(35F) Janata, J., Lab. Pract.. 20, 865 (1971). (36F) Jee, R . D., Fresenius’ 2. Anal. Chem.. 264, 143 (1973). (37F) Kalvoda, R., Holub, I., Anal. Chem., 44, 2252 (1972). (38F) Kalvoda. R . , Chem. Anal. (Warsaw), 17, 1101 (1972). (39F) Kalvoda, R . , Holub, I., Chem. Listy, 67, 302 (1973). (40F) Kelley, P. C., Horlick, G., Anal. Chem., 45. 516 (1973). (41 F) Kir’owa-Eisner, E., Tshernikovski, N., Eisner, U., J. Electrochem. SOC., 120, 361 (1973). (42F) Kirschner, G. L., Perone. S. P., Anal. Chern., 44, 443 (1972). (43F) Kogoma. M., Nakayama, T., Aoyagui. S., J , Electroanal. Chem., 34, 123 (1972). (44F) Lakshmanan, S..Ramakrishnan, K. R., Trans. SOC. Advan. Electrochem. Sci. Techno/., 8, 66 (1973) (45F) Li, Chia-Yu, Ferrier, D., Schroeder, R . R . , Chem. Instrum., 3, 333 (1972). (46F) Marecek, V., Honz, J., Collect. Czech. Chem. Commun., 38, 487 (1973).
Acta, 57, 473 (1971). Martin, R . F., Davis, D. G.. Comput. (61F) Van Leeuwen, H . P.. Sluyters. J. H.,J. Chem. Instrum., 2, 103 (1972) EIectroanaI, Chem.. 39, 233 (1972) McAllister, D. L., Dryhurst. G.. Anal. Chim. Acta., 64, 121 (1973). (62F) Vassos, B. H.. Anal. Chem., 45, 1292 (1973). Miller, B., Bellavance, M. i., Bruckenstein, S., Anal. Chem., 44, 1983 (1972). (63F) White, W. R . , Analyt. Letters, 5, 875 (1972). Mumby, J. E., Perone, S. P., Chem. Instrum., 3, 191 (1971). (64F) Whitson, P. E.. VandenBorn, H . W , Neilsen. C. J.. Stuart, J. D.. Anal. Chem., Evans, D. H . , Anal. Chem.. 45, 1298 (1973). 44, 1713 (1972). Oldham, K. B..ibid., 45, 39 (1973). (65F) Woodward, W. S., Ridgway, T. H..Reilley. C. N., ibid., p 435. Osteryoung. R. A., Comput. Chem In(66F) Zheleztsov, A. V., Zh. Anal. Khim., 27, strum.. 2. 353 11972). 1461 (1972). Pills. A. A,[ii/d,,p 139. Roe, D. K., Chem. Instrum., 4, 15 (1972). Sarma, N. S., Sankar, L., Krishnan. A,, Rajagopalan, S. R . , J . EIectroanaI. Noise Chern., 41, 503 (1973). (1G) Barker, G. C.. J. Electroanal, Chem.. 39, Schroeder, R . R . , Comput. Chem. In484 (1972). strum., 2, 263 (1972). (2G) Tyagai, V. A,, Elektrokhimiya, 8, 1628 Sedletskii. R . V.. Limin, B. E., Eiektrokhi(19721. miya. 8, 22 (1972). (3G) Tyagai, V. A,. Eiectrochim. Acta. 18, 229 Smith, D. E., Comput. Chem. Instrum., 2, (1973). 369 (1972). (4G) Tyagai, V. A,, Kolbasov, G. Ya., 2 . Phys. Tsuji, K.. Takahashi, K., Anal. Chim. Chem. (Leipzig).251, 123 (1972).
Analytical Electrochemistry: Methodology and Applications of Dynamic Techniques Peter T. Kissinger D e p a r t m e n t of C h e m i s t r y . M i c h i g a n State University, East Lansing, M i c h . 48824
The scope of this review takes a significant departure from t h a t of its predecessors. The banner “Polarographic Theory, Instrumentation, and Methodology” was carried here since 1951, most recently under the tutelage of Richard Nicholson (6A). In recent years, there have been radical changes in the theory and practice of electroanalytical chemistry. DC Polarography no longer occupies the preeminent position it held in the 40’s and 50’s. An extensive repertoire of more sophisticated excitation signals has been applied to the dropping mercury electrode, greatly expanding its analytical power. In addition, stationary and rotating electrodes have become equal partners with the DME. Now t h a t the profusion of new ideas and technology has subsided to some extent, it is a good time to assess the significance and usefulness of these developments to real chemical problems. Many techniques never attracted a large clientele, and some have lost status (e.g. chronopotentiometry). On the other hand, several techniques have reached maturity (e.g. pulse polarography and cyclic voltammetry) and are being used by chemists whose objectives do not include the study of electrode reactions per se. Spectroscopic and chromatographic methods have long dominated analytical chemistry in the United States. This will continue to be true; however, there are some examples where electrochemistry can stand alone, and many where it can be advantageously combined with another methodology. Electroanalytical chemistry remains an exciting field in which to work, and the future looks bright for many novel and very useful applications. Hopefully, it won’t be too long before all chemists carry electrodes in their toolbox. In order to complement the accompanying review, the emphasis here will be placed on nonelectronic experimental aspects and the more unique applications of finite current electrochemistry. References were selected based on the author’s interpretation of their relevance to analytical chemistry. Publications in physical electrochemistry have in general been slighted, as have those dealing with the more routine applications. Most references were selected from Chemical Abstracts, Chemical Titles, and the Interface Newsletter from January 1972 through December 1973. A number of books have appeared in the last two years which contain chapters of interest to applications oriented
electrochemists. The 6th volume of “Electroanalytical Chemistry’’ is begun with an excellent effort by Underwood on the study of molecules of biological interest ( 2 A ) . The 3rd volume of “Progress in Polarography” includes useful chapters on complex ions, intermetallic compounds, free radicals, double layer effects, and the use of proton availability to establish mechanisms in organic electrochemistry (IOA). Bauer’s little book on “Electrodics” (3A) contains a limited amount of experimental advice as does “ A Guide to the Study of Electrode Kinetics” by Thirsk and Harrison (8A). The latter is primarily a guide to the literature with a brief survey of mathematical conclusions for various excitation signals. A new series with a physical slant, “Techniques of Electrochemistry,” has been initiated by Yeager and Salkind (9A). The first volume emphasizes techniques for studying electrode processes and contains very little of interest to most analytical chemists. Purportedly designed for the “nonspecialist,” future volumes will be oriented toward applications to other disciplines. Fry has written an excellent introductory text on “Synthetic Organic Electrochemistry” which will be of great value to anyone using electrode reactions to study organic chemistry, whether their objectives be synthetic, mechanistic, or analytical ( 4 A ) .The more ambitious work edited by Baizer on “Organic Electrochemistry” is encyclopedic in scope and contains something for everyone ( I A ) . While neither of these organic books deals explicitly with analytical problems, they illustrate many analytically useful electrode reactions and techniques. The small book by Smith and Zimmerli introduces a few basic principles and surveys commercial instruments for “Electrochemical Methods of Process Analysis” ( 7 A ) . The focus will please industrial engineers, but there is little here for anyone else. Volumes 2 and 3 of the exhaustive electrochemistry review published by The Chemical Society have appeared covering the years 1970 and 1971, respectively ( 5 A ) .
ELECTRODES Reference Electrodes. There has been imperceptible activity over the past two years in the development of reference electrodes for dynamic techniques. Reference electrodes can be crucial to static techniques, and it is not surprising that more effort has been devoted to their use in potentiometry (especially ion-selective electrodes). It is A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O .
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becoming more popular to use “pseudo” reference electrodes in connection with dynamic measurements. These can consist of almost anything that conducts and does not dissolve in t h e supporting medium. Platinum and silver wire are most popular, with the latter being preferred since in some media it more closely resembles a real reference in the thermodynamic sense. The assumption is made t h a t if the reference potential doesn’t change during the experiment a t hand, then it’s good enough. This is a dangerous assumption, particularly when t h e experiment will be repeated a t a later date. Nevertheless, t h e pseudo reference is convenient for work in small volumes of solution and on vacuum lines where complicated liquid junctions are a nuisance to maintain. Pseudo references can be quite reproducible for given conditions as has been demonstrated recently for a platinum mesh reference in propylene carbonate and hexamethylphosphoramide (27B). Part of the problem is in the hands of manufacturers who have devised thousands of reference electrodes for potentiometry but virtually none for voltammetry. These electrodes are often too big or too leaky, or have too high a n impedance. They rarely can be used directly in nonaqueous media, but require antediluvian salt bridges prone to contaminate samples and pick u p electromagnetic radiation. Since sales of potentiostats are brisk, it shouldn’t be long before a good commercial reference is available. Reference electrodes and their proper use have been reviewed recently ( I B , 13B). Working Electrode M a t e r i a l s a n d Design. If anything characterizes developments in this area, it is t h e greater use of carbon as an electrode material for analytical, mechanistic, a n d synthetic purposes, both in aqueous and nonaqueous media. The various forms of carbon often have superior properties for studies of organic molecules, particularly when used as a n anode. If the number of publications is any indication, the use of carbon substrates for the mercury coated electrodes used in stripping voltammetry far outweighs all other applications. This subject will be discussed separately. Three types of carbon are most popular for electrochemical studies: pyrolytic graphite (PG), glassy carbon (GC), and carbon paste (CP). All three offer advantages under given conditions; however, it is clear t h a t the electrochemical details a t any one of them and the differences in behavior between them, are very poorly understood. I t is common to mechanically pretreat t h e surfaces or even prepare a totally fresh surface before each use. Panzer and Elving have published an interesting survey of this problem for glassy carbon and pyrolytic graphite (40B). They investigated the voltammetric behavior of several test systems in both aqueous and nonaqueous media and found the influence of mechanical preparation to be very significant. Some reasons were suggested and a thorough review was promised. Morcos reported on electrocapillary phenomena at stress annealed pyrolytic graphite (36B).The P G E has recently been evaluated for cyclic voltammetry in D M F (29B). and for analysis of vitamin E in vegetable oils (34B).Dryhurst has reviewed the extensive work on oxidation of purines a t the PGE and correlated these developments with biological oxidations (17B). What happens to the electrode itself is of more t h a n casual interest, but few detailed papers have appeared. One good example is a recent report on anodic oxidation of a PGE in sulfuric acid (43B). Electrochemical and X-ray diffraction techniques were used to study formation of a graphite bisulfate lamellar compound. Taylor and Humffray have studied electron transfer kinetics on glassy carbon as a function of surface pretreatment and compared these results with those obtained a t platinum, wax impregnated graphite, and carbon paste (47B). The voltammetry of iron(II1) has been thoroughly examined a t a glassy carbon rotating disk in various aqueous media (46B). The results contrasted favorably with those of many other electrode surfaces. Oxygen reduction a t the GCRDE has also been given much attention (BB). The GCE compares very favorably with platinum for work in aprotic media (148) and sometimes yields more reversible voltammetric waves. Even coulostatic experiments have been performed with glassy carbon and compared with the HMDE results ( 2 5 B ) . 16R
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Carbon paste has both fascinated and frustrated electrochemists since its invention by Ralph Adams in 1958. No one seems to know how or why it works, b u t there is no question t h a t its popularity has increased during the period of this review. The influence of paste composition has been studied (38B) and a new formulation devised which has unusually low background currents when used as a n anode (32B).As a substitute for mineral oil or wax, thermoplastics can be mixed with graphite and set t o a very satisfactory electrode (33B). This sort of thing may very well represent the wave of the future. Bobbitt et al. have espoused the use of carbon paste for preparative purposes and recommend the use of a quarternary liquid anion exchanger to formulate the paste ( 3 B ) . Chronoamperometry at the C P E has been used t o determine diffusion coefficients (39B). The hydrodynamics of carbon paste extruded through a nozzle has been given a mathematical treatment (28B),but no remedy was given for the mess this can make. Carbon paste electrodes with diameters as small as 30 Km have been made, and these are suitable for microanalysis (41B)and in uiuo voltammetric measurements (30B). The so-called Ruzicka Selectrode and variations on this theme are, in essence, carbon paste electrodes used potentiometrically for ion selective probes. These will be reviewed elsewhere in this volume. Carbon cloth (21B) and mechanically activated carbon (45B) have also been recommended of late. There is little new in the realm of dropping mercury electrodes. Cover has reviewed the vibrating dropping mercury electrode (16B)and then later went on to espouse the spinning D M E (37B). Bond and the group a t Melbourne have published a series of papers on what they call rapid polarography (4B, 7B, 9 B ) . The essential idea is that adequate resolution and sensitivity can be obtained when the drop time is on the order of 100-200 msec, thus enabling shorter experiments and greater analytical throughput for both dc and ac polarography. They have also continued to work in H F and describe fabrication of a Teflon DME by electrical discharge ( 6 B ) and voltammetry at a mercury pool electrode in a Kel-F cell ( 5 B ) . Canterford and Waugh have devised a simple modification of an HMDE for media corrosive to glass (IOB). A Teflon extension was pressfitted onto t h e commercial glass capillary. As mercury films grow in popularity, it is appropriate t h a t someone investigate their structure and chemistry. Two fascinating papers have appeared with regard to mercury deposited on platinum. Heineman and Kuwana were able to prepare optically transparent films and study metal deposition, stripping, and adsorption by spectroelectrochemistry (23B).Untereker and Bruckenstein studied the interaction of Hg and Pt by deposition and dissolution of Hg using a Pt RRDE (48B).They identified different intermetallic compounds (PtzHg, PtHg, and PtHgz) and conclude t h a t prospects for a stable Hg-Pt film electrode were bleak. Additional papers on composite mercury electrodes are cited with those on stripping voltammetry. Mounting metal electrodes continues to be a problem with glass-to-metal seals being more of a nuisance t h a n ever. La1 and Bauer studied platinum-glass seals for a variety of giasses and concluded that microcracks along the interface are difficult to avoid (31B). Capon and Parsons ( I I B ) and Ruby ( 4 4 B ) have described practical solutions which should be viable for a wide variety of solid electrode materials. Holub and Loucka recommend sealing gold wire in polyethylene rods (24B). Heat shrinkable Teflon also works pretty well for this purpose. Speaking of gold, Blaedel and Boyer have studied t h e characteristics of Murray Minigrids in a flow-through cell ( 2 B ) .Graves has revealed his design for a unique hemispherical electrode suitable for simultaneous electrochemical and DTA measurements (19B). Speaking of hemispheres, Chin devised a rotating ring-hemisphere electrode with an easily replaceable central hemisphere (15B).I n a similar vein Harrington et d.described a demountable ring-disk electrode (22B). Guilbault et al. recommend glass-metal composition electrodes in which a metal powder formulation is fused to a glass substrate (20B).The principal advantages would seem to be rigidity and low cost. There have been few analytical studies with semiconductor electrodes with the exception of spectroelectro-
Peter T. Kissinger IS an assistant professor at Michigan State university, East Lansing Mich He received his BS degree from Union College, Schnectady. N.Y., in 1966 and the PhD degree from the University of North Carolina at Chapel Hill in 1970 where he worked with Charles N. Reilley. He joined the faculty at Michigan State University following two years of postdoctoral study with Ralph Adams at the University of Kansas. Dr. Kissinger has research interests and experience in neuropharmacology, the rates and mechanisms of organic electrode reactions, electrochemistry in microliter volumes including thin films, and synergistic combinations of chromatography and spectroscopy with electroanalytical techniques. He is also involved in the determination of trace amounts of drugs by the combination of gas and liquid chromatography with mass spectroscopy for biomedical and forensic purposes. He edits a popular newsletter, "Interface," for the international electrochemical fraternity and is cochairman of the Western Electroanalytical Theoretical Society. He I S a member of the ACS, AAAS. and the American Association of Clinical Chemists
chemical studies. They continue to be of greatest interest to physical electrochemists. Memming and Mollers have published a series of papers on SnOz electrodes (35B) and Pleskov has reviewed the work in this area from 1964-71 (42B). The lead dioxide electrode, never analytically import a n t , has been exhaustively reviewed by Carr and Hampson ( I 2 B j . Mechanical surface pretreatment can cause serious problems with boron carbide electrodes (26Bj, and this may be related to their equivalent unpopularity among analytical chemists. In an unrelated miscellany, Flanagan and Marcoux simulated edge effects a t planar disks for chronoamperometry and chronopotentiometry (28B). This is a very useful paper for those wishing to avoid nonlinear diffusion effects in studies of homogeneous reaction rates. CELLS Semi-Infinite Electroanalytical Cells. A great deal of effort continues to be expended on t h e design of cells for exhaustive controlled potential coulometry and coulometric titrations, b u t these are the subject of another review. The principles involved in optimization of the most popular coulometric cells have recently been given a quantitative treatment (5C, 1I C ) . Cells for electrosynthesis have similar problems and these have also been reviewed ( IOCj. Schmulbach and Oommen have modified Anderson's vacuum line cell of a few years back in order to accommodate the larger volumes needed for preparative work ( 1 4 C ) . Hanzlik described a simple modular approach to cells useful for a variety of electroanalytical techniques ( 4 C ) . A fascinating cell has been developed by Hawkridge and Kuwana for enzyme kinetics and rapid coulometric determination of n values for biological electron transport intermediates (6Cj. A well-behaved redox system acts as a mediator between a n optically transparent electrode and a n electron carrier enzyme, permitting a n indirect coulometric titration of the enzyme. The progress of the titration is followed by optically monitoring t h e enzyme itself or the decrease in absorbance of the electrogenerated mediator, which is typically a species of very high molar a b sorptivity ( e . g . methyl viologen cation radical). The amount of charge required to produce the mediator in the first place can thus be related to the amount of charge assumed by t h e enzyme. It all sounds so simple, b u t it really works great! The cell has provision for vacuum degassing of the sample solution (only 2.5 ml is required), and it is apparently possible to reduce the oxygen level to below 5 X M . Heineman et al used this approach to study the redox cycle of cytochrome c oxidase ( 7 C ) . Conventional spectroscopy of electrolysis products is readily accomplished by using a cuvet as part of the electrochemical cell either with circulating (15C) or static solutions (8C).A number of other interesting cells have been reported in the past two years, and these will be mentioned in the sections to follow. If there i s , a n y trend, it is toward using smaller and smaller volumes of solution.
This makes good sense since most dynamic electroanalytical experiments take place in only a few microliters of solution anyway. A maxim we work by in our laboratory is t h a t if you want to compete with spectroscopists for detection limits, you can do so best by reducing your cell volume to zero (well, almost). One way to do this is to confine t h e sample to a thin layer of solution. Thin-Layer Electroanalytical Cells. Electrochemistry in t h i n films of electrolyte appears t o have finally spread well beyond t h e laboratories of its origin, notwithstanding the practical problem of constructing cells to contain them. Theoretical and practical aspects of this area have been reviewed by Tyurin e t al. (17C) and by Hubbard (9C). A new cell was devised specifically for the study of coupled chemical reactions ( I C ) and two others employing optically transparent electrodes were recommended for spectroelectrochemistry (12C, I6C). Schmidt and Stucki placed a gold minigrid between a n Ag/AgCl electrode and a polycrystalline Ag (or Au) electrode as a scavenger for Ag+ species which would have interfered with their study of chloride adsorption (I3Cj. There are undoubtedly other applications where this interesting idea could be put to good use. Goldberg et al. have done a beautiful job of quantitating t h e effects of uncompensated resistance in thin layer cells via finite difference simulation (2C, 3 C ) . These results are particularly pertinent to the generation of radical species for ESR or optical studies in nonaqueous media, where potential gradients can be enormous. MEDIA Nonaqueous Media. With the exception of molten salts and solid electrolytes, which will not be covered by this review, there have been few papers devoted to new nonaqueous media which have clear advantages over those presently in wide use. In contrast to the feverish activity of a few years back, there is a tendency now to focus on the relatively few solvents and electrolytes which have proved themselves. Voltammetry in dimethylsulfoxide has been reviewed recently (100). More attention has been given to purification of acetonitrile ( 1 3 0 , 280) and understanding the anodic background reaction in this solvent ( 2 7 0 ) . The latter seems to be a complicated function of the adventitious water and the supporting electrolyte chosen. A voltammetric study of hydrogen ion reduction in moist acetonitrile was reported by Lanning and Chambers ( 1 1 0 ) . This is a n important investigation since much of organic electrochemistry in nonaqueous solvents generates protons which can play a n important role in these unbuffered systems. Cauquis and Serve have discussed oxidation of N-phenyl-p-phenylenediamines in chloroform ( 6 0 ) . The reduction of alkali metals in nonaqueous solvents has received much attention lately, for example, in propylene carbonate ( 1 2 0 ) and N-methylpyrrolidone (30). Keszthelyi and Bard have evaluated molten scintillator compounds ( e g , 2,5-diphenyloxazole and 2,5-diphenyloxadiazolej as solvents for electrochemical and electrochemiluminescence studies of polycyclic aromatic hydrocarbons (919j . Molten dimethyl sulfone ( m p 109.5") has also been shown to be a useful solvent for electrochemical studies of a variety of metals a t Hg, P t , and C electrodes ( 4 0 ) . Electrochemical generation of solvated electrons is finally beginning to receive quite a bit of attention. Along with this trend comes the greater use of hexamethylphosphoramide (HMPA) which was found to be useful for this purpose years ago. For example, Avaca and Bewick studied the indirect reduction of acetamide by electrochemically generated solvated electrons in HMPA with LiCl as supporting electrolyte ( I D ) . Pavlov e t al. reported on dynamic nuclear polarization of HMPA protons by solvated electrons as further evidence supporting their actual electrogeneration ( 1 6 0 ) . Low T e m p e r a t u r e s . Electroanalytical chemists long held a fascination for 25 "C to an extent often unwarranted by their experimental objectives. In more recent years, the constant temperature baths have been set aside in a general disregard for thermodynamics. Neither extreme is likely to be judicious. Experimentalists ought to pay more than lip service to their activation parameters. While physical electrochemists have dealt a t some length with thermal influences on electron transfer, there have been A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 5 , A P R I L 1974
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few studies on the temperature dependence of coupled chemical reactions. The subject is not a n easy one, since so many aspects of the electrochemical experiment are temperature-dependent. Particularly intriguing is the possibility of slowing down or “quenching” reactions which are too fast to study at 25 “C. This might be done to evaluate mechanisms, determine activation parameters, increase t h e lifetime of radical species, or even to improve the yield of a n electrosynthesis by quenching side reactions. Van Duyne and Reilley have attacked the problem of low temperature electrochemistry both experimentally and theoretically (190-210). They emphasized cyclic voltammetry and potential pulse (ie., “double potential step”) chronoamperometry and chronocoulometry and evaluated cell designs, electrolyte systems, and electronic considerations for temperatures as low as -130 “C. Such experiments are far from routine, b u t there is no doubt t h a t useful information can be obtained (210). Bechgaard et al. have demonstrated the virtues of low temperature cyclic voltammetry and ESR in connection with their studies of 1,2-dithiolylium ions ( 2 0 ) . Miller’s group earlier reported on electrosynthesis in SO2 and now describe interesting coupling reactions of one-benzyltetrahydroisoquinolines a t subambient temperatures (150) and oxidation of aromatic hydrocarbons in methylene chloride a t -70” ( 5 0 ) . Matsumoto and Sat0 described the cyclic voltammetry and spectroelectrochemistry of 9,10-dichloroanthracene down to -60 “C at a n optically transparent electrode and interpreted electrochemiluminescence phenomena on the basis of these results ( 1 4 0 ) . T h a t electrochemistry in liquid ammonia at -50” is not impractical has been demonstrated by Demortier and Bard ( 7 0 ) and Jove and Pages ( 8 0 ) . Although there have been few examples to date, low temperature analytical electrochemistry has a promising future and certainly deserves wider attention.
ANODIC STRIPPING VOLTAMMETRY (ASV) It seems a fair assessment t h a t there is no analytical method for trace metals which can compete with stripping analysis on t h e basis of sensitivity per dollar investment. What other technique can determine part-per-billion levels using a ball of wax, a carbon rod, and a couple of integrated circuits? At least 300 papers have appeared over the past two years dealing with ASV and many laboratories consider the technique so routine that they have nothing to publish. One company performs well over 100,000 analyses per year on trace metals in food, water, pharmaceuticals, etc. and has an excellent reputation for precision and accuracy in a national pediatric (100 pl) blood lead screening program(l5E). While the track record for ASV is good, there is much room for improvement. Some controversy exists over which electrode and which excitation waveform afford superior results. In general, the best resolution and highest sensitivity are attained with a small amplitude excitation (sinusoidal or pulse) a t a mercury film electrode. Often the sensitivity is such that linear scans at mercury films or pulse excitation a t the hanging mercury drop are more than sufficiently sensitive. As in most trace methods, sample preparation is often more troublesome than the actual measurement and usually accounts for most of the error. The fundamentals of stripping voltammetry and commercially available instruments have been reviewed (8E) as has t h e use of different types of carbon electrodes ( 2 E ) . One persistent problem is intermetallic compound formation between deposited metals. This subject has also been reviewed recently (16E, 27E). Relatively few workers use unamalgamated carbon or metal electrodes (2E, 18E), presumably because surface conditions are difficult to reproduce. Nevertheless, stripping analysis of mercury itself has attracted quite a bit of attention lately and in this case a clean carbon surface is the electrode of choice ( I E , I3E, 24E, 26E). The preconcentration (deposition) step in stripping voltammetry is usually facilitated by stirring the sample solution. The overall precision and sensitivity has a lot to do with the effectiveness and reproducibility of the hydrodynamics. Clem et al. have developed a n improved stirrer and describe its use with a digital potentiostat and a cell which can be deoxygenated in short order ( 3 E ) . They also 18R
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discuss the merits of various electrodes as substrates for the mercury film which is often deposited concurrently with the metals of interest. There are problems with the reproducibility and lifetime of wax impregnated electrodes, presumably due t o crystallization of the interstitial wax and adsorption of trace surfactants. The “film” is actually a collection of little Hg globules (miniature hanging mercury drops) adhering to exposed graphite points. This is also true for glassy carbon, even though it has a much lower surface porosity. Stulikova has illustrated this very nicely by means of photomicrographs (25E).It is apparent t h a t t h e surface sites have a broad range of activities. Relatively high overvoltages are required to ensure that they are all covered with mercury. Probably if we knew more about the structure of these things, a truly reproducible and long lasting surface could be achieved. The use of synthetic polymers rather than waxes might be a place to start looking (33B, 3 E ) . One way to attack the hydrodynamic problem is to use a flow through cell (17E,21E). Another possibility is to rotate t h e electrode in the manner of Stulikova (25E, 26E) and Allen and Johnson ( I E , 12E). The latter authors have come u p with the neat idea of using a ring disk electrode. The preconcentration and stripping are carried out a t the disk. As the disk potential is scanned positively during the stripping step, the metal ions are released and are detected a t the ring which is maintained at a fixed potential. While collection efficiency is never perfect, the sensitivity is still enhanced because background currents at the fixed-potential ring are relatively small and constant. This promising idea has been used in metal deposition and dissolution studies, but application to analysis has previously been ignored. Determinations of mercury and silver have been used as examples. Yet another way to increase the sensitivity is t o work in smaller solution volumes without any mechanical stirring. A high percentage of the available metal ions can be preconcentrated in a shorter period of time. This idea has been advanced by Stulik and coworkers using hanging drops ( I I E ) and glassy carbon electrodes ( 2 4 E ) . They’ve been able to detect as little as 1 pg Pb2+ in 5 pl using 2nd harmonic ac voltammetry (24E). Stripping via small amplitude pulse voltammetry has also attracted quite a bit of attention (5E-7E, IOE, 22E), particularly since a good pulse voltammeter is finally available commercially in the U S . (22E). The sensitivity is extraordinary and, once truly reliable electrodes are available, the technique should enjoy a large following. Fiddling with mercury drops, leaky reference electrodes, and deposited films has never been too keen with the analytical service labs, where human engineering can be just as important as sensitivity. Nevertheless, there is no shortage of applications for stripping analysis. A few select examples include T1 in urine ( 1 4 E ) ;P b , Cd, Cu, T1, and Bi in blood ( 2 3 0 ; P b in blood ( 7 E ) ; Hg, Zn, Cd, Pb, Cu, Co, and Ni in water (13E); P b and Cd in water ( 3 E ) ;P b in blood and urine (20E);Au in drugs and serum ( 1 9 E ) ;Zn, Cd, Pb, and Cu in airborne particulate matter ( 4 E ) ;and P b in evaporated milk ( 9 E ) . The latter was a collaborative study with atomic absorption. It would be nice t o have more face-toface encounters of these two techniques so that the advantages of both could be looked a t more objectively. Workers (and manufacturers) in both areas are prone t o an evangelical spirit which bewilders those of us caught in the middle.
SPECTROSCOPIC TECHNIQUES The use of optical spectroscopy to elucidate electrode processes is definitely in vogue. An enormous body of literature has appeared in the last decade and is now ripe for review. Volume 9 of “Advances in Electrochemistry and Electrochemical Engineering” deals exclusively with optical techniques in electrochemistry (SF). The emphasis here is very physical with virtually no mention of solution chemistry. Reflection spectroscopy, ellipsometry, interferometry, and microscopy are examined in great detail. The large number of individual papers dealing with these subjects will not be cited here. Additional reviews in this genre have been prepared by Conway (4F), Plieth ( 2 5 f l , and McIntyre (20F). Strojek’s brief survey of optically
transparent electrodes emphasizes their preparation and use in studies of coupled chemical reactions ( 2 8 0 . Wells has described a computer-controlled rapid-scanning spectrophotometer for visible spectroelectrochemistry ( 3 0 0 . Laser and Ariel modified a commercial instrument for modulated internal reflection spectrometry in the infrared region at a rotating electrode ( 1 4 0 . They also advocate separating stationary electrodes and reflecting elements by using a gold minigrid and a germanium plate (13I.3. I n a fascinating application of infrared IRS, M a t t son and Smith investigated the potential dependent adsorption of porcine fibrinogen onto a germanium surface and discuss its relevance to thrombogenesis ( I 8 F ) .Matsumot0 and Sato described an extremely simple semi-infinite specular reflectance cell and demonstrated its use in measuring the visible spectra of the 9,lO-diphenylanthracene radical ions in acetonitrile (17F).Blount looked a t anodic pyridination of the same molecule by chronoabsorptometry and evaluated the mechanisitic details with the aid of digital simulation ( 2 0 . He concludes t h a t the half-regeneration mechanism fits better than other possibilities (the so-called E C E and ECC mechanisms). The controversy in distinguishing between these radical ion decay mechanisms continues under a full head of steam. A t least, spectroelectrochemistry affords enough selectivit y to distinguish the time course of a particular species. This would seem to be a great advantage over conventional chronoamperometric approaches. Other interesting applications papers include studies of the kinetics of methylene blue reduction (26F), the electrocatalytic oxidation of copper(I1)-ethylenediamine (19F), the three step reduction of tris(2,2’-bipyridine)chromium(II1) (19F), and the classical titanium(II1)hydroxylamine reaction ( 2 4 0 . This last paper is most unusual in that kinetic results were obtained from polarography and stopped-flow as well as current step chronoabsorptometry a t Murray Minigrids. A comparative study using different techniques is always a smart idea, but particularly when rate constants are involved. The theoretical response for potential step chronoabsorptometry was examined in detail by Li and Wilson for a variety of coupled chemical reactions ( 1 5 0 . Normalized working curves were developed and diagnostic criteria proposed. The newest thing in spectroelectrochemistry is Raman spectroscopy. Using a laser striking a n amalgamated platinum surface a t 45”, it was possible to observe Raman scattering for Hg2C12, HgzBr2, and HgO generated electrochemically ( 7 0 . Total reflection Raman spectroscopy has been proposed for use with transparent film electrodes ( 8 F ) , and there is reason to suspect a lot will be done with this idea. The application of electrochemistry to atomic spectroscopy is not exactly burgeoning, but the way is clear. Electrolysis is used to prepare the sample, and spectroscopy provides the final quantitation. The utility of electrochemical preconcentration has been reviewed ( I 1 F ) and recently Fairless and Bard demonstrated this idea by determining ppb copper in sea water via flameless atomic absorption ( 6 0 . Davis and his group apparently wanted to improve matters by electrodeposition onto a tungsten alloy loop, but surprisingly found t h a t the metals deposited without electrochemical assistance (21F). Jones and France have taken a different tack ( 9 0 . They advocate electrolytic dissolution as a means t o preparing metallic samples for AA and used it to measure magnesium in cast iron. The advent of atomic absorption virtually decimated polarography as far as trace analysis of metals is concerned. It’s encouraging to find grounds for renewing friendships among old adversaries. Vassos et al. used electrodeposition on pyrolytic graphite followed by X-ray fluorescence to determine six elements in water ( 2 9 0 . Brinen and McClure deposited the metals on mercury coated platinum and then determined them by ESCA (3F). Barnes et al. separated lead from a variety of sample matrices by electrodeposition and followed this up with isotope ratio mass spectrometry ( I F ) . Modern high vacuum methods of surface analysis are, of course, very useful for examining changes occurring on electrodes, even if they must be removed from solution. A few relevant investigations include the ESCA studies of Winograd’s group on lead and platinum surfaces (IOF);
the ESCA of some complex porous Teflon/active carbon/ Fe-phthalocyanine battery electrodes ( 1 2 0 ; and the Auger electron spectroscopy and scanning electron microscopy of electrodeposited nickel ( 1 6 0 . There are many other similar publications which won’t be cited here, since they have little bearing on electroanalysis. A few years ago Bruckenstein and Gadde described a porous Teflon based electrode, supported by a glass frit, which could be interfaced directly to a mass spectrometer. Volatile electrolysis products are swept into the MS inlet and analyzed. The viability of this concept has now been clearly evinced by Petek et al. in two recent papers. In one case, they studied methanol elimination accompanying anodic oxidation of methoxyphenols in aqueous acid ( 2 3 0 . I n the other case, the electrooxidation of hydrazine a t platinum was investigated using isotopic labeling tricks (220.
CHROMATOGRAPHIC TECHNIQUES The applicability of electrode reactions to chromatographic detection has been realized for a long time but, until recently, there has not been much interest in this idea. The preeminence of gas chromatography and the poor sensitivity of electrochemical measurements in cells of large dead volume discouraged many workers. In addition, since most chromatographers prefer a universal detector, they have not been enthusiastic about the potential of electrochemistry. Liquid chromatography and electrochemistry have enjoyed a resurgence in the past few years, Both have derived great benefit from efforts to increase the mass transport rate of molecules involved in the controlling heterogeneous phenomena. It appears now that combinations of high performance liquid chromatography with modern electrochemical techniques ( L e . , LCEC) can be the basis for highly sensitive chemically selective analyzers. Hydrodynamic chronoamperometry carried out either coulometrically or nonexhaustively has been the electrochemical method of choice. Pulse or sinusoidal excitations may prove to be advantageous in some situations, but they always waste charge on the double layer capacitance. In all cases, it is useful to optimize the ratio of electrode area to cell volume in order to improve efficiency without increasing background current unnecessarily. Thin-layer electrochemistry provides one practical solution ( I 3 G ) , and we have recently been able to detect biogenic amines and a variety of pharxaceuticals in the picogram range using this approach. Dead volumes
