general electrochemical theory, are treated in a separate review by Dr. W. H. Reinmuth, see page 211R. The present review covers, somewhat selectively, the literature published since the previous review ( f O O ) , November 1961 to Sovember 1963. As usual, no effort has been made to mention papers on applications unless they embody a new development in methodology or a novel line of approach. Reviewing the literature is made easier by the existence of the continuing bibliographies of the polarographic literature, particularly the one which appears under the direction of Heyrovskf ( 9 6 ) , and the one founded by Semerano (108). Unfortunately these bibliographies are always a t least 1 or 2 years behind the current literature. Bibliographic and abstracting work in the field of polarography is being done by a number of groups in different countries with the inevitable results of duplication and wasted effort. It is to be hoped that these groups will be able to organize a cooperative undertaking, a. for example, through the agencv of IUP.iC, and thereby provide not only a faster but a more complete coverage than is now possible. It is interesting to note how the preferences in journals change over the years among electroanalytical chemiqt.; Lqho are submitting papers for publication. h perusal of the Literature Cited section of this review shows how strikingly the preference has fallen on a few journals while others, formerly popular, are now almost completely ignored. The Journal of ElectroanHE COXTINUED GROWTH
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alytical Chemistry discontinued its extremely useful abstracts section after Volume IV, but the abstract portion under the same editorship as before is now being published separately by another publisher (Birkhhuser Verlag) as a new journal, Electroanalytical Abstracts. The first issue appeared in August 1963 and maintains the same high standards as before, but unfortunately about a year’s time was lost during the transition. Many readers will be pleased to know that the Russian publication, Zavodskaya Laboratoriya, which carries a wealth of useful information about techniques and methodology, is now available in English translation through the Instrument Society of America. Among the books published in the review period, particular mention should be made of the two-volume set, “Progress in Polarography,” edited by Zuman and Kolthoff (265), which comprises a series of short monographs on a wide variety of up-to-date topics in polarography, written by outstanding authorities all over the world. There is much information in this book which does not seem to have been published elsewhere. Published in honor of Professor Heyrovsk? on the occasion of his 70th birthday, it is a far more scholarly work than the usual heterogeneous assembly of superficial reviews, which is all too often produced on such occasions. The excellent book “Modern Polarographic Methods” by Schmidt and von Stackelberg is now available in English translation (227). A new edition of the textbook on electrochemistry by Milazzo (163) should also be mentioned. Outstanding amont the numerous reviews which have appeared are those by Yurnberg and von Stackelberg (186, 187), which are installments of an extensive survey of the methodology and applications of d.c. polarography, these portions being devoted to reaction kinetic measurements, and doublelayer and adsorption phenomena, respectively. A good review of modern methods in d.c. polarography has also been given by Nurnberg (185), and newer techniques have also been discussed by Kargin (121). Johnson (109) has given a three-part survey of techniques in direct and alternating current polarography, and later (111) a critical discussion of the advantages and dis-
advantages of polarography as an analytical tool, viewed from both economic and chemical standpoints. CLASSICAL POLAROGRAPHY
Instruments and Apparatus. E. H. Sargent and Co. now offers the FS Model polarograph which is specifically designed for hanging drop and anodic stripping voltammetry. Metrohm, the Swiss firm, distributes in America their Polarecord E261A, which, beaides normal direct current polarography, is equipped to do socalled rapid polarography with a drop knocker giving very short drop times, stripping voltammetry with hanging drop electrodes, and alternating current polarography with the aid of an auxiliary power supply. Gulton Industries in the United States is marketing the newest model of the Mervyn Mark IV square-wave polarograph. This is a modular assembly and may be used with any electrode stand and suitable recorder, thereby lowering the investment necessary. hh‘fEL in Milan offers a well engineered oscilloscopic polarograph, Model 451, which besides regular single sweep polarography is equipped to do derivative and stripping work. The design and functioning of the Tinsley h‘fark 19 pen-recording polarograph ha. been described (138). Kokhman has devised a versatile apparatus for general electroanalytical application (129). The use of modular electronic components, particularly operational amplifiers, for making electroanalytical apparatus has been growing rapidly in importance. A good idea of the scope and utility of the operational amplifier in analytical chemistry may be gathered from the papers in the operational amplifier symposium published in the November 1963 issue of Analytical Chemistry. The papers shom application to practically every electroanalytical technique and demonstrate how multi-purpose apparatus may be made from simple modules (30, 4 2 , 67, 14 2 , 1 6 8 , 1 r l , 221,231 , 2 4 7 ) . The book by Malmstadt, Enke, and Toren (151) is of importance not only in terms of the subject matter which it contains but also in view of the influence which it is having on the teaching of instrumentation in the universities. Of considerable significance in research as well as in
teaching is the availirbility of modular apparatus such as thE,t made by Heathkit of Benton Harbor, Mich., for instrument synthesis. Mention should also be made of two articles by Lewin (144, 145) on electrical acd electronic tools for analytical,chemists. Xnnino and Hagler ( 2 ) have described a simple electronic device using an operational amplifier circuit similar in design to the conventional Kelly-JonesFisher circuit for compensation of iR drop in high resistance solutions. It may be attached dirwtly to a conventional d.c. polarograph for use in systems of high resistance. Mark and Reilley (156) have pointed o J t that the device suggested by Arthur et al. ( 4 ) for compensation of iR drop in high resistance solutions is actua1l:i a current-scan polarograph, although not recognized as such by its inven.;ors. As a consequence, it is subject to the usual disadvantages of current-scan polarographic apparatus, which makes it less desirable than alterr ative methods of compensating for high resistance. Schober and Rehak (21Y) have described a modernized transistorized version of Kalousek's commutator apparat'us used in testing polarographic reversibility. Raaen and Jones (2G3) have given details of instrumentation for the automatic and simultaneous measurement of mass, drop" time, drop weight, and drop count. Svestka and Kalous have described the automa tic registration of electrocapillarity curves (24%. Zagroski in the chapter on cells for polarographic electrolysis in the ZumanKolthoff book (265) remarked, "If an album could be prepared consisting of the photographs of all the polarographic cells which have been published in the literature, it would run to many volumes." This is not much of a n exaggeration. In the current period Ullmann et al. (246) have described a convenient cell for Flolarography with mercury pool electrodes, Rusina (210) has described two cells for use with water-free solvents providing complete exclusion of the :itmilsphere, and Bersier and Bersier (24) have published the description of a microcell for pool polarography, dealing witk 0.03- to O.O?-ml. samples absorbed in Elter paper. Straf6lda (256) uses a modified Kalousek cell connected with the reference elect'rode to a three-way st,opcock. Reininger and Grams (267) have developed an all-Teflon cell asscmbly for general electrochemical use, and Toren (245) has given a description of a circulating pump and cell for electrochemical study of gases involving a closed system, requiring only a few milliliters of gas a t pressures as low as 50 mm. Although not proposed for the purpose, it is evident that the cell could be applied to polarography.
Although most work is done with a dropping mercury electrode which is essentially identical with t h a t used by Heyrovski in 1922, modifications are still being proposed and studies made to further the understanding of the phenomena which take place at the dropping electrode. Briggs and Knoules (38) cite advantages in favor of a wide-bore dropping electrode, particularly for the determination of oxygen. Raaen (202) has given a detailed account of the techniques of fabricating and using capillaries made entirely of Teflon. A properly made Teflon capillary shows a behavior very similar to that of a glass capillary and it may be applied in media which would attack ordinary glass. Successful results have been obtained, for example, in 10M sodium hydroxide and in 28.11 hydrofluoric acid. Mesari6 and Hume (159) were able to use ordinary glass capillaries in acidic fluoride medium after coating them with a layer of Tygon. The technique is very simple and for solutions not too acidic, very effective. Treibel and Berg (244) have shown some very interesting Schlieren pictures of various polarized dropping mercury electrodes. The study of electrodes under various conditions of d.c. and a x . polarization led to the development of a pointed capillary (Spitzkappilare), the diameter at the tip being of the order of 0.4 mm. This was found to reduce depletion and shielding effects markedly, and current-time curves of the successive drops agreed much better with that of the first drop than would be expected with an ordinary dropping electrode. Kalvoda and Smoler (115) have discussed the advantages of the Smoler capillary : the parabolic i-t curve of individual drops, the absence of depletion effect, and the fact that the short drop time makes it possible to do the voltage scan more rapidly than with the conventional equipment. Several authors have been examining the details of drop detachment with high speed motion picture cameras. Chao-Ortega and Laforgue-Kantzer (45), using high speed cinematography on dropping mercury electrodes a t low pressure were unable to see withdrawal of the mercury up the capillary after detachment of the drop. This was contrary to observations reported by several authors in the past who examined the electrode microscopically. I n passing, it might be remarked that the revieu-er in collaboration with W. Boardman was likewise unable to detect withdrawal of the mercury with high speed photography. Knowles and Keen (128), on the other hand, did observe retreat of mercury in the capillary. They were using a wide-bore capillary upturned at a 45" angle, however, rather than a conventional dropping electrode. Doppelfeld and von Stackel-
berg (60) studied the current-time phenomena in the first, millisecond of drop life by means of a n oscilloscope. They concluded that there exists, in effect, a streaming maximum at the inst'ant the drop breaks off. Theory of Electrode Behavior. AIthough lacking in immediate applicability to classical polarography, the new theories of Gerischer of the nature of electron transfer processes at electrodes will be of interest to those seeking a deeper understanding of t h e nature of electrochemical reactions. In a series of three papers (82, 83, 84) he describes electronic transfers between solid-state electrodes and redox electrolytes in terms of a tunnel effect from quantized levels in the solid to the species in solution. The study of semiconductor electrodes (66) has been particularly fruitful in elucidating the processes involved. For those who find language a barrier in the above papers, a good introduction to the theory can be gleaned from Gerischer's paper on the behavior of germanium electrodes which was published in The Record of Chemical Progress (82). Brodd (53) describes a new theory of the kinetic and electrical properties of electrode reactions based on their description as relaxation processes. Kolthoff and Khalafalla (130) in a study of the effect' of iodide-bromide ions on the polarographic reduction of the cobalt hexammine complex gave a picture of iodide and mercurous iodide acting as an electron transfer bridge which may have considerably more general significance. Buck (41) has derived a general kinetic model for electrode processes with prior solution reactions, obtaining a n equation valid for all values of preelect'rochemical reaction rate constants. This has lead to a modification of the Lingane-Freyhold-von Stackelberg equations for the half-wave potential of a reversible complex which is valid for low concentrations of ligand also. Hale and Parsons (91),using a graphical interpolation method based on the Kouteck? theory of irreversible waves, have shown how it is possible to determine both standard potentials and rate constants of electrode reactions, even though the reduced form-i.e., reaction product-is unstable. Kamalkar (1 16) has described a modification of the u.wal mathematical methods for the calculation of consecutive formation constants from polarographic dnt,a; Schupp, Youness, and Watters (220) have extended the conventional method so that consecutive formation constants may be determined by the use of amalgam dropping electrodes. Lopushinskaya and Pamphilova (118) have reviewed the subject of irreversible waves. Currents. A general theory of diffusion-limited, charge-transfer proc' VOL. 36, NO. 5 , APRIL 1964
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esses in electroanalytical techniques has been developed by Reinmuth (208). Subrahmaya (241), by introducing certain mathematical refinements in the derivation of the IlkoviE equation, has obtained an improved form which has slightly different numerical values of the constant terms. Bearman ( I S ) , after a careful examination of the assumptions concerning diffusion coefficients which are used in the derivation of equations for current-voltage curves, concluded that a given system actually has a t least two different kinds of diffusion coefficients applicable : the neutral-diffusion and the tracer, or self-diffusion, coefficient. The ordinary derivations of diff usion-current equations do not distinguish between these diffusion coefficients. He decided that, strictly speaking, the two diffusion coefficients are not equal, but a close examination of the derivations leads to the prediction that, within the range of error of limiting current equations, the two should effectively be equal. Studies by R. N. Adams are under way to see if the two types of diffusion coefficient are satisfactorily identical in practice. Glietenberg, van Riesenbeck, and Nurnberg (85) have studied the influence of viscosity and other solvent properties on the limiting current and discussed their significance in the determination of trace diffusion coefficients by use of the extended IlkoviE equation. Mukherjee, Gosh, and Chakravarti (170) have likewise studied the effect of viscosity, examining limiting currents of cadmium and zinc in the range of 12 to 90% glycerine. They observed that the product of limiting current and the square root of viscosity was a constailt throughout the range. Okada et al. (190), in a study of the contribution of the migration current to the polarographic limiting current, derived an equation for the limiting current as a function of supporting electrolyte composition and concentration. They obtained good agreement with experimental results for the reduction of thallium and lead in potassium chloride solutions. Zembura, Fulinski, and Bierowski (264) likewise studied migration current contributions and developed an equation by which they could calculate the limiting current a t a dropping mercury electrode with good accuracy for three-ion electrolytes. The predictions from the equation agree well with the experimental results of Lingane and Kolthoff. The effect of cell resistance on acute maxima was examined by Kolthoff, Marshall, and Gupta (131). They showed that the internal resistance behaved according to the theoretical equation R = kt-’/3 where k depends on the diameter of the capillary tip. Using various pointed capillary electrodes [cf. Treibel and Berg ( 2 4 4 ) ] $ they found that the current agreed 202 R
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with the IlkoviE equation if a sufficiently small capillary tip were used. VagramGan and Fatueva (248) have studied the mutual influence of different metal ions undergoing simultaneous deposition at the surface of a dropping electrode. They observed that the velocity of discharge for metal ions being deposited simultaneously a t a given potential was frequently not equal to the simple sum of the single discharge rates for separate deposition a t the same potential. I n the simultaneous deposition of cobalt and nickel, for example, there was marked diminution of the rate of both ions, whereas for the simultaneous deposition of nickel and iron only nickel was reduced in rate and iron was accelerated. The authors interpret the effects in terms of changes in the electrical double layer and variation in the solvation of the ions compared with the case of separate deposition. It seems to the reviewer, however, that the change in the wrface nature of the electrode due to the deposition of insoluble metal upon i t would be expected to be more important than either of these factors. DraEka (62) has considered the effect of electrostatic repulsion of the depolarizer in the double layer and concluded from theoretical grounds that the effect would be similar experimentally to the kinetic current of a preceding rapid first-order reaction. Biegler (26) has examined the effects which adsorption and deposition of impervious films have on polarographic limiting currents. He points out that the current due to a film deposited on the surface of the drop depends on the height of mercury in the same way as a classical adsorption current, although the mechanism involved is different. One cannot distinguish between film formation and classical adsorption prewaves by current us. time or current us. h”* measurements, and he suggests a re-examination of the pre-waves which have been described in the literature and a search for additional criteria which may permit a distinction. I t has been the reviewer’s experience that in practice deposited films usually tend to behave differently from adsorbed solutes, inasmuch as the films crack and break as the drop grows. I t is also possible to observe film formation quite readily with a microscope so that the problem mentioned here is perhaps not so serious as i t might a t first sound. Schmidt and von Schorlemer (216) have looked a t the capacitative residual current of an electrode, the drop time of which was being regulated by a drop knocker. For the capacitative current, no difference was observed from that with a n unregulated dropping electrode, and the theoretical equation for capacitative current was observed to hold. Differences in the faradaic current were,
however, observed as would be expected from the depletion effect. Willeboordse (259) has made a study of the influence of electrode shape and capillary size on the polarographic maximum. Factors important in this phenomenon include the concentration of supporting electrolyte, the capillary diameter, the shape of the end of the tube, and the rate of mercury flow. He attributes the motion of the solution during maxima as being due to differences in interfacial tension, or to vortices within the drop itself. Studies of current-time relationships a t individual drops have continued to be fruitful. Bockei (28) has developed a transistor-switching apparatus for plotting polarographic current-time curves of “first” drops. Von Stackelberg and Toome (236) have studied the effect of various factors on the tearing-off process as the drop falls and discussed the effect of variable back pressure on the current-time curve. Newcombe and Woods (183), using high speed cinematographic techniques combined with oscilloscopic current-time studies have confirmed the earlier results of Smith (232) on the relationship between drop area and time. K i t a and Smoler (137)have made a theoretical and experimental study of instantaneous currenttime curves at dropping amalgam electrodes. They find the nature of the tip of the capillary to be significant in terms of flow a t the surface of the drop. Weronski (258) has used the currenttime curve technique to study the rate and extent of adsorption of various reaction inhibitors on the dropping electrode. Weber and Kouteck? (266) have considered theoretically capillaryactive substances undergoing slow adsorption at the d.m.e. and their influence on the instantaneous and mean currents a t individual drops. KGta and Smoler (136) had previously studied the effect of rapidly adsorbed substances, both charged and uncharged, and their influence on irreversible electrode processes was examined in particular. METHODOLOGY
Alternating Current Polarography. Alternating current polarography, together with square wave polarography, has shown a marked increase in interest over the past 2 years as shown by a myriad of papers discussing practical applications. The book by Breyer and Bauer (%), “Alternating Current Polarography and Tensammetry,” provides an extensive review of the principles and practice. On the theoretical side, Rauer (8, IO) has amended his previous derivation (9) to allow for inequality of diffusion coefficients of oxidized and reduced species, a point which had been criticized by Reinmuth
and
Smith. Rangal*ajan and
Doss
($05) have reported, without giving
detail, modified equations for the alternating current polarographic wave which differ from those previously published. Smith (2’30) has made a theoretical study of alternating current polarographic proces3es v, ith coupled homogeneous chemical reactions. Firstorder preceding and following reactions and catalytic r e a c t i o ~ ~are s considered. The rrsults were verified experimentally. Smith (231) has also suggested the use of several new altern2 ting current techniques, particularly 1 he automatic recording of phase angles and the phase selective detection of second harmonic currents, Of potential significance in the theory of alternating current polarographic processes is the series of papers by Erdey-Gruz and his collaborators (64, 65, 66) on the effects of sinusoidal alternating current cn electrode processes. Frumkin and Danaskin (72) have criticized the tensammetric wave concept of Rreyer and H acobian, disliking both the terminolog] and the mechanism proposed to explain the observed phenomena. They contend that the application of relations deduced for faradaic currents to adsorption-desorption processes leads to erroneous results. Breyer and Hacohian have replied (37) admitting that one cif their equations was inexact, but defending other points. I n the line of instrumentation, Hayes and Aylward (94) have described a modification of the Leeds and Northrup Electrochernograph similar in principle to the approach of W:dker, Adams, and Alden (264, 255) in Nhich operational amplifiers are used to (ontrol the applied potentials. Spahr arid Knevel (234) have described a n up-scale and downscale compensator which can be applied to the familiar hliller modification of the Sargent Model XXI F’olarograph. Cohen and Lordi (48)have pointed out in connection with the Miller modification of the Sargent Model XXI for alternating current polarogi aphy, that the recorder response time has a greater effect in alternating current than in direct current polarography. The recorder usually used in the Model XXI does not have so fast a response as would be desirable, with the result that there is nonlinearity of p w k height with sensitivity setting. They suggest that a 1-second recorder 3s necessary and advise caution in the use of converted d.c. instruments which do not have high speed recorders. Hayes and Rauer (J5) have made a comparison of the Cambridge I‘nivector Unit-a phase-selective de\w’ e measuring only the alterne ting current in phase with the alteriating voltagewith the conventional alternating current polarograph where base current is not eliminated in this way. They ob-
served that for good reversible waves such as t h a t of cadmium, a ten- to a hundred-fold increase in sensitivity was possible by phase selection. For tensammetric waves, however, the conventional approach was superior and gave about the same sensitivity because of the shift in phase angle and distortion of shape interfering with the application of the phase selective apparatus. Smith (231) has described operational amplifier-based instrumentation for alternating current polarography. An instrument with tuned amplifiers and an oscillator with high frequency stability has given excellent performance: relative standard deviations of +O 4% for fundamental-harmonic currentamplitude measurements, and *0.8% for phase angle determinations have been attained. The performance on second harmonic a x . polarography was comparable. Saito (211) has described an a.c. polarograph with a compensating circuit for the charging current. Biegler (65) has pointed out the applicability of a cathode-ray oscilloscope in alternating current polarography and tensammetry. Observation of the wave form of the alternating current and the relationship between alternating current and time during the growth of a drop gives useful information about the nature of the electrode processes. He shows evidence for a characteristic change in wave form between tensammetric and ordinary alternating current faradaic processes and suggests the technique as a diagnostic tool in distinguishing adsorption processes. Imai and Delahay (102) have described a special cell with low inductance and very small stray capacitance which is useful for high frequency studies using the dropping mercury electrode. Set0 and Yamazaki (s24) report a survey of 10 kinds of common supporting electrolytes and their behavior for square wave and alternating current polarography. The importance of the supporting electrolyte is greater in alternating than in direct polarography because of the sensitivity of the former to small differences in reversibility. Senda, Senda, and Tachi (223) have likewise studied the effect of supporting electrolytes and capillary active substances on alternating current polarographic processes. Bourzeix, Robert, and Viet (32) have compared the conventional dropping electrode and the hanging drop electrode with respect to their behavior in the ax. polarography of cadmium. Their results are much as would be expected. Laforgue-Kantzer (139) studied the growth of the mercury drop under ax. polarization and observed that whereas under d.c. polarization the radius is proportional to the one-third power of time, under alternating current polarization it is proportional to the square root of time. This he sought to explain in
terms of displacement of the electrocapillary maximum during part of the time of growth of the drop. Bauer and Goodwin (11) have summarized the available information on the temperature coefficient of alternating current wave heights. Temperature effects can, as in direct current polarography, be used as an indication as to whether the electrode process is under diffusion or kinetic control. Breyer (34) has observed a new type of alternating current wave at positive polarization. This wave shows no corresponding direct current step and the alternating current base line indicates adsorption to take place on both sides of the wave. Observed with riboflavin and other substances which adsorb, or form mercury compounds which remain adsorbed on the drop, the exact nature of the wave is not yet understood. The wave obtained with 8-quinolinol is very sensitive to low concentrations of metals, and it was observed that some metals can be determined in the range of 10-6M by titration with 10-5M oxine to a n end point given by observation of the positive wave. Breyer, Beevers, and Bauer (36) have shown the applicability of alternating current polarography to the study of the dismutation of pentavalent uranium, the results obtained being the same as those found with direct current techniques. The use of higher harmonics in alternating current polarography has received a fair amount of attention. Neeb (180) has pointed out again the advantage of using the higher harmonics to eliminate the effect of the capacitative current. I n a later paper (181) he studied the effect of the individual characteristics of a number of ions on the harmonic components and characteristics of the alternating current, noting for example that elements of higher charge gave considerably greater harmonic diffusion currents. The work of Smith (231) on apparatus for harmonic measurement and the proposal of phaseselective detection of second harmonic currents has already been mentioned. Paynter and Reinmuth (196) have studied the third and fourth harmonic alternating current polarography, which they point out shares with the second harmonic technique the advantage of diminishing the contribution from the nonfaradaic charging current. The amplitudes of the harmonics decrease with the inverse power of the harmonic itself, a fact which seems to balance out the advantage of going to higher harmonics and severely limits the analytical applicability of the technique. Lordi (149) by an approach, which he terms ‘‘tuned alternating current polarography,” has been able to improve the faradaic-to-capacitative current ratio. H e places an inductance in parallel with the cell and when it is the proper value VOL. 36, NO. 5 , APRIL 1964
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with respect to the capacitance of the cell, resonance is obtained in which the inductive and capacitative currents are 180” out of phase and cancel. In this way he finds i t possible to diminish the capacitative current by a factor of two or three, although variations in the characteristics of the circuit due to drop growth make compensation only approximate. Neeb (179) has given the name of “intermodulation polarography” to a new low-frequency alternating current technique based on simultaneous polarization with two sine waves of differing frequency. If one has a frequency in the range of 3 to 30 cycles per second and the other, 50 to 150, the desired effect is achieved. Because an alternating current impedance consists of a nearly linear capacitance and a nonlinear resistance component, one can use the distortion of intermodulation caused by the resistive component for measurements. Curves and peaks ahich depend upon the concentration of the depolarizer and the frequencies used are obtained. Elimination of capacitative current gives higher sensitivity than simple alternating current polarography. It should be noted, however, that polarization due to the whole frequency spectrum of basic frequencies, to their harmonics, and to harmonic sequences of all possible combined frequencies takes place. The curves are, therefore, complicated by a number of small peaks which can make interpretation difficult. The square wave technique has received relatively little theoretical attention recently although many practical applications have been described. Geerinck et al. (79) have published a circuit for a simplified square wave polarograph, very similar in principle to that proposed by Hamm (92). Kaplan and Sorokovskaya (11Q), and von Sturm and Ressel (239) have discussed the various factors important in the application of square wave polarography and pointed out in particular the role of foreign electrolyte concentration in sensitivity. Potential Sweep Chronoamperometry. This term is suggested by Delahay, Charlot, and Laitinen (56) as better descriptive of the techniques involved than the traditional designation “single sweep oscilloscopic polarography.” It is the technique normally meant when English writers speak of “cathode-ray polarography.” An instrument for differential chronoamperometry is marketed by Southern Analytical Instruments in England. Matched capillaries are used and after 5 seconds of drop growth there is a sweep a t approximately 0.25 volt per second for 2 seconds. The drop life is then terminated by a pulse and a second cycle begins. Davis and Rooney (52) have
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described the instrument and its use for “subtractive polarography” in which the background current may be eliminated, and in “comparative polarography” in which the small difference between a sample and a standard solution may be amplified for precise measurement. Derivative curves are obtainable by slightly offsetting one sweep with both electrodes in the same solution. Shalgosky and Ratling (225) have studied the comparative technique and report that by taking special precautions, one can obtain under ideal conditions relative standard deviations of less than 0.1%. Higher precision is limited by the random variations in individual drops. Davolio, Guerzoni, and Papoff (53) have described an electronic, triangular voltage sweep generator with a wide range of scanning rates and initial potential settings, and Cowell and Styles (49) have described the design of a linear sweep apparatus for chronoamperometry with solid electrodes. Osteryoung, Lauer, and Anson (193) have suggested the use of integration of chronoamperometric waves. The current-time integral has proved its utility in the study of materials adsorbed on electrodes and may have more general applicability. Bakes, Gregory, and Jeffery (12) have warned that the use of mercury pool anodes may lead to appreciable errors when concentration is being determined by peak height measurement in chronoamperometry. Douglas and Magee (61) have suggested the use of the ratio of peak current to diffusion current as a means of determining the number of electrons in a reduction without measurement of the diffusion coefficient. Oscillographic Polarography. I n the present review, this term is restricted to the technique introduced by Heyrovski and Forejt about 20 years ago, in which a sinusoidal alternating voltage is applied and a function, usually d E / d t = f(E), is observed on the screen of a n oscilloscope. T h e technique, although very extensively used in eastern Europe and particularly in Czechoslovakia, has received comparatively little attention elsewhere. One factor in this lack of popularity is probably the fact that the method is known in the West largely through journal articles which have shown the oscilloscopic traces reproduced as tiny and quite unconvincing pictures. This reviewer must confess to having been skeptical of the claims for the accuracy and reproducibility of the method, and it was not until he saw a demonstration of the apparatus during a visit to HeyrovskG’s laboratory that this prejudice was removed. I n view of the present state of the electronic art, oscillographic polarography should be deserving of more consideration. Besides innumerable practical analyt-
ical applications, a considerable amount of methodological and theoretical work has been done in the present period. Woggon and Spranger (260) have described a new type of streaming mercury electrode for oscilloscopic polarography, in which the mercury jet is directed into a large capillary counter electrode, stabilizing its length and making it independent of the volume of solution. Fiby (6Q),and Horyna and JehliEka (99, 107) have described vibrating dropping mercury electrodes, both of the conventional and the tangential (Smoler) type, which vibrating at 50 cycles per second give a drop time of l/bo of a second. This, the authors claim, results in a very stable, motionless oscillographic trace. Not only is the oscillographic polarogram stable (it was claimed that the depth of incisions showed no change after 6 hours of operation), but the consumption of mercury is only 1 or 2y0 of that of a streaming electrode, and about 10% of that of a conventional dropping electrode. The oscillograms obtained by this type of electrode agree well with those obtained from a mercury jet. Micka (161) has used a mercury drop hung on a silver wire as a n electrode, and Skobets and Shapoval (229) claim that a mercury-coated silver electrode gives excellent results. Kalvoda (114) has made use of a vibrating platinum electrode covered with a thin film of deposited mercury. This, he reports, does not give as good polarograms as a mercury electrode. Pechan (197) reports obtaining oscillographic polarograms using a copper amalgam electrode and a n unpolarized platinum electrode on filter paper. Schmidt (215, 214) has done oscillographic polarography with platinum electrodes in a potassiumlithium chloride eutectic while Matysik (158)finds that with mercury electrodes, oscilloscopic polarography in anhydrous ammonium nitrate ammoniate is much the same for organic compounds as in an alkaline aqueous medium. Gorlich, Srzednicky, and Kowlaski (87) utilized two streaming mercury electrodes in a differential circuit, and by regulating the streaming electrodes very carefully so that their lengths agreed within 10.005 mm., obtained stable images of differential polarograms. Beran (17 ) has described a device for timing the photographing of the oscillographic trace a t a predetermined point after switching from pre-electrolysis to alternating current oscillopolarography. Micka (162) has developed a mathematical expression of the theory of kinetic currents in oscillographic polarography; basically it is the same as for regeneration of the depolarizer by a chemical reaction. Other Polarographic Methods. “Tast polarography” and methods related to i t have received a fair
amount of attention. Kronenberger and Nickels (134) h3ve described the Selector V instrument for Tast polarography and outlined the various advantages of the technique. One feature of the instrument is that current voltage times may be measured on alternate drops, which then avoids the usual depletion effect. The principles of Tast polarography have been discussed again by Kane ( 1 17) and a time delay circuit for switching current into the measuring circuit at, a Predetermined point in the life of the mercury drop has been described by Xarayanan and Venkatachalam (176). Mark and Reilley (155) have described a technique which involves measurement of the diffusion current with an oscilloscope during the last part of t,he drop life. -4 “first drop” is used and the predetermined residual current is blitnked out. They reported that a singla species, such as lead, copper, or zinc in the range of to 10-5-11 was determinable to a precision of +0.4 x 10-6M. Becker ( 1 4 ) eliminates dep‘etion effects by making h e r m i t t e n t measurements so as to :get “first” drops and reports a n absolute precision arid accuracy of 2 to 37,. Wolf (261) has further developed the technique known as “rapid polarography” in which a drop knocker is used to give a drop time of 0.2 to 1 second, permitting a faster scanning rate. Over this range of drop times, he found both for diffusion and kinetic currents good agreement with the theoretical dependence on drop time. Bodyu, Kozlova, and Lyalikov (29) have reviewed the principles and practice of “pulse polarogrsphy.” Oka (188, 189) reports freedom .+om maxima and other effects of liquid streaming when “potential step voltainmetry” is used, and Heyrovski (97) has derived a relat,ion for the mean charging current involved in using the Kalosek apparatus for changing applied potential. The charging current is more important here than in classical polarography, and the derived relat,ionship aids in distinguishing charging and faradaic currents. Mark and Reilley (156) in investigating “current-scan polarography” have studied systems which give polarographic maxima unde-- ordinary conditions. By introducing a large capacitor in parallel with the cell to reduce voltage fluctuations during the drop life, they noted a hysteresis efl’ect-e.g., in the polarography of nickc-in which the forward and reverse cui~ent-scanpolarograms differed considerably. The forward scan is not very reproducible and is affected by the factors which cause maxima in conventional potential-scan polarography. The reverse scan gives a very good and reproducible limiting current whicn is linear with
concentration. Olver and Ross (192) likewise noted streaming effects on current-scan polarograms and pointed out the necessity of using maximum suppressors here and in other circumstances where, although conventional maxima are not observed, disturbing effects are actually taking place. Ieutsu (104) has studied the effect of factors such as residual current, cell resistance, maxima, migration currents, mixed potentials, kinetic currents, and adsorption on current-scan polarograms. With Fujinaga (Ye),he has checked experimentally the effect of reversibility of the electrode processes and other factors and suggested that current-scan polarography will be useful for analytical purposes. VOLTAMMETRY WITH OTHER ELECTRODES
Mercury Electrodes. Johnson (110) has reviewed the various types of mercury a n d metallic microelectrodes used in polarographic methods. Treibel and Berg (244) have discussed the advantages of a sharply pointed dropping mercury electrode and several workers (69, 99, 107) have utilized vibrating dropping mercury electrodes, particularly in oscilloscopic polarography. A streaming mercury electrode with a constant surface area has been described (260) and the method of fluid dynamics has been used to calculate mass transfer at mercury jet electrodes with sufficient success t h a t limiting currents may be predicted with an accuracy of a few per cent (6). Kapoor and Tiwari (120) have described a mercury drop pool electrode which is used in stirred solutions. It is stated to be more sensitive than the dropping electrode, but the reproducibility is poor. Imai et al. (101, 103) placed two platinum electrodes on opposite sides of a capillary and applied a n alternating field which vibrates the electrode. An increased current results which depends on the charge of the mercury solution interface and is zero at the electrocapillary maximurn. The effect of vibration rates, surface active agents, and maxima have been examined. Narayan (174) has studied this type of vibrating dropping mercury electrode in polarography and claims that electrocapillaryphoresis is the prime cause for the increased diffusion current. The use of platinum electrodes coated with thin films of mercury has been popular for some time, but there has always been difficulty in getting a reproducible thin film to cover the platinum completely. Moros (167) described a technique by which a platinum electrode submerged in mercury is rubbed with very fine abrasive paper. A film of mercury estimated to be of the order of 10 microns in thickness is formed on the
electrode with stability characteristics which recommend it for precision voltammetry using ordinary polarographic equipment. Ramaley, Brubaker, and Enke (204) recommend instead reducing any oxide film on a platinum electrode by holding a t or near the potential of the hydrogen electrode for several minutes and then dipping in mercury. They claim to get a n adherent coat which is superior in properties to that obtained by abrasion and more conveniently attained. Franklin and Bradford (71) have described an unusual mercury electrode in which the contact between the electrode and the solution takes place wit,hin a fritted-glass disk. The electrode was proposed for chronopotentiometry and was said to be quite insensitive to vibration. It should be equally useful for other electroanalytical techniques including potential sweep chronoamperometry. Campanella and Scarano (43) have deposited rhodium on a dropping mercury electrode, thereby greatly lowering the hydrogen over-voltage. I n the presence of rhodium chloride a well developed polarographic wave for hydrogen ions is obtained a t -1.35 volts, and it is said to be useful for the study of hydrogen reduction and the determination of acids. Hanging Drop Electrodes. The hanging drop techniques have continued t o be extremely popular. Gokhstein and Gokhstein (86) have described an apparatus for the automatic removal and replacement of uniform hanging drops in a hermetically sealed system. Kaplan and Chigirev (118) have used a piston-type mercury electrode fabricated from a fluorinecontaining polymer for preparing hanging drops. Narayan (175) gives a method of getting stationary drops of known size by use of a capillary and microsyringe micrometer. Kowalski (133) has a similar technique. The principal application of the hanging drop is, of course, the technique which is sometimes called “anodic stripping polarography” or “inverse polarography.” A metal or metals are d a t e d on the drop during an extended electrolysis period and then redissolved over a short period of time by a reversed potential scan. Kemula and Goldolvsky (122) have studied the influence of surface-active substances on both the electrodeposition and the electro-reoxidation of metals a t the hanging drop electrode. Surface-active agents affect, particularly the deposition step, shifting some metals to more negative potentials. Vasil’eva (249) has studied the relationship between metal concentration in a mercury drop and the enrichment time with allowance for impoverishment of the solution. Vasil’eva a,nd T’inogradova (250, 2551) have studied the distribution of metal inside a mercury drop, taking into consideration rates of amalVOL. 36, NO. 5, APRIL 1964
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gamation and their effect upon reversal of the process when the stripping is to be done. Kemula, Kublik, and Taraszewska (123) have studied anodic passivation of the hanging drop electrode in chloride, bromide iodide, fluoride, sulfate, and hydroxide solut,ions. Stromberg and Stromberg (238) and S e e b (178) have reviewed the techniques of anodic st,ripping voltammetry and its application to trace analysis. Ficker and Meites (70) have studied intermetallic compound formation in mixed amalgams. X rather stable CObalt-zinc compound is formed in mercury and the result suggests that reactions of this sort, if prevalent, may have a considerable effect on anodic stripping waves. Von Sturm and Ressel (640) have test.ed extensively the anodic stripping technique a t hanging drops using a square-wave scan. They find that only reversible electrode processes are well adapted to this technique and that intermetallic compound formation lowers the sensitivity considerably. Jacobs (105) reports successful application of the carbon paste electrode to anodic stripping voltammetry of gold and silver. Oka (188) has done potential step voltammetry a t the hanging drop electrode, observing the current oscilloscopically. He claims a 70-fold increase in sensit,ivity over ordinary polarographic methods. Solid Electrodes. Delimarskii and Gorodyskii (56) have reviewed the use of solid electrodes in polarography. Sheridan (227) has developed a simple technique for sealing silver electrodes into glass tubing with the aid of melted polyethylene. Cozzi, Raspi, and Nucci (60) have made studies on the properties of the dipping platinum electrode, a platinum microelectrode periodically immersed in the solution. For the polarographic determination of electroactive substances in gases, Lestienne (143) has used a platinum electrode covered with a thin layer of liquid, continuously renewed and in contact with t,he gas and a reference electrode. It is said to be robust, sensitive, easily made and useful for the determination of acids, bases, and carbonyl compounds. Blaedel, Olson, and Sharma (27) have found that a short length of fine platinum tubing makes a very useful polarographic electrode, especially for flowing systems. The tube contains 2 to 10 111. of solution, is stable, and has very well defined geometry. The operating characteristics mere predicted and verified experimentally. Because of the well defined flow characteristics and high surface-to-volume ratio of the electrode, it provides a very high sensitivity--e.g., of the order of 10-831 for many metal ions. Schmidt and Gygax (216) have proposed t,he use of a "rhamher elect,rode" (Kammerelek206 R
ANALYTICAL CHEMISTRY
trode) consisting of a space formed by two close-set parallel electrode surfaces. The conditions approximate linear diffusion. A number of workers have concerned themselves with developing theoretical expressions for the currents a t solid electrodes. Asada et al. ( 6 ) gave relations for the current distribution for deposition and dissolution of metals a t vertical electrodes under free convection. Jordan and Javick (118) have continued earlier work on hydrodynamic voltammetry with both rotating and circulating electrolysis cells with stationary, conical, and cylindrical microelectrodes under controlled flow conditions. Ideal laminar flow is achievable up to velocities as high as 7 meters per second. Ozaki and Nakayama (194, 195) have studied current relationships a t a platinum wire electrode with a rotated disk stirrer. The convection current was found to be linear with concentration and the rate of convection. The profound effect of oxide films on platinum and other noble metal electrodes has been the subject of a number of studies. The claim by Laitinen and Enke (1.40) that oxide growth occurs preferentially on the grain boundaries of platinum electrodes was disputed by Mohilner, Argersinger, and Adams (166) who, through photomicrographic studies, found no evidence of selective formation, but instead observed a random distribution of oxide on platinum anodes. Anson and King (3) interpret the increased reversibility of various reactions a t reduced platinum electrodes to the platinieation of the platinum surface through reduction of the oxide. Feldberg, Enke, and Rricker (68) have made a detailed study of the formation and dissolution of platinum oxide films on platinum electrodes. They distinguish between the oxidized platinum surface, the half-reduced active state, and the clean platinum surface. They observe oxidation to be a two-step process, with reduction following a different path. The half-reduced surface, which they visualize as consisting of platinum oxide with hydrogen added, converting the oxide to hvdroxide groups alorg with the reduction, they feel to be a n active surface which enhances electron transfer with redox species in the double layer. Peters and Lingane (199) have studied both electrochemically and by chemical analysis the oxychloride films which form on platinum anodes in chloride media. Membrane-covered platinum electrodes for the determination of electroactive gases, particularly the determination of oxygen, have been of considerable interest. An apparatus utilizing the well known Clark cell is commercially available from Beckman Instruments. Mancy, Okun, and Reilley
(166) developed a modification of the Clark type cell, using a silver cathode, a lead anode, and potassium hydroxide electrolyte. Because of the nature of the cell, it operates spontaneously without the need of a n external potential source. The apparatus is now marketed by the Precision Scientific Co. as an oxygen analyzer. Rayment (206) studied the temperature effect in Clark cells and observed that the increase in the limiting current was greater than expected simply from the rise in diffusion coefficient of oxygen with temperature: polyethylene and poly(tetrafluoroethylene) membranes have increased permeability with higher temperatures. A number of microelectrodes have been described for the determination of oxygen: Sommerkamp and Oehmig ( W S ) have described one in the form of a cardiac catheter. Rice (209) has a microprobe with an outer diameter of only 70 microns, suitable for penetrating brain tissue. Naylor and Evans (177) use a wire covered with a rubber membrane. Hagihara (90) recommends a collodion-coated rotating platinum electrode for measurement of mitochondrial respiration. He also describes a special cell to be used with the electrode which permits the addition of reagents and other manipulations without contamination by oxygen. Longmuir and Allen ( I d ? ) , on studying the determination of oxygen with solid microelectrodes, concluded that the method measures partial pressure of oxygen rather than concentration. The rotating disk design of solid electrodes has been gaining in popularity because of its desirable electrochemical characteristics. Galus, Olson, Lee, and Adams (76) have pointed out the theoretical and practical advantages of rotating electrodes, particularly as applied to studies of electrode mechanisms. Galus and Adams (75) have applied the reaction layer concept to the rotating disk electrode and provided simplified derivations of the equations of Levich and Kouteck? for electron transfer followed by chemical reaction. Zembura (862, 865) has considered the component of the limiting current due to migration and developed a n equation for the limiting current on rotating disk electrodes for any concentration of supporting electrolyte in the range up to 0.1M. The experimental results for silver, hydrogen, and zinc agree well with theory. Nagy and his coworkers (172, f 7 S ) have estimated the number of active sites on a rotating platinum disk electrode from measurements of the hydrogen diffusion current. Kholpanov and Gorbachev (164) have studied the effect of rotation rate and solution concentration on the rates of reversible systems. Delimarskii, Pantshenko, and Shilina (58), using the rotating disk in molten salts, found good agreement
with Levich's equation for current as a function of rotation speed. ilzim and Riddiford ( 7 ) have designed a new type of rotated disk electrode consisting of a microelectrode centered in a bell-shaped, nonconducting holder. This design was found l,o give theoretical behavior and has the advantage that the small working electrode surface gives currents in the r'ange of ordinary polarographic equipmant whereas the larger nonconducting disk gives hydrodynamic behavior ecuivalent to the theoretically desirabla infinite disk. I n addition, edge effects are eliminat'ed. Jahn and Vielstich (106) have published details of the design of low area, highly stable, flexible-speed, rotating-disk electrode assemblies. Speed is governed by a synchronous motor powered by a n audiofrequency oscillator and amplifier. Belyaeva (16) used a n orifice in the center of a rotating disk to place a comparison electrode, and designed a special cell to minimize hydrodynamic perturbations. Geissler and Landsberg (80) describe a rotating graphite-disk electrode, and Galus, Olson, Lee, and Adams (76) report good results with a rotating carbon paste electrode. Carbon electrodes hrtve continued to interest many workers because of their inert characteristics. Of particular significance is the introduction of the pyrolytic graphite electrode by Laitinen and Rhodes (141). P,yrolytic graphite is obtained by deposition under reduced pressure on a substrate a t 1900" to 2500" C. It consists of highly ordered crystallites formed in layers parallel to the surface of the substrate. I t is anisotropic and highly impervious to liquids and gases, inert to chemical attack under ordinary conditions, and exceptionally free of entrapped contaminants. Laitinen and Rhodes used it for polarography in molten lithium chloride-potassium chloride eutectic, and Miller and Zittel (165) have found it to have very desirak.de properties for aqueous polarography. The useable potential range is from f1.0 to -0.8 volt us. the SCE in acid medium. The residual current is low, no pretreatment is needed, and it is not susceptible to poisoning. It was estimated that determinations could be performed with a precision comparable to that obtainable with the dropping mercury electrode. The use of this material in potentiometry has also been proposed (164). Perone (198) has studied the use of the waxed graphite electrode for anodic stripping voltammetry and claims a more favorable ratio of faradaic to residual current than obtainable with the hanging drop. Olson and Adams (191) have extended the use of the carbon paste electrode to anodic stripping voltammetry. Nujol pastes were used and satisfactory results obtained. The
use of the carbon paste electrode in a rotating disk has already been mentioned (76). Mueller and Adams (169) have extended their investigations of the characteristics of the boron carbide electrode and Sawyer and Seo (212) have used it in the study of the reduction of oxygen. Pleskov (200) has studied behavior at a germanium electrode, mainly from the standpoint of examining the effect of a semi-conductor electrode on the mechanism of electron transfer. The theoretical behavior of porous electrodes, both thin and thick, has been considered by Ksenzhek (135) and by Navman and Tobias (184). Although these studies are not directly connected with polarography, they are of significance to the understanding of processes involving permeable electrodes. MISCELLANEOUS
Nonaqueous solvents continue to play an important part in polarography. Guttmann, Schober, and their associates have studied extensively polarography in anhydrous dimethylsulfoxide (88, 89, 54). Cisak and Elving (46) have used pyridine as a solvent and report the silver-silver nitrate electrode to be suitable as a stable reference. Brown and Hsiung (40) have used formamide as a solvent, and Knecht and Kolthoff (127) N-methylacetamide. They found it to have no analytical advantage over water and several other solvents. Coetzee and McGuire (47) have compared half-wave potentials in nitriles and in acetone with those in water. Korchinskii (132) has measured electrocapillarity curves of mercury in acetonitrile and found the activity series of adsorptivity of anions to be the same as in water. Voorhies and Schurdak (253) have recommended nitromethane as a solvent for solid-electrode voltammetry and chronopotentiometry. Traces of water have a n important effect. Sellars and Leonard (222) have performed polarography in the ammoniate of sodium iodide and report normal behavior for both inorganic and organic electroactive species. Schober, Guttmann, and Nedbalek (218) have reviewed polarographic behavior in ethylenediamine, acetic anhydride, and dimethylsulfoxide. Nelson and Iwamoto (182) have considered the problem of liquid junction potentials in nonaqueous systems. On the basis of experimental measurements with the ferrocene-ferricinium ion couple and various substituted phenanthroline-metal complex couples, they concluded that many liquid junction potentials were not actually so large as had been presumed. Molten salt systems have been utilized by a number of coworkers. Manning (I53) has studied voltammetry with
platinum electrodes and a lithiumpotassium-sodium fluoride melt, and Manning and Mamantov (154) the same electrolyte and also lithium-beryllium fluoride with platinum and tungsten microelectrodes. A graphite cell was found satisfactory. Kido, Ishibashi, and Hayakawa (125) used a verticallyvibrating platinum tip in a molten cryolite system. Caton and Freund (44)got satisfactory curves for a number of species using a platinum microelectrode and fused alkali metaphosphates at around 750" C. A platinum crucible was used as the cell. Shinayawa and Yanagi (228) were able to use the dropping mercury electrode in fused ammonium hypophosphite a t 120" C. Gaur and Behl (78) were successful in utilizing micro platinum electrodes in a sodiumpotassium-magnesium chloride ternary eutectic at 475" C. They noted that the best results were obtained from metals which were deposited as liquids on the electrode. Schmidt (213, 214) did oscillopolarographic studies in molten lithium-potassium chloride with stationary solid electrodes. Littlewood recommends a silver-silver chloride electrode in a glass sheath as a reference electrode in fused alkali chloride melts (146) while Harrington and Tien (93) suggested the use of cobalt amalgam or mercury aged 5 days at melt temperature in a borosilicate glass bulb. The use of polarography in flowing systems for continuous analysis has been reviewed by Turyan and Murenkov (245). Strafelda and Kimla (237) have derived a new equation for the current a t a stationary spherical electrode in a flowing electrolyte, and Kimla (126) has developed it further. Vojii; (252) has utilized a n apparatus for synchronizing the rate of dropping of mercury and electrolyte drops in a flow-through cell useful for the continuous analysis of gases. Kalous (1IS) has used a flowing cell for protein concentration determination in the eluent from chromatographic columns. The catalytic protein wave in cobalt solutions is utilized. Weininger and Grams (257) have described an all-Teflon cell assembly for electroanalytical application in flowing systems. Berg and Berg and Schweiss have developed a set of related techniques which they call photo-polarography, in which rapid chronoamperometry or oscilloscopic polarography is done during or following high intensity illumination, or flash photolysis. They have demonstrated that a photosensitized reduction of ketones and quinones can be followed continuously if the half-life is greater than 15 seconds. With shorter half lives a kinetic current is involved (20). Ultraviolet irradiation of ketones in alkaline solution produces radicals and radical ions which can be VOL. 36, NO. 5, APRIL 1964
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determined by oscillographic examination of current-time curves on single drops. The rate of dimerization of the radicals may be followed readily (18, 23). Continuous polarographic examination under irradiation showed growth and decay of radicals (phot’odepolarizers). A flash lamp technique permits disproportionations and recombinations to be followed. With oscilloscopic recording, reactions with half lives equal to or greater than a millisecond can be studied (21, 22). The comparison of oscilloscopic photopolarography using the dropping and the streaming electrodes provides additional insight into the rates of the reactions involved (19). Pligka (201) has analyzed the errors in polarography and made a statistical examination of reproducibility. He concluded that the standard curve was superior to the use of the wave height quotient, which was in turn superior to the standard addition method. Eckschlager (63) studied the errors in the standard addition method, deriving a method for evaluating the precision and providing tables for estimating the uncertainty. Olver and Ross (192) studied the effect of streaming on voltammetric methods and the factors which govern streaming a t various types of electrodes. They concluded that it was desirable to use maximum suppressors even under conditions where visible maxima were not observed. Malik and Maque (150) have recommended isothiourea dodecyl ether hydrobromide and dodecyl pyridinium bromide as maximum suppressors, good for both positive and negative maxima. Micka (160) has reviewed the electrochemical behavior of insoluble depolarizers: suspensions, emulsions, surface foams, crystals, solidified melts, and porous pressed bodies. All are characterized by electron transport between a solid phase of semiconduct,or character and a fluid electrolyte; or a metallic phase and a solid semiconductor. Various elements, oxides, sulfides, selenides, tellurides, insoluble inorganic compounds, and minerals behave as insoluble depolarizers giving polarographic waves. hbbott and Collat ( 1 ) described indirect polarographic determination of acids by measurements of a diffusion current limited by acid consumed in the electrode reaction; for example, the reduction of quinone. Fujinaga and Izutsu (73) have further developed the technique of electrochemical masking in which a surface-act,ive agent adsorbed on the electrode inhibits the progress of an irreversible reaction, resulting in t,he separation of t,he waves of a reversible and irreversible system. Reference electrodes, particularly the calomel electrode, are of great importance in polarography, and it is desirable that their dynamic characterist,ics be w l l known. Shams el Din, Silsson, 208 R
ANALYTICAL CHEMISTRY
and W r a n g l h ( l d 6 ) studied the polarizability of the saturated calomel electrode, finding that a t low current densities there was very little effect, but a t high values a n anodic passivation was observed with a considerable change in potential, which might take minutes to decay to the equilibrium value. Das and Ives (51) have recently measured the temperature coefficient of the calomel electrode. Disthche (59) has described techniques for electrochemical measurements a t high pressure. Although not developed with polarography in mind, the techniques described should be adaptable. It was pointed out, for example, that glass electrodes may be used a t pressures up t o 1500 kg. per square centimeter without difficulty. .4lso suggested for the determination of gases is the highly unusual electrode system of Higuchi and Papariello (98). Made in the form of a platinum wire anode inside a Vycor, borosilicate glass, or ceramic capillary tube with a platinum wire cathode wound around the outside of the tube, i t is operated a t 500” to 700” C. When 150 to 300 volts is applied across the electrodes and argon or nitrogen is passed through the capillary as a carrier gas, the electrode system is found to be very sensitive to traces of gases such as hydrogen and methane in the carrier. Galus et al. (77) have done tritium tracer studies to determine the electrolysis products formed under conditions approximating normal polarographic practice. Beilby and Budd (15) have applied the technique they call “integral chronoamperometry”-i.e., measurement of the charge transferred during a short period of electrolysis a t constant potential with a qtationary solid electrode originally described by Booman (31)-and described a simple manual apparatus suited to its performance. Reddy, Devanathan, and Bockris (207) have applied the name “chronoellipsometry” to a new technique in which the surface of a mirror anode is observed during electrolysis. The visible formation of a film indicates a characteristic induction time analogous to the transition time in chronopotentiometry, but applicable where precipitation of a solid phase a t the electrode does not give a rapid potential increase. Braunwalder, Grubenmann, and HUgli (33) describe a technique for polarography on filter paper. Bersier and Bersier (24) have performed polarography with a micropool in contact with 0.03 to 0.04 ml. of sample on filter paper in a special microcell. Mark, Smith, and Reilley (157) describe a modification of “polarographic coulometry” (formerly known as “milliroulometry”) in which a large 20-microfarad capacitor is used in parallel with the cell. -\constant current which is less than the
diffusion current is passed through the system until the concentration is depleted by 30 to 40%. The method described, an amperostatic one, is recommended over the usual potentiostatic approach because it is easier to measure the number of coulombs passed. Delimarskii and Gorodyskii (57) have devised a current-voltage-time surface for plotting solid electrode polarograms. The surface aids in the interpretation and comparison of the various methods of solid electrode polarography. LITERATURE CITED
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Work supported in part by the U. S. Atomic Energy Commission under Contract AT(30-1)-905.
