Anal. Chem. 1980, 52. 138R-151 R (18E) Nishihara, C.; Matsuda, H. J . Electroanal. Chem. Interfacial Electrochem. 1977, 8 5 , 17-26. (19E) Saveant, J. M.; Bink, S. K . J . Electroanal. Chem. Interfacial Electrochem. 1978, 9 7 , 35-45. (20E) Speiser, B.; Rieker, A. J . Electroanal. Chem. Interfacial Electrochem. 1979, 702, 1-20. (21E) Uchiyama, S.; Muto, G.; Nozaki, .K. J . Electroanal. Chem. Interfacial Electrochem. 1978, 97, 301-308.
(14F) Engstrom, R. C.; Biaedel, W. J. Chem., Biomed.. Environ. Instrum. 1979, 9 , 61-69. (15F) Fleming, A. N.; Harrison, J. A. J. Electroanal. Chem. 1978, 87, 339-345. (16F) Garreau. D.; Saveant, J. M.; Tessier, D.J . Electroanal. Chem. Interfacial Electrochem. 1979, 703, 321-333. (17F) Holloway, J. D. L.; Senfileber, F. C.; Geiger, W. E., Jr. Anal. Chem. 1978, 5 0 , 1010-1013. (18F) Ichise, M.: Yamagishi, H.: Kojima, T. J . Electroanal. Chem. Interfacial Electrochem. 1978, 94, 187-199. (19F) O’Haver, T. C. Anal. Chem. 1978, 5 0 , 676-679. (20F) Paul, R . L. Electrochim. Acta 1978, 2 3 , 991-994. (21F) Sawatari. K.; Imanishi, Y.; Umezawa. Y.; Fujiwara, S. Bunseki Kagaku 1978, 2 7 , 180-183. C . A . 1978, 8 9 , 99008f. (22F) Schwall, R. J.; Bond, A. M.; Smkh, D.E. J . Electroanal. Chem. Interfacial Electrochem. 1977, 8 5 , 217-229. (23F) Seelig, P. F.; Blount, H. N.; Anal. Chem. 1979, 51, 327-337. (24F) Shia, G. A.; Meites, L. J . Electroanal. Chem. Interfacial Electrochem. 1978, 8 7 , 369-380. (25F) Sierra Alcazer, H. 8.: Fleming, A. N.; Harrison, J. A. Surf. Techno/. 1977,
INSTRUMENTATION
( I F ) Blutstein. H.; Bond, A. M.; Norris, A. J . Electroanal. Chem. Interfacial Electrochem. 1978, 8 9 , 75-81. (2F) BOS, M. Anal. Chim. Acta 1978, 703, 367-378. (3F) Bond, A . M.; Grabaric, B. S. Anal. Chem. 1979, 51, 126-128. (4F) Bond, A. M.; Grabaric, B. S. Anal. Chem. 1979, 51, 337-341. (5F) Bond, A. M.; Schwall, R. J.: Smith, D. E. J . Electroanal. Chem. Interfacial Electrochem. 1977, 8 5 , 231-247. (6F) Brown, S. D.; Kowalski, B. R. Anal. Chim. Acta 1979, 707, 13-27. (7F) Caja, J.; Czerwinski, A.; Mark, H. B., Jr. Anal. Chem. 1979, 57, 1328-1329. (8F) Carlsson, C.; Lindstrom, M.; Pulkkis, G. Chem., Biomed., Environ. Instrum. 1979, 9 ,21 1-218. (9F) Chow, L. H.; Ewing, G. W. Anal. Chem. 1979, 57, 322-327. (1OF) Danielson, J. D. S.; Brown, S. D.; Appellof, C. J.; Kowalski, B. R. Chem., Biomed., Environ. Instrum. 1979, 9 , 29-47. (11F) DeLevie, R.; Sarangapani, S.; Czekaj, P. Anal. Chem. 1978, 50, 110-1 15. (12F) Drake, K . F.; VanDuyne, R. P.; Bond, A. M. J . Electroanal. Chem. Interfacial Electrochem. 1978, 8 9 , 231-246. (13F) Edmonds, T. E. Anal. Chim. Acta 1979, 708, 155-160.
6 - , 61-67 -
(26F) Tanaka. N.; Tarnada, A. Kagaku, Zokan(Kyot0) 1978, 78, 47-62. C . A . 1979. 9 0 . 158895. (27F) Tryk, D. A.; Park, S . - M . Anal. Chem. 1979, 57, 585-586. (28F) Van Bennekom, W. P. Anal. Chim. Acta 1978, 707, 283-307. (29F) Woodward, W. S.; Rocklin, R . D., Murray, R . W. Chem., Biomed., Environ. Instrum. 1979, 9 , 95-105. (30F) Yamada, A.; Kato, Y.; Doi, I.; Saito, K.; Tanaka, Y.: Tanaka, N. Sci. Rep. Tohoku Univ., Ser. 11977, 50, 41-73. C . A . 1978, 8 8 , 135901. (31F) Yeakel, B. D.; Burrows, K. C.; Hughes, M. C. Chem., Biomed., Environ. Instrum. 1979, 9 , 239-259.
Analytical Electrochemistry: Methodology and Applications of Dynamic Techniques William R. Heineman Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
Peter T. Kissinger” Department of Chemistry, Purdue University, West Lafayetfe, Indiana 47907
As for previous editions of this review, we have endeavored to report on developments involving the more experimental aspects of finite current analytical electrochemistry. Representative references have been selected from a vast number of publications which for the most part appeared in print from January 1978 through December 1979. Our intent here has been t o convey what we judge to be the most novel developments which promise to have a long term impact on the field. Any selection of this sort is a matter of personal preference and due to limitations of time and space many excellent papers have not been cited. For those wishing exhaustive coverage of this area we strongly recommend that they subscribe to the Chemical Abstracts section on Analytical Electrochemistry which is available through the American Chemical Society “CA Selects” program.
ming suggests that there are three major advantages of semiconductors when compared to metal electrodes: “[l]In a semiconductor well-defined energy bands exist, their energy separation depending on the type of semiconductor; [2] t h e carrier density can be varied by doping over many orders of magnitude; [3] the occupation of energy levels by electrons or holes can be increased by optical excitation.” While these features have yet to realize important analytical results, the possibilities in this field are substantial and should be given more attention. Semiconductor electrodes are proving to be useful for the direct conversion of light into electricity and also for the photocatalytic generation of fuels and other useful chemicals. These areas have been recently reviewed by Wrighton (2A)and by Bard (3A). In order to briefly cover this very extensive area of research, we have selected some of the publications from several of the more active research groups. These citations will serve as an excellent entry to other recent work in this field. The Bard group in Texas has been extremely enthusiastic about semiconductor electrodes having published a t least 22 papers on this subject in the last few years. They have reported on electroluminescence at single crystals of n-type T i 0 2 ( 4 A ) ;aqueous photoelectrochemical cells with mixed polycrystalline n-type CdS-CdSe ( 5 A ) ;photoelectrochemistry of n- and p-GaAs ( 6 A ) and n- and p-InP in acetonitrile ( 7 A ) ; n-GaAs photovoltaic cells in acetonitrile ( 8 A ) ;electron spectroscopy of S/Se substitution in CdSe and CdS single crystals (9A); photogeneration of solvated electrons from p-GaAs in liquid ammonia (10A);characterization of n-Fe203in acetonitrile ( 1I A ) , and (finally) electrochromism and photoelectrochemistry of W 0 3 films prepared in various ways (12A).
ELECTRODE MATERIALS Semiconductors a n d O t h e r U n u s u a l Electrode Materials. Analytical chemists first became interested in semiconductor electrodes in the middle 1960’s because of their apparent value as optically transparent electrodes. At t h a t time the fact that the materials were semiconductors was of no interest per se and was normally considered to be a disadvantage. In the early 1970s physical electrochemical studies of the interface between semiconductors and electrolyte solutions gained tremendous momentum as it became clear that semiconductor electrodes had great potential for photoelectrochemistry and as substrates for covalently modified electrodes (see next section). In a n excellent recent review Memming discusses the known characteristics of charge transfer processes a t semiconductor electrodes ( 1 A ) . Mem138 R
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Wllllam R. Heineman is associate professor of chemistry and chairman of the analytical division at the University of Cincinnati I n 1964 he received his B S degree from Texas Tech University and in 1968 was awarded a Ph D from the University of North Carolina at Chapel Hill Dr Heineman was a research chemist at the Hercules Research Center and a research associate with Professor T Kuwana at Case Western Reserve University and The Ohio State University before joining the faculty at the University of Cincinnati in 1972 His current research interests include 1 bioelectrochemrstry, electroanalysis with thinlayer cells, and the development and application of thin-layer spectroelectrochemical techniques
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Peter T. Klsslnger received his B.S. degree from Union College, Schenectady, N.Y., in 1966, and the Ph.D. degree from the University of North Carolina at Chapel Hill in 1970 where he worked with Professor Charles N. Reilley. Before joining the faculty at Purdue, Kissinger taught at Michigan State University following postdoctoral study with Professor Ralph A d a m at the University of Kansas. Dr. Kissinger has research interests in the metabolism of aromatic compounds, electrochemistry in microliter volumes, and synergistic combinations of chromatography and spectroscopy with electroanalytical techniques Outside of the academic sphere, Dr Kissinger is president of Bioanalyhal Systems, Inc., a manufacturing and consulting firm specializing in chromatographic and electrochemical instrumentatign.
All of these papers present a great deal of voltammetric information which is particularly useful for those wishing a quick grasp of how semiconductor electrodes behave. In closely related studies the Bard group has evaluated the photocatalytic activity of TiO, powder with respect to the Kolbetype reaction of carboxylic acids (13A),the synthesis of amino acids from ammonia and methane ( 1 4 A ) ,and the oxidation of aqueous hydroquinone (15A). In the latter case the particles were coated with phthalocyanine dye to enhance the photocatalytic effect in the visible region of the spectrum. Earlier reports by Tachikawa and Faulkner (16A) and Fan and Faulkner (17A-19A) thoroughly evaluated the electrochemical and solid-state properties of phthalocyanine thin films deposited on glass overlaid by a gold film contact layer. It is too bad the journals do not favor color photography because these films are very beautiful! The high price of gold is forcing Faulkner’s lab to work with thinner and thinner gold films, but a t last report progress continues at a torrid pace. Murray’s group a t Chapel Hill pioneered in the covalent attachment of organic groups to metal and semiconductor surfaces, and it is not surprising to find them exploring new candidates for covalent attachment such as RuO, electrodes which have recently beer, found to be well behaved in aprotic media ( 2 0 A ) . Cyclic voltammetric results are reported for metal bipyridyls, methyl viologen, ferrocene, tetrathiafulvalene, and several nitroaromatics. Schneemeyer and Wrighton used cyclic voltammetry of similar well-behaved compounds to evaluate the flat-band potential for n-type MoS, single crystals ( 2 1 A ) . This material shows considerable promise as a photocatalyst. Similar studies were reported by Yeh and Hackerman for n-type CdS, Gap, GaAs,and p-type Ge electrodes in DMF ( 2 2 A ) . There is considerable interest in nonmetallic materials which have the electrical properties of metals. It is not surprising that these new materials would be attractive to explore from an electrochemical point of view. Mark and co-workers have studied polymeric sulfur nitride, (SN),, electrodes and considered possible chemical modificatons thereof (23A-25A). Jaeger and Bard evaluated pellets of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) as an electrode material in aqueous solutions ( 2 6 A ) . Covalently Modified Electrodes. As we begin the 1980’s, i t is fair to say that virtually all types of electrode materials (metals, semiconductors, graphite) have now been modified
by covalent attachment of both redox active and redox inert groups. While some evidence exists that covalent modification will prove to be useful for a variety of purposes, at this point the preliminary data is rather sparse. Nevertheless, we are still low on the “learning curve” and the great effort being made in this area is almost certain to bring a number of significant advances. Murray’s group at the University of North Carolina started modifying surfaces years ago and they are still a t it. They have recently extended the approach developed by Mazur (see our previous review) to attach reagents to oxide-free glassy carbon surfaces ( 2 7 A ) . Reactive surfaces were created by mechanical abrasion in an inert atmosphere as well as by argon plasma etching. Among those materials attached were vinylferrocene and vinylpyridine. The original methodology for attachment to carbon was based on the reactivity of the oxygenated surface. For example, surface carboxylates can be converted to the acid chloride (via SOC1,) and the latter reacted with amines to form an amide linkage. This approach has been used to bond tetra(aminopheny1)porphyrin and then study the influence of various transition metals subsequently placed in the macrocycle (28A, 29A). The acid chloride route can also be successful with metal oxide surfaces and has been used to attach aminophenylferrocene to RuO,, PtO, and SnO, surfaces (30A). Silanization is also a proven route to covalent modification of metal oxides, and in this case the Murray group has used alkylaminesilanes as the linking bridge for electrodes modified with ferrocenes ( 3 1 A ) ,dansyl chloride (32A),and tetrathiofulvalene (33A). They have used a variety of electrochemical and spectroscopic techniques to characterize these surfaces as well as electrodes modified only with the alkylamine ( 3 4 A ) . Ruthenium was found to be strongly chemisorbed on both native and alkylamine modified S n 0 2 (35A). Both the Ru(II)/Ru(III) and Ru(III)/Ru(IV) couples could tentatively be studied for the bound ruthenium. An important factor for the design of electrocatalysts based on surface-bound redox centers is whether or not the attached couple exhibits an E” identical with that for the corresponding couple in homogeneous solution. A number of examples have been studied and the correlation is for the most part excellent (36A). Wrighton‘s group a t M I T has been covalently attaching ferrocene to every surface they can get their hands on. They have used three different reagents to silanize surface hydroxyl groups: trichlorosilylferrocene, (1,l’-ferrocenediy1)dichlorosilane, and 1,l’-bis(triethoxysily1)ferrocene.In general, the electrodes have been derivatized to a degree that the coverage is much greater than a monolayer and very likely involves some surface polymerization of the silane monomers. Ferrocene is a well-behaved electron transfer center which is also photoelectrochemically active a t semiconductor surfaces. It has been attached to platinum and gold (37A) and the surfaces characterized by X-ray photoelectron spectroscopy (XPS) in a joint effort of the Wrighton and Murray groups ( 3 8 A ) . Bonding ferrocene centers to semiconductors was carried out with t h e motive of protecting their surfaces from “photoinduced corrosion” and thereby making them better candidates for energy conversion devices. Silicon (n-type) (39A-4IA), germanium (n-type) (42A),and gallium arsenide (n-type) (43A) have been used as substrates. As an alternate to ferrocene H a m and Armstrong bonded the photosensitizing dye erythrosin to tin oxide photoelectrodes (44A). While Anson’s group has concentrated on electrode modification by adsorption and polymer coating, they have also taken the covalent route to introduce amine functional groups on graphite as a means of attaching metal complexes (45A) such as the EDTA complex of ruthenium(II1) (46A). The ligand substitution kinetics of attached and free Ru(I1) and Ru(II1) EDTA complexes has been compared (47A). Oyama et al. later explored the bonding of aminoferrocenes directly to highly oxidized pyrolytic graphite electrodes ( 4 8 A ) . Kuwana and co-workers have continued to explore the use of cyanuric chloride (trichloro-s-triazine) as a general linking agent for attachment of redox centers to graphite electrodes. Hydroxymethylferrocene (everyone likes ferrocene) was coupled to pyrolytic graphite (49A);methylaminopropylviologen was bonded to glassy carbon (50A);and the old electrochemists favorite, o-tolidine, was attached to pyrolytic graphite (51A). Using another linking scheme, graphite electrodes modified by o-quinones were found to catalyze the oxidation of NADH ANALYTICAL CHEMISTRY, VOL. 52, NO.
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(52A). Unfortunately the electrode life was short in the presence of NADH, so further work is needed before its great potential as an analytical tool for NADH measurements can be realized. Evans and Kuwana explored the use of radiofrequency plasmas of oxygen and ammonia to introduce oxygen- or nitrogen-containing functional groups on pyrolytic graphite (edge and basal planes) and glassy carbon ( 5 3 A ) . Electrochemical and X P S experiments were used to evaluate the modified surfaces. There is now general agreement among several groups that rf plasmas provide a good method of surface preparation prior to chemically modifying electrodes. Carbon paste electrodes were among the earliest chemically modified electrodes due to the convenience of adding things to the hydrophobic oil in which the graphite particles are dispersed. Recently Yao and Musha used n-octaldehyde in the paste as a linking agent to bond NAD+ ( 5 4 A ) . An oxidoreductase enzyme method which used NAD+ as a cofactor to catalyze an oxidation (and reduce NAD+ to NADH) could therefore be followed electrochemically. For example, if the NAD’ electrode is incubated with L-lactic acid (the analyte) and its dehydro enase, bound NADH results. The latter can be measured voftammetrically while a t the same time regenerating the original NAD+ for use in a subsequent experiment. Very clever! In a totally different application of carbon paste, Cheek and Nelson illustrated how the graphite could be modified to preconcentrate metal ions prior to electrochemical analysis ( 5 5 A ) . Electrodes Modified by Adsorption or Polymer Coating. While covalently linking small molecules to specific sites on an electrode surface may well be the most elegant route to preparation of a CME, recent work suggests that there may be more practical approaches. Several groups have explored the concept of coating an electrode surface with a polymer film. Polymers with electroactive groups “built in” can be directly used for electrocatalysis, or a nonelectroactive polymer can be coated and later modified to meet a specific objective. Miller and Van De Mark appear to be among the first to report on this subject and the value of Auger and X-ray photoelectron spectroscopy to study the resulting surface (56A, 5 7 A ) . They demonstrated the potential dependent conductivity of platinum “dip coated“ with poly-p-nitrostyrene (58A). T h e polymer behaved as an insulator except when negatively charged by the addition of electrons to the nitrophenyl groups. Reduction of these groups was reversible as evidenced by the cyclic voltammetric behavior of the film. The presence of the polymer inhibited the electrochemistry of ferrocene, but when negatively charged enhanced the reduction of CC14 and catalyzed the reduction of oxygen (making the peak potential shift 200 mV more positive). Kerr and Miller showed that the same electrode was capable of catalyzing the reduction to trans-stilbene of rneso-1,2-dibromo-1,2-diphenylethane (59A). Merz and Bard used polyvinylferrocene to coat a platinum electrode ( 6 0 A ) . The stable electrode apparently consisted of the equivalent of at least 20 monolayers of ferrocene. Itaya and Bard have combined the polymer coating technique with covalent modifications by covalently attaching poly( methacryl chloride) to SnOz surfaces using silanization and then attaching hydroxymethylferrocene to the polymer (61A). The resulting electrode was considerably more stable than an analogous electrode prepared by dip-coating the same ferrocene modified polymer. Moving from Texas to the West Coast, we find that Fred Anson’s group in Pasadena has also been working hard to find a convenient route t o CME’s. Using pyridine-pentaammineruthenium(I1)complexes, Koval and Anson compared the properties of pyrolytic graphite electrodes modified with the complex by both covalent bonding and strong adsorption ( 6 2 A ) . T h e attached Ru(III)/Ru(II) was very well behaved in both cases. Adsorption provided a somewhat greater surface coverage while the covalently attached complex was more persistent. Parkinson and Anson studied the “metalactivated” adsorption on mercury of six chelating ligands containing a thioether group (63A). Pb(I1) and several other dl0 metal cations were investigated to demonstrate that neither the ligand nor the metal ions are strongly adsorbed individually. I t is suggested that the adsorbed complexes are polymeric. Oyama and Anson have dip-coated polymeric ligands (e.g., polyvinylpyridine or polyacrylonitrile) onto pyrolytic graphite and used these surfaces to coordinate ruthenium 140R
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complexes present in solution ( 6 4 A ) . The same authors later explored this subject in greater detail, finding that much greater than monolayer coverage is easily achieved and that the surfaces have excellent stability (65A). From an analytical viewpoint, it is interesting to note that bound ligands can concentrate metals on the surface from highly dilute solutions. Ultimately this concept might well provide an alternate approach to metal deposition as a preconcentration technique in analytical electrochemistry. Simultaneous coordination of the complexes of different transition metals to a single polymeric phase was investigated as was the use of surfaces simultaneously modified with two different polymeric ligands. In what would appear to be an extremely fruitful collaboration, the Anson and Collman (Stanford) groups have demonstrated that dicobalt porphyrin dimers adsorbed on graphite can catalyze the electroreduction of oxygen to water without passing through hydrogen peroxide as an intermediate ( 6 6 A ) . This result has important ramifications for fuel cells and air batteries and furthermore is an excellent example of the utility of ring-disk (platinum-graphite) electrodes for mechanism studies. Daum and Murray examined solvent effects on platinum electrodes coated with thin films of polyvinylferrocene deposited from radio-frequency plasmas (67A). Dubois and co-workers have electrochemically generated polyphenylene oxide (PPO) (68A) and polyacrolein films on metals (Pt, Au, Fe, Ni, Cu, and Al) and examined the properties of these modified electrodes by elemental analysis, IR, and ESCA ( 6 9 A ) . The progress of film formation was followed by polaromicrotribometry (PMT). P M T was also later used to study the formation of polyacetonitrile films anodically formed a t Pt in CH3CN-MC104 or CH3CN-LiBF4 electrolytes (70A). Miyasaka et al. have studied the photoelectrochemistry of SnO? electrodes coated with a monolayer of chlorophyll (71A), and Umeza and Yamamura (also from the University of Tokyo) reported on the photoactivity of platinum electrodes derivatized by strong adsorption of porphyrins (72A). Catalytic electrodes for the amperometric determination of hydrogen peroxide (73A) and nitrite (74A) have been reported. The catalytic reduction of oxygen has been demonstrated a t gold electrodes modified by underpotential depositon of transition metal adatoms ( 7 5 A ) . The photoelectrochemistry of glassy carbon modified by a film of a binuclear ruthenium complex was reported by Pool and Buck ( 7 6 A ) . Hern and Strohl demonstrated that compounds with ion-exchange or chelation properties could be adsorbed on graphite and that the solution pH and electrode potential could be used to control the affinity of these materials for transition metal ions ( 7 7 A ) . Packed bed electrodes were demonstrated to have analytical utility. Kaufman and Engler of IBM carried out spectroelectrochemical experiments on polymer films deposited by evaporation on optically transparent platinum thin-film electrodes (78A). Cheng et al. examined the anodic oxidation of the fused ring catecholamine apomorphine and discovered that a product of the oxidation irreversibly adsorbs on a carbon-paste electrode ( 7 9 A ) . Heterogeneous electron transfer between electrodes and redox proteins has generally been too slow to be useful. This subject was discussed in our previous review. I t has now been shown by several groups that the kinetics can be improved by proper choice of the electrode surface chemistry. For example, Eddowes and Hill demonstrated a rapid direct electron transfer between a gold electrode and cytochrome c when the gold was modified by adsorption of 4,4’-bipyridyl and 1,2-bis(4-pyridyl)ethylene(80A). Eddowes et al. then pointed out some analogies between the reaction of cytochrome c with the electrode and with cytochrome oxidase ( 8 I A ) . In an earlier communication Yeh and Kuwana showed that cytochrome c exhibited a reversible electron transfer a t an indium oxide electrode ( 8 2 A ) . M e r c u r y Drops. While various forms of hanging (HMDE) and dropping (DME) mercury electrodes continue to be advantageous for a great many applications, the art has matured to the point that there are very few new methodological improvements being published. Over the years several groups have addressed the problem of utilizing the D M E in media corrosive to glass such as hydrofluoric acid. Capillaries constructed from Kel-F or Teflon were developed for this purpose a number of years ago. Now Menard and LeBlond-Routhier advocate a simple design based on coating a conventional glass
METHODOLOGY AND APPLICATIONS OF DYNAMIC TECHNIQUES
capillary with molten polyethylene through which the capillar) orifice is extended first by nitrogen pressure and then (after the polymer cools) by means of a high speed (17000 rpm!) drill (83A). Another equally obscure area of application for the DME involves the use of metal amalgams rather than pure mercury. Electrodes of this type can have significant advantages as has recently been noted by Dieker et al. with respect to the dropping lead amalgam electrode (84A). They demonstrate application of this electrode to amperometric titration of metal ions in the presence of high concentrations of various halides. The influence of halide ions on the endpoint detection is greatly reduced when compared to the use of a traditional DME for this purpose. There is frequently some question as to the definition of the electrode surface area used for electroanalytical experiments. This is certainly a problem with electrodes of poorly defined surface roughness because the “effective electrode area” will vary with the time scale of the experiment. Problems arise even with smooth mercury drops, and Cummings and Elving have given some attention to the use of trioxalatoiron(II1) to calibrate HMDE areas and Cd(I1) for the DME where amalgam formation is less problematical (85A). T h e results are evaluated with respect to several theoretical expectations. The HMDE has often frustrated electrochemists whether home grown or commerical devices are used. Typically, the “drops won’t hang” of if they do, they “fall off‘ just as the experiments are giving birth to exciting results. Bonelli et al. suggest a straightforward modification of the most popular capillary based HMDE which will undoubtedly help in many cases (86A). Perhaps the whole concept should be redesigned and Princeton Applied Research has done just that (87A). Their “static mercury electrode” is both an HMDE and DME at the same time. This device not only advances polarography from a fundamental point of view (because the drop is static when the current is measured, minimizing capacitance effects), but it also avoids much of the mess associated with the traditional glass reservoir bulbs and Tygon tubing filled with mercury. During the period covered by this review several papers have espoused the use of the DME in flowing streams. This topic will be briefly covered under Hydrodynamic Techniques. I n Vivo Electrodes. In spite of the obvious problems associated with poor selectivity and adsorption, electrodes remain the most promising methodology for continuous in vivo monitoring of important clinical variables (e.g., oxygen and glucose). The problems and promises have recently been reviewed by Koryta and co-workers (88A). Adams and his group at the University of Kansas pioneered in the use of small implanted graphite electrodes to follow changes in the concentrations of metabolites of neurochemical interest in living animals. The compounds of primary interest are derived either from the amino acid tyrosine (Le., the catecholamines and their metabolites) or the amino acid tryptophan (i.e., serotonin and 5-hydroxyindoleacetic acid). The goals of this research have been briefly reviewed by Adams (89A). Cheng et al. described the chronoamperometric behavior of micro graphite electrodes in brain tissue from a mass transport point of view (90Aj. A simple restricted diffusion model was assumed based on the existence of a layer of extracellular fluid between the brain tissue and the electrode tip. The Adams group has used their electrodes for following neurotransmitter release in response to chemical and electrical stimuli (91A93A). Curzon et al. used similar implanted carbon paste electrodes to follow the “effect of stressful manipulations” (94A). A “tail pinch” on Sprague-Dawley rats evoked a response at an electrode implanted in the caudate nucleus which was presumably related to dopamine release. When electrochemists resort to pinching rats, it may be time to seek another profession! Ross Lane and co-workers have also been playing an important role in the development of in vivo voltammetric techniques. Particularly noteworthy is their work with differential pulse voltammetry (95A) and semidifferential electroanalysis (%A), the latter apparently being preferred due to its low cost. While most recent in vivo voltammetry experiments have been carried out using carbon paste electrodes formed in Teflon or glass capillaries, there are difficulties in preparing reproducible surfaces of very small dimensions. Perhaps “carbon fibers” would be better. Ponchon et al. have recently
studied 8-pm carbon fibers mounted in glass microcapillaries using both pulse voltammetry and pulse amperometry techniques (97A). Many electroanalytical chemists are unaware of the impact of in vivo voltammetric measurements in the neurochemical community. While the quality of the data is very crude compared to that obtained by most physical electrochemists working in well-defined chemical systems, it is among the best that is available for the living rat brain. With all of the interest generated by the pioneers in the 1970’s, we can expect a great deal of progress in this area in the 1980‘s.
HYDRODYNAMIC TECHNIQUES R o t a t i n g Disks. Rotating disk (RDE) and ring disk (RRDE) electrodes continue to play an extremely important role in fundamental studies of electrode processes due to their well-defined th-oretical basis. While hydrodynamic voltammetry a t such electrodes continues to derive analytical importance from its fundamental role in amperometric titrations, coulometry, and stripping voltammetry, it has had very little following as an analytical technique in its own right. Stationary solution voltammetry (which includes such popular techniques as cyclic voltammetry and differential pulse polarography) continues to dominate. From a traditional analytical point of view neither hydrodynamic nor stationary electrode voltammetry is particularly satisfactory. The techniques exhibit very poor molecular specificity and resolution when contrasted to most modern spectroscopy; they are very wasteful of sample; and they are frought with surface problems (adsorption, double layer charging, and electrode surface modification). Very significant advantages result from operating an electrode or electrodes at a fixed potential(s1 and bringing a concentration pulse of the sample to the electrode(s) via hydrodynamics. Most of the problems listed above can be largely eliminated. The recent success of various flowthrough hydrodynamic amperometry schemes, preceded in some cases by liquid chromatography, is such that it appears unlikely that traditional voltammetric measurements will ever catch up in terms of practical analytical utility. The value of hydrodynamic amperometry at a fixed-potential is exemplified by the ring disk electrode operated in the “ring shielding mode” whereby the disk potential is scanned while monitoring the ring current at a fixed potential. Bruckenstein and Gifford have recently advocated the use of this technique for micromolar voltammetric analysis of several metal ions (1B). While the principles are clear from this paper, in actual practice a flow cell would do the job much more effectively. The construction of disk electrodes has never been easy and papers continue to appear on this topic. Ritzler and Gross in Strasbourg describe an electrode of relatively simple construction which they demonstrated to perform well a t up to 25000 rpm (2Bj. The P-diketonates of Co(I1) and Co(II1) and ferro/ferricyanide were used as test cases. Rowley and Osteryoung reported on construction of an RRDE from irregular electrode materials such as end-plane sections of several pyrolytic graphite materials (3B). Heat-shrinkable polyolefin tubing was used to attach and seal the irregular shapes to a glass tube, but there is obviously a lot more to it! Gough and Leypoldt designed an RDE for membrane transport studies (4B). In their device both the RDE and the reference electrode are mounted in the same shaft, the end of which is covered with a semipermeable membrane. Pulsed-rotation voltammetry (switching the rotation of an RDE between two values and measuring a difference current) is a special case of the general hydrodynamic modulation techniques developed by Miller and Bruckenstein. Blaedel and Engstrom have evaluated the analytical potential of this technique using a glassy carbon RDE and ferricyanide as a test system (5B). The technique partially corrects nonconvective background currents and is particularly attractive for studies of heterogeneous kinetics. Good S / N was obtained for a lo-* M solution of ferricyanide in 0.1 M pH 7.5 phosphate buffer. C o n t i n u o u s Flow Systems. Exploration of the use of amperometric (and, in the limit, coulometric) detectors in flowing streams is one of the most active areas of analytical electrochemistry. This and the following two subsections will cover the most recent developments. We use the general term “continuous flow” to include classical air-segmented flow systems as well as nonsegmented systems. While there is nothing new about the latter, the catchy name “Flow Injection ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
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Analysis’’ (or FIA) provides an appropriate nomenclature for techniques in which a small volume sample is injected into a flowing stream containing a supporting medium with or without reactive reagents. The 1979 volume of AKALYTICAL CHEMISTRY marked the opening of the age of RVC (Reticulated Vitreous Carbon). RVC is a relatively impervious form of carbon (like the “glassy carbon” familiar to electrochemists) which is formed in a rigid three dimensional honeycomblike structure. While the matrix is available in several porosities, the material designated as being 100 pores per inch (“100 ppi”) would appear to be most suitable for electrochemical studies. RVC was apparently developed for nonelectrochemical purposes by Chemotronics International Inc. of Ann Arbor, Michigan. The advantages of this material for electrochemistry can be summarized as follows: T h e microscopic surface area is very low compared to conventional porous graphite (therefore surface adsorption and background currents are comparatively low); the matrix is rigid (therefore the permeability is good compared to a bed of loose particles of wide size distribution); RVC is extremely inexpensive (compared to “glassy carbon”); and it is quite easily machined and mounted in various cell designs. In our laboratory we often hit the stuff with a hammer (do not forget your safety goggles) and use the resulting pieces to pack coulometric flow cells for electrosynthetic purposes. Even a first year graduate student can develop the skill for that type of machinery! Strohl and Curran have evaluated intact RVC as an electrode (6R) and used it for several coulometric ( 7 B ) and FIA applications (8B).Blaedel and Wang carried out a general evaluation of the material ( 9 B ) ,explored its use as a substrate for a mercury-film flow through ASV electrode (IOR),and very recently advocated a rotating RVC disk electrode for ASV and detection of NADH ( I I B ) . The RVC material is extremely useful for electroanalytical and electrosynthetic work. It is so inexpensive and so easy to work with that you ought to order some before the sun sets. Pungor and co-workers, a pioneering group in flow electrolysis, discussed various aspects of FIA with amperometric detectors (12%). Chan and Fogg demonstrated the usefulness of amperometric FIA for determination of a phenolic analgesic, meptazinol, a t a glassy carbon electrode (13B). Up to 80 samples per hour could be accommodated, a feat which is out of reach for conventional voltammetry. Koile and Johnson addressed the problem of film formation and electrode passivation when phenolic compounds are oxidized at platinum (14B). A simple recipe for electrochemical removal of the film was devised; however, a better approach would be to use a carbon electrode where this problem is far less severe. One of the most impressive recent papers on flow-through amperometry was published by Pihlar, Kosta, and Hristovski (15B). These chemists from Yugoslavia developed an FIA method for cyanide ion using a silver wire electrode placed in the center of a stainless steel tube which served as the auxiliary electrode. A mobile phase of 0.1 M NaOH was used t o support the following anodic process: Ago + 2CN Ag(CN),- + e
-
Less than 1 ng samples of cyanide could be easily determined a t a rate of a t least 100 samples per hour with a linear range of over six orders of magnitude! This method is clearly superior to traditional potentiometric (ion-selective electrodes) and colorimetric methods and is very easily implemented with simple apparatus. The dropping mercury electrode was first used in flowing solutions about 30 years ago and yet very little of value has come out of this effort. The situation may be improving as evidenced by several new LCEC detectors and the use of FIA with the DME to determine serum proteins by pulse polarography (16B). Ariel’s group in Israel has also demonstrated the value of square-wave polarography for FIA determinations of various substances a t up to 300 samples per hour (17B). LCEC. During the period covered by this review there have been nearly one hundred papers published on the use of amperometric detectors to monitor compounds eluted from modern liquid chromatography columns (18B). Most of these reports deal with routine application of this technology to practical problem solving, where the technology itself was of secondary interest. From the liquid chromatographer’s point of view, LCEC has developed to the point where amperometric 142R
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detection is competing with fluorescence to become the second most important detector for trace organic determinations. Both LCEC and LCF, of course, remain far behind LCUV which is more convenient to use and is applicable to a wider variety of problems. While the advantages of electrochemical detectors are clear to many chromatographers, the advantages of chromatography have unfortunately not impressed very many electrochemists. Flow-through electrochemical cells coupled to the znjectmn system of an LC can provide so much information about organic electrochemical reaction mechanisms in so little time that much of the progress in organic electrochemistry from the beginning of time until the present day could be repeated in a few months. While the reviewer’s opinion (like the current price of platinum) is perhaps somewhat inflated, there can be little question that modern reverse-phase LC makes the job of the organic electrochemist much easier than it once was. In a t least one laboratory, electrochemistry is used with chromatography both before and after the column (“ECE”) to characterize the products of organic redox reactions (29R). A chromatographic system built into an electrochemical cell provides the electrochemical experiments with a degree of chemical resolution unlikely to ever be achieved by electrochemistry alone. In this way it is possible to carry out voltammetric studies on complex mixtures of natural products, crude synthetic reaction mixtures, or whatever is handy. Often a great deal of information can be gathered on only 10 pmol of total analyte! Electrochemical cells for LCEC are highly developed and all sorts of materials have been used as electrodes. T h e next major advances are likely to come in the following three areas: pre- and post-column reactions to enhance selectivity and extend application to nonelectroactive substances; the use of chemically modified electrodes to improve the detection of irreversible substances via electrocatalysis; and more work with inorganic substances. Now let us take a look a t just a few of the successes in the last two years. LCEC really got its start in neurochemistry and applications in this area continue to be legion. Representative publications include studies of the lateral distribution of norepinephrine in the human thalamus (ZOB),catecholamines in rat brain (21B),catecholamines in human urine (22R),catecholamines in blood plasma (23B),and tryptophan metabolites in tissue homogenates and biological fluids ( 2 4 B ) . Other applications of clinical and toxicological interest have been developed for measurement of diethylstilbestrol in animal tissue (25B), methylxanthines in serum (26B),penicillamine in blood (27B), and benzidine in urine (28B) and wastewater (29B). Several new electrode materials have been investigated for LCEC as alternatives to the glassy carbon, carbon paste, or mercury usually employed. Hepler e t al. considered lowtemperature isotropic carbon (3OB),the Wightman group at Indiana University evaluated the use of the basal plane of pyrolytic graphite ( 3 1 B ) ,Armentrout and colleagues a t Dow Chemical used a tubular electrode constructed from a mixture of carbon black and polyethylene ( 3 2 B ) ,and Curran’s laboratory at the University of Massachusetts toyed with a mixture of Ceresin wax and graphite powder (33B). While all of these materials worked just fine, none of them appear to have a clear advantage over previously used electrodes. While the DME was the very first LCEC detector (Kemula, 1952), it has yet to catch the imagination of very many chromatographers for obvious reasons. Michel and Zatka described a new design based on a horizontal electrode with a very rapid drop time (typically 50 ms) (34B) while Hanekamp et al. evaluated several novel designs ( 3 5 B ) . In most respects the DME does not compete with the amalgamated gold mercury film electrodes now in widespread use (36R),an advantage which is not nearly as important in LCEC as it is in classical polarography. The application of chemistry to electrochemical detection can provide unique advantages in selected cases. Kissinger and co-workers explored the potential usefulness of pre- and post-column chemical reactions (37B) and Little e t al. also considered some classical chemistry for post-column derivatization (38B). Larochelle and Johnson have taken advantage of electrocatalysis of chromium(V1) reduction by iodine adsorbed on a tubular electrode packed with platinum chips (39B). Their goal was to develop an LCEC method for chromium(V1) in drinking water. Eggli and Asper in Zurich
METHODOLOGY AND APPLICATIONS OF DYNAMIC TECHNIQUES
have developed a two-stage electrochemical detector for disulfides such as cystine (40R). The separated disulfide is first reduced in a coulometric electrode constructed from silver powder and the resulting oxygen-free thiol solution is detected amperometrically at a mercury electrode downstream. Takata et al. have published a preliminary report on the detection of Co(I1) by means of the electrocatalytic oxidation of tartrate in the eluent from an ion-exchange column (41B). One of the most unique procedures is embodied in the nitrosamine analyzer reported by Snider and Johnson (42B). Nitrosamines in a basic aqueous stream are irradiated by a xenon arc lamp causing them to decompose and thereby release nitrite. The nitrite is concentrated on an anion-exchange resin, stripped off with acid, and detected amperometrically at a platinum wire electrode. T h e Use of Enzymes. After years of trying, electrochemistry is finally beginning to give colorimetry and fluorescence some real competition as the analytical handle on enzymebased methods. While these methods do not necessarily involve hydrodynamic electrochemistry, they usually do and will therefore be covered in this section. The basic concepts of enzyme-coupled finite current electrochemical measurements were examined in the two previous reviews in this series. In almost all of the papers published to date the electrochemically active species has been either oxygen, hydrogen peroxide, ferri/ferrocyanide, or NADH. In a few cases, the redox dye intermediates used for classical colorimetric methods have been adapted to amperometric detection, but one of the advantages of electrochemistry is to avoid the need for such beasts. The determination of glucose in blood based on catalysis of its oxidation by glucose oxidase continues to provide the most important “test system” for amperometric methods. The reaction P-D-glucose + o2 D-glUConiC acid + H 2 0 ,
-
may be followed by either measuring oxygen consumption or hydrogen peroxide generation. An excellent effort in the latter category was recently described by Watson, Stifel, and Semersky who used a continuous flow system based on a column of porous alumina t o which the enzyme was covalently bound (43B). Small volumes of sample (2 pL) were injected onto the bed in the manner commonly used for liquid chromatography. The enzyme activity in the column was sufficient to completely convert the injected glucose into hydrogen peroxide before the effluent was passed through a low dead volume amperometric flow cell with a platinum working electrode at +0.5 V (vs. a Ag/AgCl reference). A cellulose dialysis membrane isolated the electrodes from protein adsorption and apparently also minimized interferences from electroactive organics such as uric acid, ascorbic acid, and cysteine. At least 60 samples per hour could be processed with results which correlated extremely well with the usual hexokinase reference method. Another interesting glucose analyzer is based on differential amperometry a t two platinum disk electrodes covered by collagen membranes (44B, 45B). Glucose oxidase was covalently bound to the collagen membrane covering the active electrode, and the hydrogen peroxide generated a t this membrane was detected. Good linearity was achieved over a wide range of glucose concentrations (10-7-10-3M). Mottola‘s group a t Oklahoma State has taken a somewhat different approach. They use a closed-loop flow system with circulating enzyme. Glucose samples are injected and oxygen consumption is measured a t an exposed platinum wire electrode. The performance of this simple system is extraordinary! A thousand or more samples can be processed per hour with a relative standard deviation of better than 2 % . In addition, the enzyme solution need not be replenished even after over 10000 serum samples have been processed! As the authors explain (46R), glucose oxidase is sufficiently inexpensive that immobilizing it does not provide significant economic advantages. In any case, no one will claim that their enzyme was wasted. The same group has also used their “closed loop” idea to measure glucose oxidase activity as well as for several other applications (47B). Karube et al. report the successful amperometric determination of phosphatidyl choline in serum using two immobilized enzymes in a flow system (48B).Phospholysase D releases choline from the phospholipid and choline oxidase converts the latter into betaine with liberation of hydrogen
peroxide which is measured by oxidation at a platinum electrode. Hahn and Olson used a tubular carbon electrode (TCE) for amperometric determination of total cholesterol in serum (49B). Three enzymes were used (cholesterol esterase, cholesterol oxidase, and peroxidase) to liberate free cholesterol, oxidize it, and convert the resulting H 2 0 2 into equivalents of ferricyanide which is reduced a t the TCE. Cheng and Christian used conventional instrumentation to follow oxygen depletion as blood and urine galactose is oxidized using (of all things) galactose oxidase (50B). The same authors developed a method for blood ethanol (51B). This time a two-step process was used. With NAD+ as the cofactor, alcohol dehydrogenase oxidized the ethanol to acetaldehyde while generating NADH. The latter was reoxidized by O2 to NAD’ using horseradish peroxidase as the catalyst. The depletion of O2 in the second step indirectly provided the quantitative measure of ethanol concentration. Attiyat and Christian used a biamperometric technique employing two TCE’s in series to determine glycerol and triglycerides in serum (5ZB). As in the paper described above, direct electrochemical measurement of NADH was avoided. This time around the NADH was converted to ferrocyanide. The fact that the electrodes were prepared from the carbon rod removed from an old dry cell battery illustrates once again that one of the primary advantages of electrochemistry is that it conserves grant money. As illustrated by the reports discussed above, much of the literature on amperometric methods for oxidoreductase enzymes and their substrates involves some attempt to convert NADH into something else. There is little question that many of these methods would be less awkward if the NADH itself were detected at the electrode. There continues to be concern that the electrochemical oxidation of NADH is not analytically useful due to surface adsorption and that various schemes need be introduced to overcome this problem (53B). It is the opinion of some of us that this concern is largely unwarranted and that an incorrect impression is gotten by attempts to measure NaDH with voltammetric scans at rotating or flow through electrodes. NADH can be detected at very low levels with excellent precision and minimal long term drift by using hydrodynamic amperometry a t a fixed potential. When low concentrations of NADH are passed tl-rough a thin-layer or tubular flow cell in the form of a brief concentration pulse, the performance is excellent (far more sensitive, far more reliable). There is no way that conventional voltammetry can compete with these experiments! Resides the, adsorption problems mentioned above, voltammetry under semi-infinite conditions is extremely wasteful of sample and is not readily adapted to automation or LCEC. T h e latter technique is extremely well suited to NADH determinations in that NADH behaves very well on modern reverse-phase LC columns and can be isolated from potential interferences in a few minutes (54B). Papers continue to appear in which enzymatically generated NADH is amperometrically recycled to NAD’, and the regenerated cofactor is used for further determinations. Malinauskas and Kulys developed sensors for alcohol, lactate, and glutamate using a platinum electrode with the cofactor trapped behind a semipermeable membrane (55B). Jaegfeldt et al. used a rotating platinum gauze electrode to coulometrically regenerate NAD+ which had been reduced in a reactor packed with alcohol dehydrogenase immobilized on glass beads (56B). It is interesting to note that the current efficiency for regeneration or enzymatically active NAD’ was 99.370, supporting the contention that NADH is very well behaved from an electrochemical point of view.
SPECTROELECTROCHEMISTRY Several new developments in spectroelectrochemical methodology and numerous applications of existing techniques have been reported in the past two years. The discussion of these developments is organized below according to technique. The combination of optical and electrochemical techniques as applied to studies of redox chemistry was recently discussed by Heineman (1C). Some examples of spectroelectrochemical methodology for studies of biological systems were examined in a critical review of bioelectrochemistry by McCreery (2C). Chronoabsorptometry. Chronoabsorptometry involves the measurement of absorbance-time response during a potential step experiment in quiescent solution. The optical ANALYTICAL CHEMISTRY, VOL. 52, NO.
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beam usually passes through an optically transparent electrode (OTE) and the solution, Le., transmission spectroscopy. An interesting new development by Pruiksma and McCreery is the observation of concentration a t specific distances from the electrode by means of a laser beam passing parallel to the electrode surface ( 3 C ) . A 10-pm slit was placed parallel to the electrode to intercept the beam. Movement of this slit relative to the electrode surface enabled optical monitoring of concentration as a function of distance from the electrode. Such spatial resolution of the diffusion layer can provide fundamental information about mass transfer and improved characterization of homogeneous reactions coupled to charge transfer. Another development in the methodology of chronoabsorptometry is optical monitoring by specular reflection at a glancing incidence. McCreery's group has shown the effective optical path length of this technique to be 100-200 times longer than the typical path length for chronoabsorptometry at an OTE ( 4 C ) . Tyson and West have passed a narrow light beam a t a grazing incidence across a smooth platinum chloride to increase t h e optical path (5C). Chronoabsorptometry has been used primarily for determining mechanisms and measuring rates of homogeneous chemical reactions accompanying charge transfer reaction(s). An analysis of 22 electrochemical mechanisms by finite difference simulation and simplex fitting of double potential step current, charge, and absorbance response has been reported by Hanafey, Scott, Ridgway, and Reilley ( 6 C ) . The ability of double step methods to differentiate between mechanisms and the probable errors in kinetic parameters obtained by fitting typical data to working curves was demonstrated. Techniques for obtaining complete spectra of intermediates by analysis of time-dependent spectra generated from a potential-step experiment have been demonstrated by Langhus a n d Wilson (7C). T h e wavelength-dependent effect of the shape of absorbance-time curves for individual species on the estimation of kinetic parameters was examined, and the effect of photometric noise on the fit of experimental and theoretical curves was illustrated. The technique of open circuit relaxation spectroelectrochemistry was evaluated by Evans and Blount for an EC dimerization model and applied to the reaction of 9,lO-diphenylanthracene cation radical with 4cyanopyridine (8C).Potential step chronoabsorptometry was used to investigate two E E mechanisms: the reduction of a porphyrin diacid (7C) and relative reactivities of nucleophiles in reaction with the cation radical of 9,lO-diphenylanthracene ( 9 C ) . Both potential step chronoabsorptometry and open circuit relaxation were used by Steckhan to study the dimerization of 4,4'-dimethoxystilbene cation radical (IOC), by Genies and Diaz to study the dimerization of a radical cation produced from a 1,3-diphenylpyrazoline derivative ( I 1C),and by Evans and Blount to investigate the effects of initial cation radical concentration on the rate-determining step in the half-regeneration mechanism of 9,10-diphenylanthracene radical cation (12C). T h e methodology of chron'oabsorptometry has now been extended to the measurement of heterogeneous electron transfer rate constants. Theory for the single potential step chronoabsorptometric determination of such rate constants was developed by Albertson, Blount, and Hawkridge and verified by using the quasi-reversible oxidation of ferrocyanide at tin oxide (13C). Transmission spectroelectrochemistry has been applied by Goelz and Heineman t o the study of electrode surface phenomena such as the specific adsorption of anions on a mercury-coated platinum O T E (14C). Double potential step experiments on aqueous solutions of NaF, NaBr, NaNO,, Na2C03, CH3COONa, NaCl, and NaSO, produced optical signals whose magnitudes correlate with the known sequence of anion specific adsorption on mercury. T h i n - L a y e r Spectroelectrochemistry. The technique of simultaneous optical and electrochemical measurements on thin solution layers by means of an optically transparent thin-layer electrode (OTTIX) has been applied to a variety of chemical systems. O T T L E techniques for the study of biological redox systems have been discussed by Heineman, Meckstroth, Norris, and Su ( 1 3 2 ) . The measurement of rate constants of homogeneous chemical reactions of electrogenerated species has been demonstrated by Blubaugh, Yacynych, and Heineman with single- and double-potential step tech1 4 4 R * ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
niques for the benzidine rearrangement (16C) and by Owens, Marsh, and Dryhurst with electrogenerated imine-alcohol intermediates (17C, 18C). The O T T L E has been used for measuring the temperature dependence of reduction potentials of copper blue proteins ( I 9 C ) and cytochrome c ( 2 0 0 . Thin-layer spectroelectrochemical techniques have proved to be a convenient means for obtaining spectra, reduction potentials and n values for Schiff base complexes of cobalt and copper in dimethylformamide @ I C , 22C) and vitamin BI2and related cobalamins (23C). Two new cells for the study of biological redox systems have been reported: a small-volume cell (24C)and a circulating, long-optical-path cell (25C). The O T T L E has been used in conjunction with characterizing a surface modified gold minigrid electrode for reduction and oxidation of myoglobin (26C). I n d i r e c t Coulometric T i t r a t i o n (ICT). The spectroelectrochemically-based indirect coulometric titration developed in Kuwans's laboratories has been employed by Szentrimay and Kuwana for studying the complex IV re ion of intact mitochondria with electrogenerated viologen racfical cation reductant and molecular oxygen oxidant (27C). Double-wavelength modulation was used to circumvent problems of turbidity and low redox component concentrations. Rickard, Landrum, and Hawkridge employed the ICT technique to measure the redox potential of ferredoxin (28C). Resonance R a m a n . Resonance Raman spectroelectrochemistry has been used to investigate both solution and surface phenomena. Van Duyne's group studied the electrogenerated dianion of tetracyanoquinodimethane (TCNQ2-) (29C). The Raman data was used to identify the oxidation state of TCNQ in a coordination complex. Wallace, Jaeger, and Bard elucidated surface processes occurring during voltammetry at the tetrathiafulvalenetetracyanoquinodimethane electrode (30"). Raman spectra of pyridine and related molecules adsorbed on a roughened silver electrode (31C)and p-nitrosodimethylaniline adsorbed on silver (32C)have been obtained. Anderson and Kincaid developed a circulating cell and applied it to resonance Raman and visible absorption spectroelectrochemical studies of heme proteins (33C). Reflectance Techniques. Reflectometry and ellipsometry have been used in conjunction with a H M D E or a DME positioned in a homogeneous broad beam of light (34C). Reflectance and/or ellipsometry continue to be used primarily for the study of interfacial phenomena such as the effects of P b and Cd adatoms on the oxidation of formic acid on rhodium (35C), the adsorption of adenine derivatives on gold (36C),cation adsorption on oxide layers of gold and platinum (37C),and the adsorption of barbiturates on mercury (38C). S u r f a c e Spectroscopy of Electrodes. Surface spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS, ESCA), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) have been invaluable in the development and characterization of chemically modified electrodes, vide supra. Photoacoustic spectroscopy has also been used to detect molecules covalently bonded to electrode surfaces (39C-41 C). Some examples of surface spectroscopy studies of nonchemically modified electrodes are a comparison of electrochemical and AES analyses of the surface composition of platinum-rhodium alloys (42C),AJ3S characterization of spherical Au electrodes and Pt electrode surfaces prepared in the gas phase (43C),and LEED characterization of iodine adsorbed on Pt single-crystal surfaces (44C) and of the electrochemical activity of Pt surfaces (45C). X-ray emission spectroscopy was used in conjunction with cyclic voltammetry to characterize hydrogen and oxygen adsorption properties of ruthenium surfaces (46C). A new technique, dynamic X-ray diffraction, involves a continuous in situ measurement simultaneous with electrochemical measurements (47C). The technique was employed to study the discharge of a Tis2 electrode in a Li/TiS, organic electrolyte cell. Optically T r a n s p a r e n t Electrodes (OTE). A new metallized plastic OTE consisting of a polyester sheet covered with a thin film of gold, titanium oxide-coated gold, or indium/tin oxide has been reported by Cieslinski and Armstrong ( 4 8 0 . T h e electrode exhibits low sheet resistivity and high transparency to visible light. T h flexibility of these OTEs may facilitate their use in unusual cell geometries. An optically transparent bubbling gas electrode based on a tin oxide or gold OTE has been used to studv the photoelectrochemistry of the
METHODOLOGY AND APPLICATIONS OF D Y N A M I C TECHNIQUES
rhodamine B--hydroquinone system ( 4 9 3 . The mercurycoated platinum OTE was evaluated with respect to the nature of its surface (.50C) and its utility with internal reflection spectroscopy ( 5 1 C ) . Miscellaneous. Visible absorption spectroelectrochemistry has been used to study films coated on OTE. A P t OTE, chemically modified by coating with a polymer of the potassium salt of 1 , 3 - d i ( p - m e t h o x y p h e n y l ) - S - ( p h y d r o x y phenyl)-A*-pyrazoline attached to cross-linked chloromethylated polystyrene resins ( 5 2 0 . and n-heptylviologen radical cation films on transparent oxide electrodes (53C) have been examined. Optical absorption measurements have been made in the studv of the electrochromic Drocess a t thin-film WO, electrodes (54C). T h e O T E has proven useful for monitoring excited state electron transfer processes (55C). Spin trapping techniques have been used for elucidating electrode processes bv ESR spectroscoT;)y . . of electrogenerated radicals (56C). A flow-through s~ectroelectrochemicalcell was used for a study of the chimica catalysis of the electrochemical reduction of alkyl halides (57C). Fiber optics have been used to record spectra inside an electrolytic cell containing molten salt (58C, 59C). An optical phase contrast method has been developed for observing the dropping mercury electrode (60C). Flameless atomic absorption spectrometry has been used to study the reduction of mercury at a glassy carbon electrode (6lC). Corrosion inhibition of copper by mercaptobenzothiazole has been studied by spectroelectrochemical techniques (620.
STRIPPING VOLTAMMETRY Stripping Techniques. Potentiometric stripping is a relatively new technique in which metals preconcentrated into a mercury electrode are stripped by a chemical oxidation reaction with Hg(I1) added to solution as the oxidant ( I D ) , M(Hg)
+ (n/2)Hg(II) -*
M ( n ) + (n/2)Hg
T h e potential of the mercury electrode, when monitored as a function of time, gives an experimental curve which is analogous to a normal redox titration curve. A sharp potential change accompanies depletion of the metal a t the electrode surface. T h e time required to reach the equivalence point is proportional to the amount of metal ion in solution. Analytical procedures have been reported by Jagner's group for Bi, Cd, T1, P d , Cu, and Zn in various media ( I D ) ;P b in urine ( 2 0 ) ; Zn, Cd, Pb, and Cu in seawater ( 3 0 ) ;and Zn, T1, Cd, Pb, Bi, and Cu using dissolved oxygen as the oxidant ( 4 0 ) . Derivative potentiometric stripping analysis with a thin film of mercury on a glassy carbon electrode (50), a computerized data acquisition'technique involving multichannel potentiometric monitoring (60),and microcomputer-controlled instrumentation for automation ( 7 0 ) have been reported. Pseudopolarograms are plots of stripping peak current as a function of deposition potential. Brown and Kowalski have shown them to be useful for determining ligand number and stability constants for metal complexes a t concentration ranges in natural waters (80). The technique has been evaluated with P b and Cd with chloride and carbonate, the structure determination of As(II1) in acidic media, and the speciation of Pb in geothermal water. Theoretical correlations between pseudopolarogram half-wave potentials and pre-electrolysis time, heterogeneous rate constants and electrode radius, and relationships between applied potential and log [(ii - i ) / i ]were calculated by Shuman and Cromer for reversible and nonreversible systems ( 9 D ) . T h e theory for reversible systems was in good agreement with that obtained for TI(1). Semidifferential electroanalysis has been applied by Goto and co-workers as a stripping technique ( 1 0 0 , 1 1 0 ) . Results obtained with a thin mercury film formed in situ on a rotating glassy carbon disk electrode shows the method to be nearly comparable in sensitivity and resolution with differential pulse stripping, but to be a much faster technique (IOD). Kankare and Haapakka investigated a symmetric doublestep waveform superimposed on a voltage ramp with subtraction of the areas under adjacent current peaks t o cancel the contribution of double layer charging. A reasonably flat base line was obtained down to nanomolar levels of Cd, Cu,
and P b a t a mercury film electrode ( I 2 D ) . The recursive estimation technique known as the Kalman filter has been experimentally verified by Seelig and Blount for real time analysis by linear sweep stripping voltammetry a t the HMDE and MFE ( 1 3 0 ) . Synthetic lead samples were used for a critical comparison between this technique and traditional nonreal time digital filtering methods. The application of the filter to the analysis of data for the determination of trace levels of P b in municipal and seawater using differential pulse, pulse, and linear sweep ASV a t the HMDE and linear sweep ASV a t the M F E has been demonstrated (140). A minicomputer-controlled ASV technique reported by Brown and Kowalski uses differential voltammetry a t one electrode in conjunction with a rapid data-averaging algorithm to improve sensitivity (152)). Cd, Pb, and Cu could be determined down to 1L10 pg mI, in 15 min. The linear learning machine method has been applied t o ' by ASV the determination of Cd, Pb, and TI down to 1 0 ~ M a t a HMDE (160). T h e detection limit of anodic stripping coulometry a t mercury-film glassy carbon electrodes was evaluated ( I 72)). Subtractive ASV with twin-cell twin-electrodes has been investigated as a means of minimizing background current. T h e technique has been evaluated with twin-mercury film electrodes ( 1 8 0 ) and a rotating mercury coated glassy carbon split-disk electrode ( 1 9 0 ) . Preconcentration Techniques. Adsorption as the preconcentration step was used by Kolpin and Swofford to determine heme in the 1-100 nM concentration range ( 2 0 0 ) . The rate of deposition of heme a t a H M D E from stirred solution was shown to be nearly a linear function of the bulk solution heme concentration when the electrode surface coverage is less than half of its maximum value. Preconcentration in flow-through cells continues to be investigated as a means of enhancing sensitivity. ASV with collection a t two mercury-coated glassy carbon tubular electrodes in series was applied by Schieffer and Rlaedel to the determination of Cd, Pb, and Cu in tap water at subnanomolar levels ( 2 1 0 ) . T h e rotating disk electrode has been adapted by Wang and Ariel for ASV in a flow-through cell ( 2 2 0 ) . A flow-through system for automatic ASV analysis of discrete samples with a rotating M F E deposited on a glassy carbon substrate allows the simultaneous determination of P b , Ccl, and Cu a t the ppb level a t a rate of 10 samples per hour (230). Flow injection analysis by stripping voltammetry has been used by Ruzicka and Hansen to determine heavy metals in the 10 "--lO~' M range ( 2 4 0 ) . Electrodes. New electrodes for use in stripping voltammetry continue to be explored. A porous mercury-coated reticulated vitreous carbon electrode in a flow-through configuration has been evaluated by Blaedel and Wang ( 2 5 0 ) . The electrode shows good promise for use with ASV. A study by Ostapczuk and Kublik of the silver-based M F E compares theoretical and experimental results for Cd stripping (260). Deviation from theory is attributed to interaction between Cd dissolved in the mercury film and the substrate silver amalgam. The behavior of Cu a t a rotating carbosital (a new carbon material) electrode has been reviewed (27D). A simple and inexpensive modification which facilitates filling a common type of H M D E has been reported ( 2 8 0 ) . Interferences. Problems such as intermetallic compound formation and contamination have been investigated. Metallic bonding in liquid amalgams has been reviewed by Ben-Bassat and Azrad; the amalgams Cu-Hg, Zn-Hg, and Cu-Zn-Hg were studied electrochemically ( 2 9 0 ) . Roston, Brooks, and Heineman eliminated Cu-Zn and Cu--Cd intermetallic interferences in ASV in a twin-electrode thin-layer cell by selective removal of Cu onto one electrode with subsequent determination of Cd and Zn on the second electrode ( 3 0 0 ) . Elimination of the Cu-Zn interference by addition of Ga was investigated for potentiometric stripping ( I D ) . Donnan dialysis has been used by Cox and Cheng to minimize matrix effects by transferring test ions from the sample into a controlled electrolyte prior to stripping analysis (310). The method was evaluated on the determination of phosphate and arsenate by cathodic stripping voltammetry. Interferences originating from siliconization of the HMDE capillary ( 3 2 0 )and from Teflon vessels used in pressurized digestion of samples ( 3 3 0 ) were investigated by Oehme.
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Applications of Anodic Stripping Voltammetry (ASV). T h e determination of metal ions in natural waters continues to be an area of considerable investigation ( 3 4 0 ) . Of particular interest is the possibility of distinguishing between free metal ions and metal ions complexed by naturally occurring complexing agents, i.e., speciation. Recent papers have discussed the following aspects of metal speciation: defining the electroanalytically measured species in a natural water sample ( 3 5 0 ) ;reversibility of Cu in dilute aqueous carbonate and its significance to ASV of Cu in natural waters ( 3 6 0 ) ;voltammetry of inorganic Pb(I1) complexes in seawater ( 3 7 0 ) ;studies of Cd-ethylenediamine complex formation in seawater ( 3 8 0 ) ; reversible electrodeposition of trace metal ions from multiligand systems ( 3 9 0 ) ;base-line concentrations of Cd, Pb, and Cu in selected natural waters ( 4 0 0 ) ;iron interference on the ultratrace determination of Cu in natural waters ( 4 1 0 ) ;determination of labile trace metal fractions in aqueous ligand media by a Chelex resin method compared to ASV ( 4 2 0 ) . Clem and Hodgson investigated ozone as an oxidant for the destruction of trace organic sequestering agents in water (430). T h e method was applied to the determination of P b and Cd in sewage effluent and bay water. Some typical reports of other ASV applications include determinations of Cd, P b , and Cu in urine a t elevated temperature ( 4 4 0 ) ;toxic metals in urine ( 4 5 0 ) ;Cu, Pb, Cd, and Zn in teeth ( 4 6 0 ) ;As a t the nanogram level by deposition on a gold film on pyrolytic graphite electrode ( 4 7 0 ) ;Hg a t an impregnated graphite electrode ( 4 8 0 ) ;hexavalent molybdate ( 4 9 0 ) ;Se and T e in electrolytic copper (500); Te(1V) in aqueous solution and mixed solvents ( 5 1 0 ) ;Co ( 5 2 0 ) ;Sn in the presence of P b in pyrogallol medium ( 5 3 0 ) ;Cu, Pb, Cd, and Zn in zirconium metal and zirconium dioxide ( 5 4 0 ) ; brightener concentration in copper pyrophosphate plating baths ( 5 5 0 ) . The application of ASV to the analysis of food and related products has been discussed by Gajan ( 5 6 0 ) . A p p l i c a t i o n s of C a t h o d i c S t r i p p i n g V o l t a m m e t r y (CSV). Some typical reports of CSV applications include determinations of phosphate and arsenate ( 3 1 0 ) ; organic sulfur compounds, flavins and porphyrins at a Hg pool electrode ( 5 7 0 ) ;the release of inorganic sulfide from proteins during denaturation in alkaline media ( 5 8 0 , 590); organic halides in drug dissolution studies ( 6 0 0 ) ;selenium in soils ( 6 1 0 ) ;arsenic in industrial ZnSO, solutions for electrolysis (620). Electrochemical P r e c o n c e n t r a t i o n f o r Spectroscopy. Preconcentration of material on an electrode with subsequent investigation by a spectroscopic technique has been used to determine substances with a microprobe ( 6 3 0 ) ,Se with atomic absorption ( 6 4 0 ) ,and cyano metalates with X-ray emission (650). Miscellaneous. Approximate expressions have been derived for the faradaic and nonfaradaic current observed during the stripping of reactants from an electrode surface or from a thin liquid film on the electrode surface ( 6 6 0 ) . T h e standard addition method has been evaluated with respect to optimization for ASV ( 6 7 0 ) . Stripping voltammetry and cyclic voltammetry were used to determine the solubility of S b in Hg ( 6 8 0 ) . Digestion procedures for the determination of heavy metals in blood by ASV have been compared ( 6 9 0 ) . Mercury toxicity has been discussed ( 7 0 0 ) . VOLTAMMETRIC TECHNIQUES D i f f e r e n t i a l Pulse Voltammetry ( D P V ) a n d Pulse Voltammetry (PV). An upsurge in the use of the differential pulse waveform in conjunction with stationary electrodes is occurring. Cyclic DPV, which is analogous to the widely used cyclic voltammetry, was recently reported by Drake, Van D u p e , and Bond ( I E ) . Such a cycling technique is a valuable extension in the currently available methodology for stationary electrodes. The redox properties of bacteriochlorophyll and bacteriopheophytin in aprotic solvents were investigated by cyclic DPV ( 2 E ) . Single sweep DPV has proved to be a useful technique for studying electrodes which have been chemically modified by a n electroactive group, vide supra. Differential pulse voltammograms have been recorded for electroactive species which have been attached by covalent binding ( 3 E ,4 E ) , polymerization ( 5 E ) , or adsorption ( 4 E ) . Adsorption of a nonelec146 R
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troactive species at solid electrodes has been monitored by DPV from the shift of potential of zero charge as a function of concentration ( 6 E ) . Yeh and Kuwana used DPV and cyclic voltammetry to obtain well-defined voltammograms for the reduction of cytochrome c a t an indium oxide electrode ( 7 E ) . This is one of the few observed reversible electrode reactions for a heme protein. P V and DPV have shown potential utility for detecting neurotransmitters in vivo. Lane, Hubbard, and Blaha evaluated DPV a t a graphite paste electrode ( 8 E ) ,and Pujol et al. evaluated PV with a carbon fiber working electrode (9E). The behavior of stationary electrodes (glassy carbon, carbon paste, paraffin-impregnated graphite with and without a mercury film, gold, platinum, and HMDE) has been evaluated with PV and DPV (10E). Typical analytical applications of DPV include the determinations of polyinosinic acid and its components at pyrolytic graphite ( I I E ) ;theophylline in plasma at carbon paste (12E); butylated hydroxytoluene in transformer oils a t platinum (13E);xanthine and xanthosine 5’-monophosphate at pyrolytic graphite ( 1 4 E ) ;aminopyridines a t a bubbling solid electrode (15E);phenylbutazone and oxyphenbutazone at glassy carbon (16E);polynuclear aromatic hydrocarbons at platinum (I 7 E ) ; and tocopherols in plant oils at glassy carbon (18E). Ac Voltammetry. The application of ac voltammetry to electrodes modified by irreversible adsorption, polymer coating, and chemical binding has been discussed by Laviron (19E). Second harmonic ac voltammetry showed good promise as an electroanalytical tool for the study of reactive species chemically bonded to a n electrode surface as evidenced by voltammograms of 2-pyrazoline bonded to tin oxide (20E). Lennox and Murray used cyclic phase-selective ac voltammetry to study glassy carbon-bound tetra(aminopheny1)porphyrins ( 3 E ) . Eddowes and Hill employed cyclic ac voltammetry to investigate the redox properties of cytochrome c a t gold modified by 4,4’-bipyridyl adsorption (21E). L i n e a r S w e e p Voltammetry. A few analyses based on linear sweep voltammetry have been reported. Au has been determined to the M level by reduction of AuCl, a t carbon paste (22E). Morphine in poppy seeds, crude morphine, and pharmaceutical preparations has been determined on graphite or platinum ( 2 3 E ) . Cyclic Voltammetry. Cyclic voltammetry continues to be the workhorse technique for studying electrode processes. Innumerable examples of its use in the study of inorganic, organic, and biological systems have been reported in the past two years. An interesting analytical application of cyclic voltammetry at a DME by Miller and Werber is the determination of ferredoxin a t concentrations down to 5 X lo-’ M (24E). Cyclic voltammetric ejection and redeposition of solvated electrons in hexamethylphosphoramide has been studied ( 2 5 E ) . Cyclic voltammetry was used to study the process of intercalation for transition metal disulfide intercalates (26E). Stopped-flow voltammetry has been investigated as a method for observing fast chemical reactions (2723). POLAROGRAPHIC TECHNIQUES ( V O L T A M M E T R Y A T THE D M E ) P u l s e a n d D i f f e r e n t i a l P u l s e Polarographies. Differential pulse polarography (DPP) and pulse polarography (PP) have, to a large extent, supplanted conventional polarography in the recent electroanalytical literature. This is attributed to the lower detection limits of the pulse techniques. D P P and PP are being widely employed for the determination of organic compounds and metal ions for which stripping voltammetry is unsuitable. Some advances in the application of D P P and PP are reviewed in this section. Developments in the theory and instrumentation of these techniques are reported in the companion article of this issue ( I F ) . Improving the detection level of D P P and correcting for background current with a quadratic least-squares fit of data removed from the faradaic peak current of interest provides a general method of predicting the base line (227). Bond and Grabaric have tested the method for Cd in the 10~6-10-sM range. A large number of applications of D P P and PP to the determination of drugs have been reported. Some typical examples are determinations of the following drugs: N-nitroso derivative of a tripeptide in pharmaceutical dosage forms (3F);
METHODOLOGY AND APPLICATIONS OF DYNAMIC TECHNIQUES
disodium chromoglycate in urine (4n;penicillamine ( 5 F ) : benzylpenicillenic acid ( 6 F ) ;cephalosporins and their degradation products ( 7 F ) ;derivatives of dibenzodiazepines and dibenzothiazepines ( 8 F ) ;2-benzimidazolyl 2-pyridyl methyl sulfoxide in a pharmaceutical formulation (9F):hydrocortisone in pharmaceutical preparations (Ion; corticosteroids in single-component solutions, suspensions, ointments, creams, and single-component tablets (IIF-13F);and methaqualone and some of its metabolites ( 1 4 F ) . Numerous organic compounds hake been determined by pulse techniques. Many examples of determinations of organic compounds of biological significance by PP, DPP, and other voltammetric methods are contained in the review by Smyth and Smyth (I5F). Typical methods for pesticides, carcinogens, and organic compounds are determinations of parathion, its major metabolites, and other nitro-containing pesticides (16F); azomethine-containing pesticides ( I 7 F ) , nitrosamines ( I 8 F ) ; aflatoxins in various foodstuffs ( I 9 F ) ; and ammonia, p aminophenol and sulfanilamide by indophenol derivatization (20F). DPP and PI' have also been applied extensively to the determination of inorganic compounds. Speciation of As(III), As(V), and total As has been achieved with D P P (21I.3. PP has been evaluated for detecting quasilability in metal complexes using CdL+/EDTAas a model system (22F). Hydroxide has been determined by Kirowa-Eisner and Osteryoung in the concentration range 0.6 pM-0.4 m M by PP a t the DME and pulse voltammetry, vide infra, a t a RDE (23F). The method is based on oxidation of the mercury electrode to form Hg(OH),. Other determinations of inorganic compounds include biologically active organoarsenic acids (24F); phenylarsine oxide (25F);chlorite ion (26E);tellurium by a catalytic hydrogen current resulting from deposition of T e on the DME (27F);selenium and tellurium and their mixtures (28F3,molybdenum in plant samples grown on fly-ash amended soils by reduction of a Mo(V1) complex with 8-quinolinol in aprotic solvent (29F);molybdenum down to 2 X lo-* M by a catalytic Mo(V1) wave (30F);germanium(1V) in the presence of 3,4dihydroxybenzaldehyde (32F);tin in zinc-aluminum alloys ( 3 2 3 ;lead complexes of humic and fulvic substances (33F); complexation of heavy metals with natural polyelectrolytes ( 3 4 F ) ;silicon in steel (35F);zinc dialkyldithiophosphate in lubricants (36F); ammonia and primary amines (37F); the degradation of cephalexin by the measurement of hydrogen sulfide evolved (38F);SO,, NO, and NO,,in air (39F);carbon disulfide in water (40F);and sulfuric acid in acetonitrile (41F). Pulse polarographic techniques have been applied to biological systems such as heme proteins. T h e electrochemical behavior of cytochrome c3 was found t o be reversible and diffusion controlled by PP and D P P (42F). Surfactants can be determined from the magnitude of tensammetric (adsorption/desorption) peaks on differential pulse polarograms (43F). Ac Polarography. Several analytical methods based on ac polarography have been reported. Schaar and Smith have demonstrated clear advantages in determining organic pharmaceuticals in aprotic organic solvents as compared to the usual procedures involving protic aqueous or alcoholic media (44F). Tablet assays were developed for the alkaloids colchicine and reserpine with acetonitrile and 0.1 M tetraethylammonium perfluoroborate as the electrolyte. Other determinations by ac polarography iiiclude folic acid in pharmaceutical preparations (45F),indium(II1) after solvent extraction into acetonitrile (46F),hydroxylamines in photographic processing solutions ( 4 7 F ) ,and Ge(1V) in perchloric acid solution (31F). Polarography. Conventional dc polarography continues t o be used for studying electrode reactions of organic and inorganic compounds and in analytical methods. T h e polarographic characteristics of metal ions in various supporting electrolytes have been compiled by Meites et al. (48F). Nitrohumic acids have been characterized by the E,, for reduction of the nitro group (49F). The formation constants for the binding of molecular oxygen to several cobalt(I1) Schiff-base complexes have been measured polarographically from the diffusion current for reduction of the oxygenated complex i5OF). An analytical procedure for pilocarpine and some related imidazoles is based on the effect of ligand (imidazole) concentration on the El for reduction of the copper complex ( 5 I F ) . The antidepressant drug chlorimipramine can
he determined by a catalytic hydrogen evolution wave (52F). Microamounts of vanadium(V) and molybdenum(V1) can be determined by their catalysis of the reduction of bromate ( S F ) . Other determinations include ampicillin in capsules and tablets (.54F), nitrate in fertilizer with N,N-dimethylformamide solvent (55F), metal ions extracted as complexes into an organic phase (56F). therapeutically important 1,4benzodiazepines (57F1, folic acid in tablets (5887, and organic compounds of arsenic ( 5 9 0 .
MISCELLANY Immunoassay by Electrochemical Techniques. The possibility of immunological methods based on labeling an antigen with an electroactive group detectable by an electrochemical technique has been explored in two laboratories. Weber and Purdy described a homogeneous voltammetric immunoassay in which free antigen was detected in the presence of antibody-bound antigen using continuous flow amperometric detection ( I G). A morphine immunoassay system with ferrocene as the electroactive tag was studied. Heineman, Anderson, and HalsaU labeled estriol with mercuric acetate as the electroactive group and monitored the reaction of this labeled antigen with estriol antibody (2G). Differentia! pulse polarography was used t o detect the electroactively tagged estriol. In both cases, separation of free-labeled antigen from antibody-bound-labeled antigen was unnecessary. Molecules Containing Multiple Redox Centers. Electrochemical techniques are useful for the study of molecules containing multiple centers that are capable of accepting or giving up electrons. Flanagan, Margcl, Bard, and .4nson have given a general analysis of the voltammetric behavior of molecules containing identical, noninteracting redox centers (33).Such a molecule gives a voltammogram with the same shape as that obtained for the corresponding molecule containing a single center. The additional electroactive centers enhance only the magnitude of the current. Vinylferrocene and poly(kiny1ferrocene) were used as experimental examples. Another interesting example of multiple, noninteracting centers reported by Saji, Pasch, Webber, arid Bard is the reduction of poly-2-vinylnaphthalene and poly-9-vinylanthracene ( 4 G ) . Voltammetric waves with the overall shape of a one-electron transfer reaction are obtained although up to 1200 electrons may be transferred per molecule as demonstrated by coulometry. Fenton, Schroeder, and Lintvedt reported a binuclear copper(I1) complex which exhibits a peak potential separation of 42 mV for cyclic voltammetry (5G). This is the value predicted by Polcyn and Shain for sequential transfer of electrons a t the same potential. Chronoamperometry. Ryan, Wei, Feinberg. and Lau have used chronoamperometry to determine homogeneous small molecule-redox protein reaction rates (6G). Lecithin has been determined by a procedure based on the extent of inhibition of oxygen reduction by the adsorption of lecithin as measured by current-time curves during the growth of single mercury drops ( 7 G ) . Piezoelectric Measurements. A piezoelectric element which detects the derivative of the surface stress with potent,ial has been used by Malpas, Fredlein, and Bard to observe surface processes a t solid electrodes (8G,9G). Adsorption a d film formation have been observed a t P t , and elwtrocapillary curves have been measured for gold. Low-Temperature Electrochemistry. Lowtemperature cyclic voltammetry has been used to measure equilibrium and rate constants for conformational changes in organic compounds (10G). Rates, mechanisms,, snd apparent reaction orders of electrohydrodimerization reactions have been determined by low-temperature double-pdential step chronocoulometry ( Z I G ) . Electrochemical investigations of liquid sulfur dioxide solutions at -40 "C have been reported (12G). ELECTROCHEMICAL NOMENCLATURE, F I N I T E CURRENT TECHNIQUES Voltammetry. Finite current methods in which a current response is measured as a function of a potential waveform increasing in amplitude in a positive or negative direction. The waveform may be a ramp, a pulse train?a sine wave, or various combinations thereof. Polarography. Voltammetric methods carried out using the dropping mercury electrode. ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
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Hydrodynamic Voltammetry. Voltammetric techniques in which mass transport is controlled by convection as well as by diffusion, including stirred solution voltammetry and rotating disk voltammetry among others. Amperometry. An electroanalytical approach in which current is measured a t a fixed potential under conditions where the faradaic reaction is mass transport limited. T h e current may be constant ( e g , some amperometric oxygen or enzyme electrodes) or may vary as a function of time (e.g., chronoamperometry) and/or added reactant (e.g., amperometric tit rat ion). Anodic S t r i p p i n g . Techniques (usually voltammetric) in which an analyte is preconcentrated by reduction onto or into an electrode and is subseqriently determined by electrochemical oxidation t o form a soluble species. Cathodic S t r i p p i n g . Techniques (usually voltammetric) in which an analyte is preconcentrated by oxidation onto or int,o an electrode and is subsequently determined by electrochemical reduction t o form a soluble species. D i f f e r e n t i a l T e c h n i q u e s . (Not subtractive). Require measurement of a difference between two signals either instantaneously or separated by a fixed time period (as in differential pulse polarography). D e r i v a t i v e T e c h n i q u e s . (Not differential). Those in which a rat,e of change is continuously measured. Anodic C u r r e n t . T h e rate of charge flow in a direction which results in oxidation of an electrode material or so!ution component. Cathodic C u r r e n t . The rate of charge flow in a direction which results in reduction of an electrode material or solution component. Positive Potentials. (Not "anodic potential"). Potentials positive with respect to a specified reference electrode or redox couple. Negative P o t e n t i a l s . (Not "cathodic potentials"). Potentials negative with respect t u a specified reference electrode or redox couple. Semiinfinite Diffusion Techniques. Methods in which a concentration gradient of reactant and/or product extends outward from an electrode surface but does not reach the walls of the container. R e s t r i c t e d D i f f u s i o n T e c h n i a u e s . A concentration gradient extends from an electrode surface to the walk of the container or occurs in the pores of a packed bed electrode. T h i n - L a y e r T e c h n i q u e s . Restricted diffusion methods in which the concentration gradient exists perpendicular to parallel walls (two electrodes, or one electrode and a diffusion barrier). Working Electrode. (Not "indicator" or "test" electrode) T h e electrode a t which the reaction of interest takes place. A u x i l i a r y Electrode. (Not "counter" electrode). T h e electrode in three-electrode experiments Rhich provides a current path in series with the solution and the working electrode. T h e potential of the auxiliary electrode is not controlled with respect t o the reference electrode. C o u n t e r Electrode. An electrode used In two-electrode experiments which serves the purposes of both a reference electrode (facilitates potential control) and an auxiliary electrode (carries current). Counter electrodes are generally useful only for analytical experiments in which low currents are passed across a medium of relatively high conductivity. C h r o n o a b s o r p t o m e t r y . A spectroelectrochemical technique in which optical absorbance is measured as a function of time. Chemically Modified Electrode. An electrode whose surface chemistry has been deliberately altered (by covalent reaction, adsorption, polymer coating, etc.) to enhance its performance for a specific objective. ACKNOWLEDGMENT T h e authors wish t o thank Karl Bratin and Beth Stinson for their assistance in preparation of the manuscript and the National Institutes of Health and National Science Foundation for generous financial support. LITERATURE CITED ELECTRODE MATERIALS (1A) Mernrning. R Electroanal Chem 1979, 7 7 , 1-84 (2A) Wrighton, M S Acc Chem Res 1979. 12 303-310
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F.; Heineman, W. R.; Deutsch, E. Znorg. Chem. 1979, 18, 2536-2542. (23C) Mark, H. E., Jr.: Kenyhercz, T. M.; Kissinger, P. T. I n "Electrochemical Studies of Biological Systems", Sawyer, D. T., Ed.; American Chemical Society: Washington, 1977; pp 1-25. (24C) Anderson. C. W.: Halsall. H. B.: Heineman. W. R. Anal. Biochem. 1979. 93,366-372 (25C) Anderson, J L Anal Chem 1979, 57, 2312-2315 (26C) Stargardt, J F , Hawkridge, F M , Landrum. H L Anal Chem 1978, 50 930-932 (27C) Szentrimay, R.; Kuwana, T. Anal. Chem. 1978, 5 0 , 1879-1883. (28C) Rickard, L. H.; Landrum. H. L.; Hawkridge, F. M. Bioelectrochem. Bioenerget. 1978, 5 , 686-696. (29C) Van Duyne, R. P.; Suchanski, M. R.; Lakovits, J. M.; Siedle. A. R.; Parks, K. D.: Cotton. T. M. J . Am. Chem. Soc. 1979, 101, 2832-2837. (30C) Wallace, W. L.; Jaeger, C. D.; Bard, A. J. J . Am. Chem. Soc. 1979, 107, 4840-4843. (31C) Albrecht. M. G.; Creighton, J. A. Electrochim Acta 1978. 2 3 , 1103-1 105. (32C) Hagen. G.; Glavaski, B. S.; Yeager. E. 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(34C) Pieterse, M. M. J.; Siuyters-Rehbach, M.; Siuyters. J. H. J . Electroanal. Chem. 1978. 91. 55-62. (35C) Adzic, R.'R.; Tripkovic, A. V. J . Electroanal. Chem. 1979, 99,43-53. (36C) Takamura, K.: Mori, A.; Watanabe. F. J . Electroanal. Chem. 1979, 102, 109-1 16. (37C) Adzic, R . R.; Markovic, N. M. J . Electroanal. Chem. 1979, 102, 263-273. (38C) Kunimatsu, K.; Parsons, R. J . Electroanal. Chem. 1979. 100,335-363. (39C) Fujihira, M.; Osa, T.; Hursh. D.; Kuwana, T. J . Electroanal. Chem. 1978. 88,285-288. (40C) Fischer, A. E.; Kinney, J. E.; Staley, R. H.; Wrighton, M. S. J . Am. Chem. SOC. 1979. 101. 6501-6506. (41C) Iwasaki. T.; Sawada, T.; Kamada, H.; Fujishima, A,; Honda, K. J . Phys. Chem. 1979, 83,2142-2145. (42C) Baker, E. G.; Rand, D. A. J.; Woods, R. J . Electroanal. Chem. 1979, 97. 189-198. (43C) 'Civiiier, J.; Chauvineau, J. P. J . Electroanal. Chem. 1979, 97,199-210; 100,461-472. (44C) Felter, T. E.; Hubbard, A. T. J . flectroanal. Chem. 1979, 100,473-491. 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(39D) Turner. D. R.; Whitfieid, M. J . Elecfroanal. Chem. 1979, 103,43-60; 61-79. (40D) Poldoski. J. E.; Glass, G E. Anal. Chim. Acta 1978. 101, 79-88. (41D) Bonelli. J. E.; Skogerboe, R. K.; Taylor, H. E. Anal. Chim. Acta 1978, 10 1 , 437-440. (42D) Figura, P.; McDuffie, E. Anal. Chem. 1979, 51, 120-125. 43D) Ciern, R . G.; Hodgson, A. T. Anal. Chem. 1978, 5 0 , 102-110. (44D) Lund, W.; Eriksen, R. Anal. Chlm. Acta 1979, 107,37-46. (45D) Golirnowski, J.; Vaienta. P.; Stoeppler, M.; Nurnberg, H. W. Talanta 1979, 26, 649-656. (46D) Oehme, M.; Lund, W.; Jonsen. J. Anal. Chim. Acta 1978. 100, 389-398. (47D) Davis, P. H.: Duiude, G. R.: Griffin, R. M.; Matson. W. R.; Zink. E. W. Anal. Chem. 1978, 50, 137-143. (48D) Bilewicz, R.; Stojek, 2.; Kublik. Z . J . Electroanal. Chem. 1979, 96, 29-44. (49D) Ogura, K.; Enaka. Y. J . Electroanal. Chem. 1979, 95, 169-175. (50D) Hamilton, T. W.; Ellis, J.; Florence, T. M. Anal. Chim. Acta 1979, 110, 87-94. (51D) Kopanica, M.; Stara, V. J . flectroanal. Chem. 1978, 91,351-357. (52D) Bloom, H.; Nolier, 8. N.; Richardson, D. E. Anal. Chim. Acta 1979, 109, 157- 160. (53D) Glodowski, S.; Kublik. Z . Anal. Chim. Acta 1979, 104,55-65. (54D) Stulik, K.; Beran, P.; Dolezai, J.; Opekar, F. Talanta 1978, 25,363-369. (55D) Tench, D.; Ogden, C. J . Electrochem. SOC. 1978, 125, 194-198. (56D) Gajan, R. J. FDA By-Lines 1978, 9 , 113-130. (57D) Florence, T. M. J , Electroanal. Chem. 1979, 97,219-236. (58D) Florence, T. M. J . Electroanal. Chem. 1979, 97,237-255. (59D) Florence, T. M. Anal. Lett. 1978, B f 1 , 913-924. (60D) Davidson, I . E.;Smyth, W. F. Anal. Chem. 1979, 51, 2127-2133. (61D) Forbes, S.; Bounds, G. P.; West, T. S. Talanta 1979, 26, 473-477. (62D) Monama, T.; Duyckaerts, G. Anal. Lett. 1979, 12,219-229. (63D) Bock, R.; Zimmer. E.; Weichbrodt, G. Z . Anal. Chem. 1978, 293, 377-387. (64D) Lund, W.; Bye, R. Anal. Chim. Acta 1979, 110,279-284. (65D) Wundt, K.; Duschner, H.; Starke, K. Anal. Chem. 1979, 51,1487-1492. (66D) Barker, G. C.; Gardner A. W. J . Electroanal. Chem. 1979, 100, 64 1-656. (67D) Franke, J P.; de Zeeuw, R. A.; Hakkert. R . Anal. Chem. 1978, 50, 1374- 1380. (68D) Verpiaeste, H.; Donche, H.; Temmerman, E.; Verbeek. F. J . Electroanal. Chem. 1978. 93,213-219. (69D) Oehme, M.; Lund, W. Fresnius' Z . Anal: Chem. 1979, 298,260-268. (70D) Anal. Chem. 1979, 51, 1329A-1330A. VOLTAMMETRIC TECHNIQUES (1E) Drake, K . F.; Van Duyne, R. P.; Bond, A. M. J . Electroanal. Chem. 1978, 89, 231-246. (2E)7605-761 Cotton, 1.T. M.; Van Duyne, R. P. J . Am. Chem. SOC. 1979, 107, (3E) Lennox, J. C.; Murray, R. W.. J. Am. Chem. SOC.1978, 100,3710-3714. (4E) Koval, C. A.; Anson, F. C. Anal. Chem. 1978, 50, 223-229. (5E) Oyama, N.; Yap, K. E.; Anson, F. C. J . Electroanal. Chem. 1979, 100, 233-246. (6E) Weber, J. H.; Cheng, K. Anal. Chem. 1979, 51, 796-799. (7E) Yeh, P.; Kuwana. T. Chem. Lett. 1977, 1145-1148. (8E) Lane, R. F.: Hubbard, A. T.; Biaha, C. D. Bioelectrochem. Bioenerg. 1978, 5. 504-525. (9E) Ponchon, J.-L.; Cespugiio, R.; Gonon, F.; Jouvet, M.; Pujol, J.-F. Anal. Chem. 1979, 51, 1483-1486. (10E) Dieker. J. W.; van der Linden, W. E.; Poppe, H. Talanta 1978, 25, 151-155. (11E) Karber, L. G.; Dryhurst, G. Anal. Chim. Acta 1979, 108, 193-204. (12E) Muson, J. W.; Abdine, H. Talanta 1978, 25, 221-222. (13E) Foley. L.; Kimmerle, F. M. Anal. Chem. 1979, 51, 818-822. (14E) Dryhurst, G.; Karber, L. G. Anal. Chim. Acta 1978, 100, 289-300. (15E) Desideri, P. G.; Heimler, D.; Lepri, L. J . Electroanal. Chem. 1978, 87, 275-282. (16E) Chan, H. K.; Fogg, A. G. Anal. Chim. Acta 1979, 109, 341-349. (17E) Burrows, K. C.; Hughes, M. C. Anal. Chlm. Acta 1979, 170,255-260. (18E) Deldime, P.; Jacobsberg, E.; Belhassine, M. Anal. Lett. 1978, A l l , 63-72. (19E) Laviron. E . J . flectroanal. Chem. 1979, 100,263-270. (20E) Diaz, A. F.; Kanazawa. K. K. J. Electroanal. Chem. 1978, 86,441-444. (21E) Eddowes, M. J.; Hili, H. A. 0. J . Am. Chem. SOC. 1979, 101, 446 1-4464. (22E) Alexander, R.; Kinseila, B.; Middieton, A. J . Electroanal. Chem. 1978, 93, 19-27. (23E) Proska, E.; Moinar. L. Anal. Chim. Acta 1978, 97, 149-154. (24E) Miller. I. R.; Werber. M. M. J . Electroana/. Chem. 1979, 100,103-110. (25E) Martin, G. W.; Murray, R. W. J . Electroanal. Chem. 1979, 149-157. (26E) Suib. S. L.: Faulkner, L. R.; Stucky. G. D.; Biattner, R. J. Anal. Chem. 1979, 51, 1060-1064. (27E) Mohammad, M.; Razaq, M. J . Electroanal. Chem. 1979, 98,335-338. POLAROGRAPHIC TECHNIQUES (1F) Johnson. D. C. Anal. Chem., preceding paper in this issue. (2F) Bond, A. M.; Grabaric, E. S. Anal. Chem. 1979, 51,337-341. (3F) Prue, D. G.; Gernmiil, F. Q., Jr ; Johnson, R . N. Anal. Chim. Acta 1979, 107,59-66. (4F) Fogg, A . G.; Fayad, N. Anal. Chim. Acta 1978, 102, 205-210. (5F) Jemal, M.; Knevei, A. M. J . Electroanal. Chem. 1979, 95, 201-210. (6F) Jemal, M.; Knevel, A. M. Anal. Chem. 1978, 5 0 , 1917-1921. (7F) Fogg, A. G . ; Fayad, N. M.; Burgess, C.; McGlynn, A. Anal. Chlm. Acta 1979, 108,205-211.
Anal. Chem. 1980, 52, 151 R-161 R (8F) Vire, J.-C.; Patriarche, G. J.: Patriarche-Sepulchre, J. Anal. Lett 1978, 8 1 1 , 681-695. (9F) Johansson, B.-L.: Persson, B. Anal. Chim. Acta 1978, 702, 121-131. (IOF) Jacobsen, E.; Korvald, B. Anal. Chim. Acta 1978, 99, 255-261. ( 1 1F) de Boer, H. S.; den Hartigh, J.: Ploegmakers, H. H. J. L.: van Oort, W. J. Anal. Chim. Acta 1978, 102, 141-155. (12F) de Boer, H. S.; Lansatt, P. H.; van Oort, W. J. Anal. Chim. Acta 1979, 708,389-393. (13F) de Boer, H. S.; Lansatt. P. H.: Kooistra, K. R.: van Oort, W. J. Anal. Chim. Acta 1979, 1 7 1 . 275-279. (14F) Chatten, L. 6.:Moskalyk. R. E.; Locock. R. A,; Schaefer, F. J. Analyst (London) 1978, 103. 837-841. (15F) Smyth, M. R.; Smyth, W. F. Analyst (London) 1978, 103, 529-567. (16F) Smyth, M. R.; Osteryoung, J. G. Anal. Chim. Acta 1978, 96, 335-344. (17F) Smyth, M. R.; Osteryoung, J. G. Anal. Chem. 1978, 5 0 , 1632-1637. (18F) Samuelsson, R. Anal. Chlm. Acta 1978, 102, 133-140; 1979, 108, 2 13-219. (19F) Smyth, M. R.; Lawellin. D. W.: Osteryoung, J. G. Analyst(London) 1979, 104, 73-78. (20F) Fogg, A. G.; Ahmed, Y . 2. Anal. Chim. Acta 1978, 701, 211-214. (21F) Henry. F. T.; Kirch, T. 0.;Thorpe, T. M. Anal. Chem. 1979, 57, 215-218. (22F) van Leeuwen, H. P. J . Electroanal. Chem. 1979, 99, 93-102. (23F) Kirowa-Eisner, E.; Osteryoung, J. Anal. Chem. 1978, 5 0 , 1062-1066. (24F) Elton, R. K.; Geiger, W. E . , Jr. Anal. Chem. 1978, 50, 712-717. (25F) Lowry, J. H.; Smart. R. B.: Mancy, K. H. Anal. Chem. 1978. 50, 1303-1309. (26F) Masschelein, W. J.: Denis, M.; Ledent, R. Anal. Chim. Acta 1979, 707, 383-386. (27F) Kopanica, M.; Stara, V. J . Electroanal. Chem. 1979, 98, 213-221. (28F) Stara, V.; Kopanica, M. J . Electroanal. Chem. 1979, 101, 171-175. (29F) Bosserman, P.; Sawyer, D. T.; Page, A. L. Anal. Chem. 1978, 50, 1300-1303 .- - - . - (30F) Christian, G. D., Vandenbalck. J L ; Patriarche, G J. Anal C h m Acta 1979. 108. 149-154. (31F) Alam, A. M. S.;Vittori, 0.;Porthault, M. Anal. Chem. Acta 1978, 702, 113-1 19. (32F) Hitchen, A. Talanta 1979, 26, 369-372. (33F) Greter, F.-L.; Buffle, J.; Haerdi, W. J . Electroanal. Chem. 1979, 101, 21 1-229. (34F) van Leeuwen, H. P. Anal. Chem. 1979, 5 1 , 1322-1323. (35F) Fogg, A. G.; Osakwe, A. A. Talanta 1978, 2 5 , 226-228. (36F) Alam. A. M. S.: Martin, J. M.; Kapsa, Ph. Anal. Chim. Acta 1979, 107, 39 1-393. (37F) McLean J. D.; Stenger, V. A.; Reim. R. E . ; Long, M. W.; Hiller, T. A. Anal. Chem. 1978, 50, 1309-1314. (38F) Fogg, A. G.; Fayad, N. M.; Burgess, C. Anal. Chim. Acta 1979, 110, 107- 115. (39F) Bruno, P.; Caselli, M.; Monica, M. D.; di Fano, A. Talanta 1979, 2 6 , 1011-1014. (40F) Hu. H.-C. Anal. Chim. Acta 1979, 707, 387-390
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(41F) Hanck, K. W.; McGaughey, J. F. Anal. Chim. Acta 1979, 107, 75-82. (42F) Niki, K.; Yagi, T.: Inokuchi, H.; Kimura, K. J . Am. Chem. SOC.1979, 10 1 , 3335-3340. (43F) Canterford, D. R.; Taylor, R. J. J . Electroanal. Chem. 1979, 98, 25-36. (44F) Schaar, J. C.: Smith, D. E . J . Electroanal. Chem. 1979, 100, 145-157. (45F) Jacobsen, E . ; Bjornsen. M. W. Anal. Chlm. Acta 1978, 96, 345-351. (46F) Nagaosa, Y . Talanta 1979, 26, 987-990. (47F) Canterford, D. R. Anal. Chim. Acta 1978, 98, 205-214. (48F) Ficker. H. F . ; Ostensen, H. N.; Schlossei, R. H.; Scott, F.: Spritzer, M.; Meites, L. Anal. Chlm. Acta 1978, 98, 163-169. (49F) Green, J. B.; Manahan, S. E . Anal. Chem. 1979. 5 1 , 1126-1129. (50F) Hammerschmidt, R. F.: Broman, R. F. J . Hectroanal. Chem. 1979, 99, 103- 110. (51F) Clark, G. C. F.; Moody, G. J.; Thomas, J. D. R. Anal. Chim. Acta 1978, 98,215-220. (52F) Brunt, K. Anal. Chim. Acta 1978, 98, 93-99. (53F) Rao, V. S. N.; Rao, S. B Talanta 1979, 26, 502-504. (54F) Squella, J. A.; Nunez-Vergara, L. J. Talanta 1979, 26, 1039-1040. (55F) Braun, R. D.; LoVerso, M. R . Talanta 1979, 2 6 , 185-188. (56F) Toropova, V. F.; Budnikov, R. G. K.; Ulakhovich, N. A. Talanta 1978, 25, 263-267. (57F) Smyth, W. F.; Smyth, M. R.; Groves, J. A.; Tan, S. B. Analyst(London) 1978. 103, 497-508. (58F) Rozanksi, L. Analyst (London) 1978. 103, 950-954. (59F) Watson, A. Analyst (London) 1978, 103. 332-340. MISCELLANY
(IG) Weber, S. G.; Purdy, W. C. Anal. Lett. 1979, 12, 1-9. (28) Heineman, W. R.; Anderson, C. W.; Halsall; H. B. Science 1979, 204, 865-866. (3G) Flanagan, J. B.; Margel, S.: Bard, A. J.: Anson. F. C. J . Am. Chem. SOC. 1978, 100. 4248-4253. (4G) Saji, T.; Pasch, N. F.; Webber, S. E.; Bard. A. J. J , Phys. Chem. 1978, 8 2 , 1101-1 105. (5G) Fenton, D. E . ; Schroeder. R. R.; Lintvedt. R. L. 1978, 100, 1931-1932. (6G) Ryan, M. D.; Wei, J.-F.; Feinberg, 8. A.; Lau, Y.-K. Anal. Blochem. 1979, 96,326-333. (7G) Hernandez-Mendez, J.; Sanchez-Perez, A.: Rubio-Miron. A. Anal. Lett. 1979, 72,1315-1337. (8G)Malpas, R. E.: Fredlein. R. A.; Bard, A. J. J . Electroanal. Chem. 1979, 98, 171-180. (9G) Malpas, R. E.; Fredlein, R. A.; Bard, A. J. J . Electroanal. Chem. 1979, 98,339-343. (10Gl Nelsen. S. F.; Clennan, E. L.; Evans, D. H. J . Am. Chem. SOC.1978, 100, 4012-4019 (11G) Beztlla, B. M.. Jr.; Maloy, J. T J Electrochem SOC 1979, 126, 579-583 (12G) Tinker, L. A. and Bard, A J J Am Chem SOC. 1979, 101, 23 16-23 19
Titrations in Nonaqueous Solvents Byron Kratochvil Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
This review covers approximately the literature reported in Chemical Abstracts over the period December 1977 to December 1979. As in previous reviews, emphasis has been placed on fundamental developments and on methodology, followed by those applications that appear to illustrate new approaches or trends. Space limitations have forced deletion of many references in both fundamental and applied areas, and inevitably many quality papers have had to be omitted. Overall, a small but noticeable decrease in the number of applications of nonaqueous titrations seems to be occurring. This is especially apparent in the area of pharmaceutical analysis, where nonaqueous acid-base titrations are being replaced in many cases by liquid chromatography. On the other hand, an increase in the use of constant current coulometry in nonaqueous solvents is evident. In fundamental studies, considerable work is being done on both ion-solvent interactions and on the behavior of polar neutral solutes. Much of the interest in this area is kindled by the importance of nonaqueous solvents in organic synthesis, where reaction solvents are still selected primarily on an empirical basis. Progress in explaining solvent effects on ionic and molecular reactions is slow, but a large number of experimental tools 0003-2700/80/0352-15lR$Ol .OO/O
are being brought to bear on the problem, and much information is being obtained. This area is likely to remain important to synthetic and solution chemists for some time to come. Its impact on analytical methods has not yet been felt, but is beginning to appear in new methods for organic functional group analysis, for example.
BOOKS AND GENERAL REVIEWS Books and reviews of relative breadth that have appeared over the period covered by this review are included here; those that concentrate on specific areas are included under the appropriate subject headings of this review. Two more volumes, have appeared in the series “The Chemistry of Nonaqueous Solvents”. Volume VA contains a chapter on solvation and complex formation in protic and aprotic solvents ( 1 1 ) ,and one on solvent basicity (33). T h e proceedings of a 1976 symposium on spectroscopic and electrochemical characterization of solute species in nonaqueous solvents (184),and of a 1977 symposium on solvent parameters (142),have appeared. Friedman has reviewed developments in the area of structure of electrol>Te solutions over the period 1952-1977 (88). Another survey treats electrolyte solutions C 1980 American Chemical Society
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