a/. Chem., 46, 391-7 (1973). (25F) Kies, H. L., Den Os, M., Anal. Chlm. Acta. 67, 246-50 (1973). (26F) Klein, N.. Yarnitzky, C.. J. Electroanal. Chem., 61, 1-9 (1975). (27F) Kojima. H., Bard, A. J., J. Electroanal. Chem., 63, 1 17-29 (1975). (28F) Kretscmer, K. J., Hamann, C. H., Fassbender, E., J. Electroanal. Chem., 60, 231-4 (1975). (29F) Lamy, C.. Herrmann, C. C., J. Elecfroana/. Chem.. 59, 113-34 (1975). (30F) Lindstrom, M., Sundholm, G.. Finn. Chem. Lett., 27-30 (1975). (31F) Magno, F., Bontempelli, G., Mazzocchin, G. A,, Patane, I., Chem. Instrum., 6, 239-57 (1975). (32F) Overton. M. W., Alber, L. L.. Smith, D. E., Anal. Chem., 47, 363A (1975).
(33F) Poojari, A,, Rajagopalan, S. R., Rangarajan, S. K., Trans. SOC. Adv. Electrochem. Scl. Techno/.,6, 147-53 (1973). (34F) Poojary, A., Rajagopalan, S. R., J. flectroanal. Chem., 62, 51-8 (1975). (35F) Ramaiey, L., Chem. Instrum., 6 , 119 (1975). (36F) Senda, M.. Ikeda. T., Rev. Polarogr., 19, 51-5 (1973). (37F) Shabrang, M.. Bruckenstein, S., J. flectrochem. SOC.,121, 1439-44 (1974). (38F) Shabrang. M., Bruckenstein, S., J. flectrochem. SOC.,122, 1305 (1975). (39F) Sherwood, W. G., Untereker, D. F., Bruckenstein, S., Anal. Chem., 47, 84-8 (1975). (40F) Sturrock, P. E., Hughey, J. L., Vaudreuii, E., O’Brien, G. E., Gibson, R. H., J. Electrochem. SOC.,122, 1195-200 (1975). (41F) Thomas, L. C.. Christian, G. D., Daniel-
son, J . D. S . , Anal. Chim.. Acta, 77, 163-9 (1975). (42F) Untereker, D. F., Sherwood, W. G., Martincheck. G. A,, Reidhammer, T. M.. Bruckenstein, S., Chem. Instrum.,6, 259-66 (1975). (43F) Vittori, O., Porthault, M., Bull. SOC.Chlm. Fr., 11,Pt. 1. 2411-14(1974). (44F) Vassos, E. H., Osteryoung, R. A,, Chem. Instrum., 5 , 257-70 (1974). (45F) Willems, G. G.. Neeb, R., Fresenlus Z. Anal. Chem., 269, 1-10 (1974). (46F) Woodward, W. S., Ridgway, T. H., Reilley, C. N., Anal. Chem., 46, 1151-4(1974). (47F) Yarnitzky, C., J. Electroanal. Chem.. 5 1 , 207-10 (1974). (48F) Yarnitzky, C., Friedman, Y., Anal. Chem., 47,876-80 (1975). (49F) Yarnitzky, C., Klein, N.. Anal. Chem., 47, 880-4 (1975).
Analytical Electrochemistry: Methodology and Appl cat ons of Dynamic Techniques Peter T. Kissinger Department of Chemistry, Purdue University, West Lafayette, Ind. 4 7907
I t behooves us to begin this piece on a Bicentennial note, however, it was not until April of 1800 that William Nicholson and Anthony Carlisle first used Volta’s pile (1794) t o demonstrate electrochemical decomposition of water into hydrogen and oxygen. Although they concluded that the observation “seems to point a t some general law of the agency of electricity in chemical operations” ( I A ) , it was not until 1834 that this general law was documented ( 2 A ) . I t is clear then that electroanalytical chemistry was not part and parcel of the Colonial spirit. The situation is not dramatically different today; however, there are a number of favorable signs. The trend away from the cloistered academic fraternity of the 1950’s and 1960’s continues unabated. The users of modern techniques now far outnumber the several dozen proponents of only a few years ago. I t is clear that the Laplace transform-operational amplifier-digital simulationcomputer revolution is over and that finite current electrochemical methods are, in fact, finding many useful chemical applications. With this maturity, there has been a fall off in the rate of technique development. Over the past four years, this reviewer has found it increasingly difficult to ferret out those contributions which are more than an exercise in technique and truly hold promise for valuable future applications. There are quite a few “wheels” being reinvented, but little in the way of really novel stuff. Most of the exciting action has moved into the applications area. As in the past, the emphasis here will be directed toward nonelectronic experimental ideas and unique applications ( 3 A ) .The references listed were selected from several thousand possibilities based on the author’s subjective view. Chemical Abstracts, Chemical Titles, and the Interface Newsletter were searched from January 1974 through December 1975. Several books have been published recently which deal with the experimental aspects of electrochemistry. Sawyer and Roberts have described “Experimental Electrochemistry for Chemists” in a manner which will make it easier for “outsiders” to join in the application of electrochemistry to various chemical problems ( 4 A ) . For organic chemists Weinberg has edited an extensive two-volume treatise dealing with the “Technique of Electroorganic Synthesis” ( 5 A ) . Rifi and Covitz have written a much shorter introduction to the same subject ( 6 A ) .Gileadi and coworkers introduce
“Interfacial Electrochemistry” from the physical chemists’ point of view ( 7 A ) .Although limited in scope, this book is very readable and has many excellent sections. Meites and Zuman have organized a major work entitled “Electrochemical Data” ( 8 A ) . Anyone who doubts that this is an ambitious undertaking will be convinced by examination of the first volume on the electrochemical behavior of organic, biochemical, and organometallic substances. Data have been selected and compiled from publications for the 12year period from 1960 through 1971. The sections which follow are intended to provide only a loose structural framework for this review since many of the references cited contain information related to more than one section. ELECTRODES AND CELLS It’s often been said that electrochemistry would be useful if it weren’t for the electrodes. This shortsighted cynicism is popular with those who remember electrochemistry as clogged capillaries and dried-out agar salt bridges. These times are gone, but we really still don’t know very much about the surface chemistry of electrodes and how this influences electrode processes. Detailed electrochemical studies of surface adsorption and its effects are now really beginning to make some headway. Anson, for example, has recently reviewed his towering efforts in this regard ( I B ) . The application of spectroscopic techniques (see below) is also becoming more sophisticated and correlations with electrode kinetics are just now becoming practical. An entirely new approach has recently begun to surface. Why not prescribe the interface you want and synthesize it by covalently binding appropriate molecules to an electrode substrate? One of the neatest ideas to hatch in a few years is the “chiral electrode” devised in Larry Miller’s lab a t Fort Collins ( 2 B ) . In this case, an optically active amino acid was linked to carboxylic acid sites on air-oxidized graphite. The nice stable amide bonds afford a chiral environment in the interphase. Electrodes with both R and S configurations were prepared and demonstrated to produce optically active electrolysis products from inactive starting materials. This work is the first chapter in what promises to be one 9f the most exciting areas in preparative organic electrochemistry. ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1 9 7 6
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Royce Murray et al. have also been doing some tricky things, in this case with chemically modified tin oxide electrodes ( 3 B ) .This substrate ought to be more controllable than graphite and thus better suited to detailed surface studies. Royce takes the lead from chromatographers by binding organosilanes to surface hydroxyls. The goal is to facilitate metal ion electrochemistry by incorporating various amine ligands into the interphase. Electrodes have been successfully modified and characterized by both cyclic voltammetry and ESCA. There are so many neat things to try with these electrodes that they undoubtedly will play a big part in the 1978 review, Several areas of bioengineering are banking on the successful use of chronically implanted electrodes both as sensors and stimulators. For example, if we wish to replace some of the more complex human organs with artificial devices, it becomes essential that the artificial organ be capable of monitoring and responding to the biochemical status of the patient. A case in point is the work under way to develop an artificial pancreas for diabetics ( 4 B ) .In this effort it is necessary to have rapid continuous monitoring of blood glucose in a critically damped feedback loop so that the swing between hypoglycemia and hyperglycemia can be minimized. There is only one finite current electrode which has been widely used in vivo and its impact is not often recognized by analytical chemists. Leland Clark made one of the most important contributions since sliced bread when he developed the membrane-covered amperometric oxygen electrode in 1956. Since oxygen is just about the only electroactive molecule capable of diffusing across a hydrophobic membrane, the “Clark electrode” achieves extraordinary specificity without resorting to any fancy tricks. In the past two years, there may well have been several thousand papers published in which this idea was put to use. Every hospital clinical laboratory has one for blood poQ measurements and many operating rooms use micro p o Z electrodes for continuous monitoring. The environmental applications are also legion. The Clark electrode stimulated efforts to devise similar probes for other species, but these have necessarily involved engineering complications. Clark himself first reported on the use of enzymes in amperometric transducers in 1962. Since that time electrodes have been developed for cholesterol (5B),glucose, disaccharides, ethanol, methanol, uric acid, and several others. One approach of general utility is to take advantage of peroxide generating oxidase enzymes. At, least 40 such enzymes have been characterized. The amount of substrate (S) converted to product (P)can be related to the oxygen consumed or the peroxide produced. oxidase
S+O2 e P+H202
Both approaches have been used in the implantable glucose sensors based on glucose oxidase. The peroxide can be monitored by oxidation. This sometimes presents problems because other species (e.g., ascorbate) are likely to interfere. Clark has solved this difficulty to some extent by a differential measurement wherein one of the electrodes does not include the enzyme (6B).A more satisfactory approach is to incorporate a membrane with a very small pore size into a single electrode (5B). Me11 and Maloy have employed digital simulation to model the behavior of steadystate immobilized glucose oxidase enzyme electrodes of the type developed by Guilbault and Lubrano ( 7 B ) .The model was successful in predicting a number of experimental observables. The oxidase catalyzed reactions can also be monitored by decomposition of HzOz to 0 2 . The latter can then be measured using the traditional highly selective 0 2 electrode. The conversion to 0 2 might be carried out by catalase, but the use of an inorganic catalyst is perhaps more practical from an engineering point of view. Updike et al. have very recently advocated the latter approach ( 8 B ) . Wingo and Emerson have recommended using the catalase-catalyzed decomposition of hydrogen peroxide to calibrate amperometric p o 2 electrodes (9B). Nanjo and Guilbault have used a reaction-rate method for oxidase enzymes based on measurement of the disap18R
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pearance of dissolved oxygen. The initial rate of oxygen consumption is linearly dependent on substrate concentration over a limited range. This idea developed into electrodes for uric acid (IOB) and for alcohols, aldehydes, and carboxylic acids ( I I B ) . Yet another approach to following oxidase enzymes is to use a redox mediator (see Hydrodynamic Techniques below). Schlapfer et al. have adopted this trick for blood glucose measurements (12B). Stulikova and Stulik have reviewed the use of carbon electrodes in electroanalytical chemistry including ordinary graphite, pyrolytic graphite, glassy carbon, and carbon pastes (13B). Lindquist described the voltammetric properties of seven different carbon paste formulations with respect to several model systems (14B).Bauer et al. formulated carbon paste with solid electroactive compounds in the paste mixture and concluded that the solids could be electrolyzed directly ( I 5 B ) . Levy and Farina developed an improved technique for mounting glassy carbon (16B) and Bond et al. described the application of this material in hydrofluoric acid (17B). Virtually all aspects of controlled-potential coulometry for analytical purposes have been reviewed by Harrar in the latest of the volumes edited by Bard (18B).Both experimental considerations and applications are covered in detail as indicated by the 683 references cited. Harrar and coworkers have recently described the use of fluoropolymer ion-exchange membranes as separator tubes for electrolysis cells ( I 9 B )and a new platinum electrode controlled-potential coulometry cell (20B).Geiger et al. described a complicated yet functional vacuum electrolysis cell for studies of nickel(1) species produced by electroreduction of nickel dithiolenes (21B).Their cell should be generally useful where rigorous exclusion of oxygen and moisture is desirable. Both preparative and diagnostic electrochemical experiments were carried out.
MEDIA Parker et al. have made significant strides toward stabilizing radical ions in nonaqueous electrolyte solutions. Their approach involves scavenging water by addition of trifluoroacetic anhydride ( I C ) or suspended alumina (SC, 3C). The former reagent is used for enhancing cation lifetimes, and the latter is used when anion radicals are generated. Parker continues to pursue the question of how to distinguish between disproportionation and classical ECE mechanisms ( 4 C ) . In a particularly nice series of papers, this same group examined the mechanism of some interesting anodic coupling reactions of hydrocarbons and methoxy derivatives (32).Evidence is presented for intramolecular cyclization reactions involving disproportionation of the radical cation to form a dication-diradical and a molecule of starting material. Certainly, an attractive feature of these papers is that many approaches are brought to bear on the mechanistic questions, including a study of temperature effects. Several groups have begun to follow up on the low temperature work of Van Duyne and Reilley ( 3 A ) . Grypa and Maloy have used low temperature cyclic voltammetry (6C) and pOtential pulse techniques (7C) to study the electrohydrodimerization of radical anions in DMF. They have described what appears to be a very practical low temperature cell for use on a vacuum line (@). In an interesting twist, Nelsen et al. have used low temperature cyclic voltammetry to detect conformational equilibria for tetraalkylhydrazines in butyronitrile (BC). For some compounds, two anodic waves show up a t low temperatures (typ. -55 “C) where only a single reversible oxidation occurred a t room temperature. Kinetic and thermodynamic data are accessible and correlations of voltammetric and NMR work will be of great value in understanding what’s happening. Study of the influence of conformation on electrode reactions is another promising way to probe the fundamentals of the heterogeneous electron transfer. Smith and Bard have continued to explore electrochemical reactions of organic compounds in liquid ammonia and have proved it to be a practical medium for voltammetric studies. Nitrosobenzene and nitrobenzene were both found to be reversibly reduced in two one-electron steps (9C). Both the anion radicals and dianions were found to be stable unless a weak (isopropanol) or strong (ammonium ion)
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 as a clinical chemist, Kissinger taught for three years at Michigan State University following two years of postdoctoral study with Professor Ralph Adams at the University of Kansas. Dr. Kissinger has research interests in neuropharmacology, the rates and mechanisms of organic electrode reactions, electrochemistry in microliter volumes including thin films, and synergistic combinations of chromatography and spectroscopy with electroanalytical techniques. He edits a popular newsletter, Interface, for the international electrochemical fraternity and is collaborating on two books. Outside of the academic sphere, Dr. Kissinger is president of Bioanaiytical Systems Inc., a manufacturing and consulting firm specializing in trace organic analysis and electrochemical instrumentation. He is a member of the ACS. AAAS, American Society for Mass Spectrometry, and the American Association of Clinical Chemists.
acid was added. In both cases, the events following protonation were studied in detail. Considering that nitrobenzene was the first, or one of the first, organic compounds to ever see a dropping mercury electrode, it’s about time that the details of its reduction are being worked out. In a subsequent paper, the same two workers described the reductive alkylation of quinoline by reaction of its anion radical with alkyl bromides in an ECEC sequence (1OC). Molten salts are becoming less dangerous for those who really get into their work. Ford sings the praises of triethyln-hexylammonium triethyl-n-hexylboride (or N2226 B2226) as an ambient temperature molten salt solvent (11C). It’s clear from his preliminary voltammetric experiments that this is a medium deserving further study. Chum et al. have done some very nice things in another room temperature molten salt, 2:l aluminum chloride-ethylpyridinium bromide (122). One of their tricks was to produce the hexamethylbenzene cation radical and show its existence by cyclic voltammetry. Mamantov’s group has been hot in the area of chloroaluminate melts. An excellent case in point is their thorough study of zirconium(1V) electrochemistry in AlC13-NaCl a t 175-220 O C ( 1 3 C ) . Fung and Mamantov have very recently written a chapter which is a good place to start learning about what has been and can be done with electroanalytical techniques in molten salts (14C). Water is still good stuff, but purifying it can be a real problem. Hassan and Bruckenstein describe a method for removing impurities from aqueous electrolyte solutions by passing the solution through a column of reduced platinum sponge a t a negative potential (15C). Another problem with water is that it is not the universal solvent as originally advertised. Some improvement can be made by the use of surfactants to solubilize electroactive materials in micelles. Fujihira et al. have used a non-ionic detergent (Tween 20) to solubilize ferrocene in aqueous buffers (16C). The ferricinium-ferrocene couple is capable of rapid electron transfer, both with electrodes and heme proteins and therefore serves nicely as a redox mediator between the two. Erabi e t al. have described some dc polarography experiments on ubiquinone-10 solubilized in micelles formed from the anionic surfactant sodium dodecyl sulfate (17C).
SPECTROSCOPIC TECHNIQUES Vibrational spectroscopy of materials a t or near electrode surfaces has been developing rather slowly because of serious instrumentation problems. Mattson and Smith have made significant progress by preparing a new carbon film electrode for use in infrared internal reflectance experiments ( I D ) .Glassy carbon was evaporated and deposited on a germanium IRS element. The resulting electrodes have sufficiently low absorption coefficients in the IR, conduct satisfactorily, and exhibit a wide usable potential range. Several Russian workers have devised a means of carrying out transmission IR on platinum electrodes in a vacuum following direct transfer from the electrochemical cell
(20). The intensity of the Pt-0 and Pt-OH vibrational modes was studied as a function of electrode potential. Laser Raman spectroscopy may ultimately be more useful than IR absorption for surface studies. Fleischmann e t al. have studied the potential dependence of pyridine adsorption on a silver electrode ( 3 0 ) . The sensitivity of ordinary Raman spectroscopy is unfortunately not very good. One can do nice things with small amounts of sample, but the concentration must be very high ( > O . l M) to achieve good signal-to-noise. The best spectroelectrochemistry paper in several years was written by Jeanmaire, Suchanski, and Van Duyne of Northwestern University ( 4 0 ) . They described “resonance Raman spectroelectrochemistry” (RRSE) which permits vibrational information to be obtained under conventional electrochemical conditions (millimolar concentrations, bulk metal electrodes). This is possible because the resonance Raman effect can enhance the scattering cross section by a millionfold in some cases. The Northwestern group demonstrated the power of RRSE using electrogeneration of the tetracyanoethylene anion radical in acetonitrile. An argon ion laser beam was aimed a t a small (1 X 4 mm) platinum electrode and the Raman scattered light was collected. Spectra were recorded under steady-state conditions and the sensitivity was such that even Raman intensity transients could be followed for single-shot potentialpulse experiments! There are, no doubt, many interesting directions to pursue from this beautiful piece of work. There have been many published applications of spectroelectrochemical techniques in the UV-Vis region over the past two years and it would appear that this kind of experiment is routine in a number of laboratories. A review has been published (50) as have several papers dealing with cytochrome c (60, 7 0 ) . Ryan and Wilson have made the point that most theoretical work in spectroelectrochemistry has focused only on the time course of the primary electron transfer product ( 7 0 ) . Since the radical ions most often studied to date have spectral transitions well removed from those of solvent or precursor, this emphasis has had an experimental justification as well as a mathematical excuse. Now that more complex systems are being studied, the opportunity for spectral overlap is far more severe, and it may be necessary to know the time dependence of the absorbance for all species a t several wavelengths. Karweik has described a vacuum line spectroelectrochemical cell which he used to study reproportionation rates for the reaction of zinc tetraphenylporphyrin with its dication in butyronitrile ( 8 0 ) . Chronoabsorptometry measurements were undertaken a t microsecond times using internal reflectance a t vapor deposited platinum films. Heineman’s team a t Cincinnati has come up with an extremely simple, yet very powerful, new application for optically transparent thin-layer electrochemical cells ( 9 0 ) . In the narrow confines of a T L E cell a t a fixed potential, a redox cou le can equilibrate rather quickly so that the ratio of [Oy to R is Nernstian throughout the entire sample solution. If 0 and [R] are stable and can be monitored optically as a function of applied potential, it is possible to assess the Eo’for redox couples with rather slow electron transfer kinetics. In the extreme case (e.g., redox enzymes) where the exchange current is very low, a redox mediator (see below) can be added which will speed up the overall equilibration process ( 9 0 ) .The simplicity of this T L E approach to formal redox potentials is beautiful to behold. We’ve tried it on several systems and it works very nicely. Furthermore, edge effect and iR drop problems are not as troublesome here as they are in traditional thin-layer experiments (e.g., voltammetry). Heineman et al. have also described an amalgamated nickel minigrid T L E cell for use in spectroelectrochemistry and anodic stripping voltammetry (both dc and pulse excitations) ( 1 0 0 ) . Piljac et al. used another new minigrid cell to study the composition and stability constants of Li+ complexes with the reduction products of 1-hydroxy-9,lOanthraquinone in DMF (110). Electrochemical generation of radicals for examination by electron spin resonance is by now routine for both spectroscopists and electrochemists (120, 130). Quantitative and/or potential dependent work has been largely hampered by the enormous iR drops encountered when elec-
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trodes are placed in traditional ESR flat cells designed for a rectangular microwave cavity. Simultaneous electrochemical ESR measurements (SEESR) have therefore been unusual. Goldberg et al. have described the advantages of using a constant current pulse for kinetic SEESR experiments ( 1 4 0 ) and demonstrated the point for second-order radical reactions ( 1 5 0 ) .Bard et al. demonstrated that spin trapping can be used to detect radical intermediates generated electrochemically ( 1 6 0 ) .As an example, they trapped phenyl radical produced by electroreduction of phenyldiazonium ion in acetonitrile using a-phenyl-N-tert-butylnitrone as the trap. Allendoerfer et al. have made significant progress by using a coaxial microwave cavity with a free standing helical gold wire electrode on the inside wall of a 6-mm i.d. quartz sample tube (170). Apparently the microwaves will only “see” solution within about one wire radius of the helix, thus the tightness of the windings is critical. The nice feature of this arrangement is that it doesn’t much matter what goes on inside the helix. Neither the bulk of lossy solvent, the central auxiliary electrode, nor the Luggin capillary reference probe disturb the performance of the spin resonance measurement. Thus, a nearly ideal three-electrode placement is achieved in a small volume, and uniform current density can be expected even for nonaqueous supporting electrolytes of low conductivity. Potential sweep and pulse experiments are very practical in the new coaxial cavity SEESR cell. Application to study of electrogenerated cyclooctatetraene radical anion has already been described ( 1 8 0 ) , and many future reports will undoubtedly appear. The use of electrochemistry in preparation of samples for atomic spectroscopy is definitely a hot area. Electrolytic dissolution is often a good way to dissolve a metallic component without resorting to the use of corrosive acids. When organic solvents are used the resulting solution can be especially advantageous for direct analysis by flame AA. For example, this approach has been used to analyze iron in zirconium and zirconium alloys used as fuel cladding in nuclear reactors ( 1 9 0 ) . Dawson et al. used electrodeposition onto an iridium wire which could be rotated into a flame for AA ( 2 0 0 ) . Electrolytic deposition prior to flameless atomic spectrometry can provide both preconcentration and separation from matrix interferences. Sacks has found a way to do this with a bang ( 2 1 0 ) . His group deposits on a wire and then literally explodes the wire by rapid joule heating. This is an effective way to atomize and to keep graduate students from napping. Atomization can also be achieved by heating wire filaments more gently. Newton and Davis studied nineteen elements on a tungsten alloy loop ( 2 2 0 ) while Lund and Larsen focused their attention on cadmium in sea water ( 2 3 0 , 2 4 0 ) . Dogan and Haerdi electrodeposited mercury on copper for flameless AA ( 2 5 0 )and Jensen et al. examined cadmium, lead,^ and zinc by deposition into a hanging mercury drop followed by atomization in a graphite furnace ( 2 6 0 ) .If a lot of samples were processed in this manner, one ought to wonder where the mercury went and take appropriate precautions. Vacuum techniques such as Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS or ESCA) and low energy electron diffraction (LEED) are becoming more accessible to electrochemists. There are many examples of poorly understood electrochemical results where electrode surface chemistry is likely to hold ultimate responsibility. Often the best recourse is to remove the electrode from its cell and examine the surface features by a high vacuum technique. The act of removing the electrode is fraught with several obvious problems related to potential control, evaporation of supporting electrolyte, and surface alteration by atmospheric contaminants. Nevertheless, it is clear from the literature that these difficulties do not preclude useful experiments if some modest precautions are observed. In some cases, very elaborate apparatus is used to interface the electrochemical and spectroscopic techniques. Hubbard et al. have described a thin-layer electrochemical cell for use with AES and LEED ( 2 7 0 ) .Revie et al. studied passivation of iron electrodes by direct transfer from an electrochemical cell into an AES instrument ( 2 8 0 ) . Studies of electrode surface oxidation have been 20R
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most amenable to examination by XPS or AES and several papers have appeared recently dealing with surface oxides (290-310) and sulfides ( 3 2 0 ) and organic intermediates (330). HYDRODYNAMIC T E C H N I Q U E S One of the most important features of faradaic electrochemistry is the ease by which it can be adapted to monitoring flowing streams. It has only been very recently that this fact has been developed in a manner which promises to see wide application in analytical service laboratories. The differential amperometric approach which has been under study in both Walt Blaedel’s and Carter Olson’s laboratories is a case in point. Nicotinamide adenine dinucleotide is an important factor in a great many enzymatic reactions and provides an attractive entrance for the use of amperometry to follow such reactions. Although progress is being made in understanding the behavior of both the oxidized (NAD+) ( 1 E ) and reduced forms (NADH) ( 2 E ) a t electrode surfaces, in neither case is the electrochemistry well behaved from an analytical point of view, particularly in complex media. Some recent work from Blaedel’s lab indicates that it is possible to obtain decent hydrodynamic voltammograms for NADH, a t least in clean solutions ( 3 E ) . Thomas and Christian have oxidized NADH and NADPH a t P t , glassy carbon, and carbon paste under stationary solution conditions and demonstrated application to measurement of lactate dehydrogenase activity and ethanol concentration (using alcohol dehydrogenase) ( 4 E ) .The carbon electrodes were found to be analytically useful. Taking the lead from colorimetric assays, the situation can be improved considerably by homogeneously coupling the NADH/NAD+ to the electrode via a “redox mediator” which reacts heterogeneously with a high exchange rate. Mediators are chosen which have stable forms on both sides of a rapid two-electron reaction, thus avoiding problematic free radical coupling reactions. In the differential amperometry experiment, one uses a continuous flow system to mix the sample and reagent together. Two working electrodes are placed in the flow stream a t the same potential, but separated by a delay line in which the enzymatic reaction proceeds. The difference in the current a t the two electrodes is used as a measure of the reaction which has taken place in the delay line. The published results have been quite impressive. Smith and Olson have monitored lactate dehydrogenase (LDH) ( 5 E )and blood alcohol using alcohol dehydrogenase ( 6 E ) . Obviously many other enzymes and substrates are suitable to this very straightforward approach. In our experience, the sensitivity of hydrodynamic electrochemistry under steady-state conditions is impressive and, indeed, often superior to far more sophisticated small amplitude pulse and sine wave techniques. The ring-disk ASV work mentioned earlier is likely to prove this out. Miller and Bruckenstein also prefer to operate in a rotating frame of reference, and they have achieved impressive sensitivities with the rotating disk, especially via hydrodynamic modulation (7E, 8 E ) . Myers et al. used a steady rotation rate but modulated the potential ( 9 E ) .Depending on the relationship between the pulse width and rotation rate, one can operate from pure diffusion to convection control. In all humility, I can do better analytical work with a stationary electrode in a thin-layer flow cell, but at this stage rotating electrodes are far more useful for fundamental electrochemical studies. One of the great remaining problems of analytical electrochemistry, and there are many, is the fact that potential changes a t carbon electrodes are accompanied by alteration of surface chemistry as well as traditional double-layer charging. The latter nonfaradaic event can be made to occur very rapidly but changing the redox state of surface functional groups results in faradaic currents which can take many minutes to settle down. Unfortunately, the faradaic currents can be orders of magnitude larger than the accompanying double-layer charging a t even moderately fast potential scan rates. The result is that stationary electrode or rotating electrode voltammetry a t bare carbon is never very useful for trace analysis. The slow faradaic surface reactions make it difficult to reproduce the surface
state prior to initiating a scan and also can lead t o hysteresis in cyclic hydrodynamic voltammetry. These problems undoubtedly explain why Blaedel and Jenkins achieved a hundredfold reduction in background current a t rotating glassy carbon by a stepwide rather than continuous potential sweep (IOE). When using such pesky electrodes, it is beneficial to keep the electrode potential constant and measure the steady-state limiting current as a function of analyte concentration. This reviewer’s enthusiasm for the use of hydrodynamic amperometry in liquid chromatography detection was adequately displayed in the previous review ( 3 A ) . Nevertheless, recent events have indicated that the power of this approach is far greater than originally anticipated. Not only are technical developments continuing, but also there are now many such detectors in routine use in both academic and industrial laboratories. Two of the coulometric detectors described in the previous review ( 3 A ) have been refined and applied t o several chemical problems. Takata et al. ( I I E , 12E) and Tanaka e t al. (13E) have reported from Japan. Dennis Johnson at Iowa State has described methods for nitrate and nitrite (14E) and antimony ( I 5 E ) using tubular electrodes. In France, Devynck et al. developed a coulometric detector based on Pt powder packed in a tube which functions as a porous separator (16E). The dropping mercury electrode continues to find application to chromatography. Stillman and Ma applied their cell ( 3 A ) to selected pesticides, vitamins, and analgesics (17E). Wasa and Musha described a new DME cell and evaluated it for assay of nitropyridine derivatives (18E). Fleet and Little also developed an improved small volume DME cell but went on to espouse the virtues of glassy carbon electrodes and present several supportive applications (19E). Thin-layer detectors using carbon paste electrodes ( 3 A ) have been evaluated for clinical use (20E) and applied to the assay of catecholamines (21E) and ascorbic acid (22E) in small animal brain, This approach has also been demonstrated for several phenolic drugs in formulations and body fluids (23E, 24E) and is under consideration as a reference method for serum uric acid (2523).A number of other applications are “in press” and will appear early in 1976. Miscellaneous chromatographic applications of electrochemistry include the use of semiintegral techniques ( 2 6 E ) , the assay of pharmaceuticals on aluminum-backed TLC plates ( 2 7 E ) ,and the measurement of HP and CO in exhaled air (28E).The latter report describes an amperometric GC detector based on a metallized (Au, Ag, or Pt) Teflon membrane. This portable system responds to partial pressure and is capable of monitoring 10 ppm H2 to 1 0 . 1 PPm. One of the more amazing efforts of recent vintage is the successful incorporation of a two-electrode preparative flow cell into a spinning 5-mm 0.d. NMR tube by Richards and Evans (29E).A porous bed (mercury-coated or uncoated platinum chips, carbon cloth, or 80-mesh glassy carbon) contained by a 1.5-mm 0.d. ion exchange tube formed the working electrode. This is surrounded by a latinum wire auxiliary coil. For modest flow rates(10.2 m l A i n ) , the residence time of product in the detection region of the magnetic field is less than 1 second. The steady-state conditions would permit the use of Fourier transform instruments to enhance sensitivity. Perhaps next year Evans will be able to build this cell into a spinning melting point capillary and save on supplies. On a serious note, this concept of cell design is viable for a number of applications which have nothing t o do with NMR. We have used the Evans approach a t Purdue and it works like a charm. The application of rotating electrodes to steady-state photochemical measurements has been developing slowly but steadily. Johnson’s group has theoretically and experimentally examined the photodimerization of benzophenone (30E) and has also studied the ultraviolet photolysis of fluorenol (31E).Our last example of hydrodynamic success is from the work of Fujishima e t al. (32E).These workers devised a rotating ring disk electrode using a gold ring with a CdS single crystal disk. Spectral sensitization of the CdS semiconductor was studied by directing light on the disk from above. Both the photocurrent a t the disk and
dark current a t the ring were measured as a function of wavelength, with and without a dilute solution of sensitizer. The results were consistent with photosensitization involving transfer of an electron from the excited dye to the semiconductor electrode.
STRIPPING VOLTAMMETRY It’s no longer satisfactory to consider only anodic strip‘ping voltammetry (ASV) since cathodic stripping (CSV) is beginning to attract quite a bit of attention. In the latter case, it is usual to anodically deposit an insoluble film of material on the electrode and to strip it off during a negative potential sweep. Colovos et al. have looked into exchange reactions involving mercurous halide films a t the HMDE ( I F ) and then used this information t o simultaneously determine bromide and chloride via CSV ( 2 F ) . Their results indicate that it is reasonable to measure bromide a t the M level with chloride present a t M. CSV has also been used for rainwater lead in the 0-100 ppb range ( 3 F ) ,for Pb02 deposited on tin oxide films ( 4 F ) ,for mercaptopyridine-N-oxide below 10 ppb ( 5 F ) , and for phosphate on glassy carbon ( 6 F ) .Problems with CSV usually occur a t high concentrations due to surface saturation effects. There is a t least one simple diversion to avoid this problem. A very fine introduction and review of ASV has been published in this journal by Copeland and Skogerboe ( 7 F ) . They describe the linear sweep, alternating voltage, and small amplitude pulse waveforms used in the stripping step and also,note the advantages of different electrode materials. An assessment is made of the relative merits of atomic absorption and ASV. The conclusion is that, in general “ASV offers greater sensitivity than flame AA techniques and is quite competitive with the nonflame systems. The primary advantage of AA consequently accrues from the wider range of elements that can be determined.” There continues to be much research and much controversy about the merits of different electrodes for various analytical problems. Batley and Florence thoroughly compared several techniques for a variety of heavy metals (8F). Stojek and Kublik advocate a silver-based mercury film ( 9 F ) ,whereas Anokhin and Ignatov like “carbon pyroceramic” (IOF). Clem and Sciamanna use a polystyrene-impregnated graphite substrate prepared by cobalt-60 irradiation of absorbed monomer ( 1 1 F ) . The latter approach is said to provide improved performance and electrode life in neutral to weakly acidic electrolytes, but unfortunately does not afford significantly better stability than a WIGE in strongly acidic solutions. Clem has been working toward both understanding and eliminating the failure of WIGE’s in strong mineral acids in order that on-line and field instrumentation could be utilized reliably. He has recently suggested that the degradation of hydrogen overvoltage which occurs for graphite electrodes in strong acid is due to the formation of surface oxides (22F).Mercuric ion is implicated as a catalyst in this process. Rotating electrodes can have advantages both for the deposition and stripping phases of ASV. Johnson’s laboratory a t Iowa State has taken the lead in this regard and recently described a method for selenium(1V) using a gold disk (13F).Sensitivity a t submonolayer coverage was such that a detection limit of ca. 0.04 ppb could be achieved. Laser and Ariel used Johnson’s ring-disk approach to ASV by depositing on a mercury-plated glassy carbon disk and collecting the subsequently stripped ions on a ring of the same material ( 1 4 F ) . Detailed instructions are given for the RRDE. Miguel and Jankowski report ppb assays for several metals using simultaneous deposition of the metals and mercury on a vitreous carbon rotating disk ( 1 5 F ) . Roux et al. used alternating and pulse voltammetry a t a rotating amalgamated copper disk electrode ( 1 6 F ) while Luong and Vydra recommended chronopotentiometric stripping on a glassy carbon rotating disk ( 1 7 F ) . Intermetallic compound formation continues to plague stripping voltammetrists and Copeland et al. have valuable advice on the copper-zinc problem (18F).Galus et al. have reported on the diffusion coefficients of 17 metals in mercury ( 1 9 F ) .This is another subject of fundamental interest to ASV. Two applications which caught my eye involved zinc. ANALYTICAL CHEMISTRY, VOL. 48, NO.
5, APRIL 1976
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Marshall assayed zinc dialkyldithiophosphate engine oil additives by TLC followed by ASV (20F). Ted Williams a t The College of Wooster led a team which was the first to analyze human eye tissue for zinc (21F). They used small amplitude pulse ASV a t the HMDE and corroborated their findings with atomic absorption. Four eye parts were assaved and found to contain between 8 and 391 wg/g of zinc o n the basis of dry tissue. LITERATURE CITED
'
General (1A) Nicholson, W., Nlch. J., 4, 183 (1800). (2A) Faraday, M., Philos. Trans. R. SOC.London, Ser. A,, 124, 77 (1834). (3A) Kissinger, P. T., Anal. Chem., 46, 15R (1974). (4A) Sawyer, D. T., Roberts, J. L., Jr., "Experimental Electrochemistry for Chemists", Wiiey-lnterscience, New York. 1974. (5A) Weinberg, N. L.. Ed., "Technique of Eiectroorganic Synthesis", Vol V, Parts 1 and 2, "Techniques of Chemistry", A. Weissberger, Ed.. Wiley-Interscience, New York, 1975. (6A) Rifi. M. R., Covitz, F. H., "Introduction to Organic Electrochemistry", Marcel Dekker, New York, 1974. (7A) Gileadi, E., Kirowa-Eisner, E.. Penciner, "Interfacial Electrochemistry", Addison-Wesley, Reading, 1975. (8A) Meites, L., Zuman, P., Ed., "Electrochemical Data", Part 1, Vol. A, John Wiley, New York. 1974. Electrodes and Cells ( l e ) Anson, F. C., Acc. Chem. Res., 8, 400 (1975). (28) Watkins, B. F., Behling, J. R., Kariv. E., Miller, L. L., J. Am. Chem. SOC.,97, 3549 (1975). (38) Moses, P. R., Wier. L., Murray, R. W., Anal. Chem., 47, 1882 (1975). (48) Maugh, T. H., Science, 190, 1284 (1975). (58) Clark. L. C., Jr., Emory, C. R., Glueck, C. J., Campbell, M., Circulation, 52, 11-170 (1975). (6B) Clark, L. C., Jr., Clark, E. W., in "Oxygen Transport to Tissue", H. I. Bicher and D. F. Bruley, Ed., Plenum Publishing Corp., New York, N Y , 1974, p 127. (7B) Mell, L D , Maloy. J. T.. Anal. Chem., 47, 299 11975) ~. -, (88) Updike, S. T., Shults, M. C.. Kosovich, J. K., Treichel, I., Treichei, P. M., Anal. Chem., 47, 1457 (1975). (9B) Wingo. W. J., Emerson, G. M., Anal. Chem., 47, 351 (1975). (1OB) Nanjo, M., Guilbault. G. G., Anal. Chem., 46, 1769 (1974). ( 1 l B ) Nanjo, M.. Guiibault, G. G., Anal. Chim. Acta, 75, 169 (1975). (128) Schlapfer, P., Mindt. W., Racine, P. H., Clin. Chim. Acta, 57, 283 (1974). (138) Stulikova, M., Stulik, K.. Chem. Listy, 68, 800 (1974). (148) Lindquist, J., J. Electroanal. Chem., 52, 37 (1974). (158) Bauer, D.. Gaillochet, M. Ph.. Electrochim. Acta, 19, 597 (1974). (16B) Levy, S. C., Anal. Chem., 47, 604 (1975). (178) Bond, A. M., O'Donnell, T. A,. Taylor, R. J., Anal. Chem., 46, 1063 (1974). (18B) Harrar, J. E., Electroanal. Chem., 8, 1 (1975). (19B) Harrar, J. E., Sherry, R. J., Anal. Chem., 47, 601 (1975). (20B) Rigdon. L. P.. Harrar, J. E., Anal. Chem., 46. 696 11974). (218) Geiger, W. E.. Jr.. Mines, T. E., Senftieber, F. C., lnorg. Chem., 14, 2141 (1975). ~
\
.
~
I
,
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22R
ACKNOWLEDGMENT
The author is grateful to Sharon Schwier, Larry Pachla, Tom (Rodney) Kenyhercz, and John Hammond for their expert assistance-in searching the literature and preparing this review for publi cation.
(7C) Grypa, R. D., Maloy, J. T., J. Electrochem.
SOC.,122, 509 (1975). (8C) Nelson, S. F., Echegoyen, L., Evans, D. H., J. Am. Chem. SOC., 97, 3530 (1975). (9C) Smith, W. H., Bard, A. J., J. Am. Chem. SOC.,97, 5203 (1975). ( l a c ) Smith, W. H., Bard, A. J., J. Am. Chem. SOC., 97, 6491 (1975). (11C) Ford, W. T., Anal. Chem., 47, 1125 (1975). (12C) Chum, H. L., Koch, V. R., Miller, L. L., Osteryoung, R. A., J. Am. Chem. SOC.. 97, 3264 (1975). (13C) Gilbert, B.. Mamantov, G.. Fung, F. W., lnorg. Chem., 14, 1802 (1975). (14C) Fung, K. W., Mamantov, G., in "Comprehensive Analytical Chemistry", Svehla, G., Ed., Elsevier, New York, 1975, p 305. (15C) Hassan, M. 2.. Bruckenstein, S.. Anal. Chem., 46, 1962 (1974). (16C) Fujihira, Y., Kuwana, T., Hartzell. C. R.. Biochem. Biophys. Res. Commun., 61, 538 (1974). (17C) Erabi, T., Hiura. H.. Tanaka, M., Bull. Chem. SOC.Jpn., 48, 1354 (1975). Spectroscopic Techniques (1D) Mattson, J. S., Smith, C. A,. Anal. Chem., 47, 1122 (1975). (2D) Markova. 2 . A,, Mikhailova, A. A,. Osetrova, N. V., Bagotskii, V. S., Electrokhimiya, 10, 1794 (1974). (3D) Fleischmann. M., Hendra. P. J., McQuilIan, A. J., Chem. Phys. Leff., 26, 163(1974). (4D) Jeanmaire, D. L., Suchanski, M. R., Van Duyne. R. P., J. Am. Chem. Soc.. 97, 1699 (1975). (5D) Kuwana, T., Winograd, N., Electroanal. Chem., 7, l ( 1 9 7 4 ) . (6D) Steckhan, E., Kuwana, T., Ber. Bunsenges. Phys. Chem., 78, 253 (1974). (7D) Ryan, M. D., Wilson, G. S., Anal. Chem., 47, 885 (1975). (8D) Karweik, D. H., J. Nectrochem. SOC., 122, 153C (1975). (9D) Heineman. W. R., Norris. B. J., Goelz, J. F., Anal. Chem., 47, 79 (1975). (10D) Heineman, W. R., DeAngelis, T. P., Goelz, J. F., Anal. Chem., 47, 1364 (1975). (11D) Piljac, I., Tkalcec, M., Grabaric, B., Anal. Chem., 47, 1369 (1975). (12D) Kastening, B., in "Electroanalytical Chemistry", H. W. Nurnberg. Ed., Interscience, New York, 1974, p 421. (13D) Goldberg, I. B., Bard, A. J., in "Magnetic Resonance in Chemistry and Biology", J. N. Herak, Ed., Marcel Dekker, New York, 1975. (14D) Goldbert, I. B., Bard, A. J., J. Phys. Chem., 78, 290 (1974). (15D) Goldverg, I. B., Boyd, D., Hirasawa, R., Bard, A. J., J. Phys. Chem., 78, 295 (1974). (16D) Bard, A. J., Gilbert, J. C., Goodin, R. D., J. Am. Chem. Soc., 96, 620 (1974). (17D) Ailendoerfer, R. D., Martinchek. G. A., Bruckenstein, S.. Anal. Chem., 47, 890 (1975). (18D) Allendoerfer, R. D., J. Am. Chem. SOC., 97, 218 (1975). (19D) Mantel, M.. Aladjem, A,, Anal. Leff., 8, 415 (1975). (20D) Dawson, J. B., Ellis, D. J., Hartley, T. F.. Evans, M. E. A,, Metcalf. K. W., Analyst (London), 99, 602 (1974). (21D) Sacks, R. D.,Holcombe, J. A., Appl. Spectrosc., 28, 518 (1974). (2213) Newton, M. P., Davis, D. G., Anal. Chem., 47, 2003 (1975). (23D). Lund, W., Larsen. 8 . V., Anal. Chim. Acta, 72, 57 (1974). (24D) Lund, W., Larsen. B. V., Anal. Chim. Acta, 72, 299 (1974). (25D) Dogan, S., Haerdi, W., Anal. Chim. Acta. 76, 345 (1975). (26D) Jensen, F. O., Dolezal, J., Langmyhr, F. J.. Anal. Chim. Acta, 72, 245 (1974).
ANALYTICAL CHEMISTRY, VOL. 48, NO.
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(27D) Hubbard, A. T., Ishikawa. R. M., Schoeffei, J. A., Proc. Symp. Electrocat., M. W. Breiter, Ed., Electrochemical Society, Princeton, N.J., 1974, p 258. (28D) Revie, R. W., Baker B. G., Bockris, J. O'M.. J. Electrochem. SOC.,122, 1460 (1975). (29D) Dickonson, T., Povey, A. F.. Sherwood, D. M. A,, J. Chem. SOC.Faraday Trans. 1, 71, 298 (1975). (30D) Kim, K. S., Sell, C. D., Winograd. N., Proc. Symp. Electrocat., M. W. Breiter, Ed., Electrochemical Society, Princeton, 1974, p 242. (31D) Johnson, W. C., Heldt, L. A., J. Electrochem. SOC., 121, 34 (1974). (32D) Evans, J. F., Blount, H. N., Ginnard, C. R.. J. Nectroanal. Chem. 59, 169 (1975). (33D) Allen, G. C., Tucker, P. M., Capon, A,, Parsons, R., J. Electroanai Chem., 50, 335 (1974). Hydrodynamic Techniques (1E) Schmakel, C. O., Santhanam, K. S. V., Elving, P. J., J. Am. Chem. Soc., 97, 5083 (1975). (2E) Braun, R. D., Santhanam, K. S. V., Elving, P. J., J. Am. Chem. Soc., 97, 2591 (1975). (3E) Blaedel, W. J., Jenkins, R . A., Anal. Chem., 47, 1337 (1975). (4E) Thomas, L. C., Christian, G. D., Anal. Chim. Acta, 78, 271 (1975). (5E) Smith, M. D., Olson, C. L., Anal. Chem., 46, 1544 (1974). (6E) Smith, M. D.. Olson, C. L., Anal. Chem., 47, 1074 (1975). (7E) Miller, B.. Bruckenstein, S.. Anal. Chem., 46, 2026 (1974). (8E) Miller, B., Bruckenstein, S., Anal. Chem., 46, 2033 (1974). (9E) Myers, D. J., Osteryoung, R. A,, Osteryoung, J., Anal. Chem., 46, 2089 (1974). (10E) Blaedei, W. J., Jenkins, R. A,, Anal. Chem., 46, 1952 (1974). (11E) Takata, Y., Arikawa, Y., Bunseki Kagaku, 23, 1522 (1974). (12E) Takata, Y., Fujita, K., J. Chromatogr., 108, 255 (1975). (13E) Tanaka, K., Ishihara,. Y., Sunahara, H., Bunseki Kagaku, 24, 235 (1975). (14E) Davenport, R. J., Johnson, D. C.. Anal. Chem., 46, 1971 (1974). (15E) Taylor, L. R., Johnson, D. C., Anal. Chem., 46, 262 (1974). (16E) Devynck. J., Pique, A,. Delarue, G., Analusis, 3, 417 (1975) (17E) Stillman, R., Ma, T. S., Mikrochim. Acta, 641 11974) (18E) Wasa, T., Musha. S., Bull. Chem. SOC. Jpn., 48, 2176 (1975). (19E) Fleet, 8.. Little, C. J., J. Chromatogr. Sci., 12, 747 (1974). (20E) Kissinger, P. T., Felice, L. J., Riggin, R. M.. Pachia, L. A . , Wenke, D. C., Clin. Chem., 20, 992 (1974). (21E) Refshauge. C. J., Kissinger, P. T., Dreiling, R., Blank, L., Freeman, R.. Adams, R. N., Life Sci., 14, 311 (1974). (22E) Thrivikraman, K. V., Refshauge, C., Adams, R. N., Life Sci., 15, 1335 (1974). (23E) Riggin, R. M., Rau, L. D., Alcorn, R. L., Kissinger, P. T.. Anal. Lett., 7, 791 (1974). (24E) Riggin, R. M., Schmidt, A. L., Kissinger. P. T.. J. Pharm. Sci., 64, 680 (1975). (25E) Slaunwhite, W. D., Pachla, L. A., Wenke. D. C.. Kissinger. P. T., Clin. Chem., 21, 1427 (1975). (26E) Briimyer. G. H., Lamey, S. C., Maioy. J. T., Anal. Chem., 47, 2304 (1975). (27E) Oelschlaeger, H. O., Bunge, K., Lim, G. T., Kraft, G., Arch. Pharm., 307, 796 (1974). (28E) Bergman, I., Chromatographia, 8, 581 (1975). (29E) Richards, J. A,, Evans, D. H., Anal. Chem., 47, 964 (1975). (30E) Lubbers, J. R., Resnick, E. W., Gaines, P.
R., Johnson, D. C., Anal. Chem., 46, 865 (1974). (31E) Gaines, P. R., Peacock, V. E.. Johnson, D. C., Anal. Chem., 47, 1373 (1975). (32E) Fujishima, A., iwase, T., Watanabe, T., Honda, K., J. Am. Chem. SOC.,97, 4134 (1975).
Stripping Voltammetry (1F) Colovos, G.. Wilson, G. S.,Moyers, J. L., Anal. Chem., 46, 1045 (1974). (2F) Colovos, G.. Wilson, G. S., Moyers, J. L.. Anal. Chem., 46, 1051 (1974). (3F) Kinard. J. T., Propst, R. C., Anal. Chem., 46, 1 W 6 (1974). (4F) Laitinen, H. A,, Watkins, N. H., Anal. Chem., 47, 1352 (1975).
(5F) Csejka. D. A,, Nakos, S. T., DuBord, E. W., Anal. Chem., 47, 322 (1975). (6F) Cox, J. A,, Cheng, K. H., Anal. Len., 7, 659 (1974). (7F) Copeland, T. R., Skogerboe, R. K.. Anal. Chem., 46, 1257A (1974). (8F) Batley, G. E., Florence, T. M., J. Electroanal. Chem., 55, 23 (1974). (9F) Stojek, Z., Kublik. 2.. J. Electroanal. Chem.. 60, 349 (1975). (1OF) Anokhin. E. A,. Ignatov, V. I . , Zh. Anal. Chem.. 29, 1046 (1975). (11F) Clem, R. G., Sciamanna, A. F., Anal. Chem., 47, 276 (1975). (12F) Clem, R. G.. Anal. Chem., 47, 1778 (1975). (13F) Andrews, R. W., Johnson, D. C., Anal.
Chem., 47, 294 (1975). (14F) Laser, D., Ariel, M., J. Electroanal. Chem., 49, 123 (1974). (15F) Miguel, A. H., Jankowski, C. M., Anal. Chem., 46, 1832 (1974). (16F) Roux, J. P.. Vittori, O., Porthault, M., Analusis, 3 , 41 1 (1975). (17F) Luong, L., Vydra, F., Collect. Czech. Chem. Cornmun., 40,2961 (1975). (18F) Copeland. T. R., Osteryoung. R . A,, Skogerboe. R. K., Anal. Chem., 46, 2093 (1974). (19F) Baranski. A.. Fitak, S.,Galus. Z., J. Electfoanal. Chem., 60, 175 (1975). (20F) Marshall, R . A. G., J. Electroanal. Chem., 56, 311 (1974). (21F) Williams, T. R . , Foy, D. R . , Benson, C., Anal. Chim. Acta. 75, 250 (1975).
Ion Selective Electrodes Richard P. Buck The William Rand Kenan, Jr., Laboratories of Chemistry, The University of North Carolina, Chapel Hill, N.C. 275 14
The history of ion selective electrodes (ISE’s) in the past decade shows the typical behavior of expansion followed by consolidation. The early rapid growth of new electrodes for ion activity measurement, new formats, and new materials of construction has given way to more introspective research on “how’s and why’s” of the functioning of various electrodes and to extensive application studies, uses of ISE’s as instrumental components and uses in diverse fields, particularly in clinical and environmental chemistry. I am restricting this review to topics which deepen our understanding of ISE’s or show new and potentially import a n t applications for ISE’s. The extensive bibliographies previously included in the reviews (49, 51) which were intended to be comprehensive, have been omitted. Nevertheless, the actual body of published material on ISE’s is as large as or larger than ever. While risking appearance of chauvanism, with few exceptions, references were selected from journals readily available in the USA. For the first time, a computer search of Chemical Abstracts tapes was used for the initial survey. Since selections were based on titles and key words, important papers involving ISE’s, but not specifically called out, may have been overlooked. For someone seeking a book on the principles and applications of ISE’s, the most comprehensive volume in English is “Ion-Selective Electrodes” by Koryta (198). I t is a small volume, 207 pages, but covers basic principles, observed responses, and many applications in tabular form. T h e book is an expansion of his 1972 review and contains literature references only through 1973. Three important, authoritative reviews have been prepared by Covington, and Moody and Thomas (74, 75, 252a). The first contains information on reversible electrodes and emphasizes new results on glass and neutral carrier membrane electrodes, while the second and third cover historically all types of ISE’s. One other extensive review in Polish is mentioned because it contains 305 references (346). Sollner, who has contributed many reviews previously, has outlined the basic electrochemistry of fixed site ion exchange membranes (339). Riande’s review on the same topic contains 305 references (302). Recommendations for nomenclature, particularly symbols to be used for the selectivity coefficient, kAB (rather than the previously used term “potentiometric selectivity coefficient”, Kpp‘) have been discussed in a new IUPAC Information Bulletin (178). Nomenclature for ISE’s of conventional types and enzyme and gas-sensing electrodes is included. IUPAC tentative conventions for symbols and terminology in the general electrochemical field are given
by Parsons (277). In Ref. 178, both the two-point, bi-ionic (or separate solution) method and the mixture method (or fixed interference method) are briefly described and advocated for measurement of kAB. In this review, 1 will use a compromise symbol k, for the selectivity coefficient. Buck ( 5 4 ) has summarized t i e numerous advantages of the mixture methods, and the indispensible features possessed by mixture methods when dealing with concentration-dependent selectivity coefficient measurements. The nearly unavoidable flaws in the two solution method (discussed below) make it an undesirable technique. The thrust of this paper (54) is not just a history of the various techniques of selectivity coefficient measurement, but rather an experimental and theoretical justification for the occurrence of activity ratio and absolute activity-dependent selectivity coefficients. Although not all cases have been diagnosed, permselective systems for common valence ions are activity ratio dependent, while kinetically controlled (irreversible) systems are activity dependent. Buck advocates plots of log k,, vs. log aJa, to distinguish these cases. Only the mixture methods are capable of showing up these commonly occurring effects. The general procedure suggests computation of hi, (app.) from experimental data for common valence ions according to In ki;(app.) = In (exp[(Eij- &)/SI - 11 - ln(aj/ai) (1) where E,, is the a,,aJ mixture response and E, is a corresponding pure a, response. For the special case of irreversibility manifest only as different slopes, S, and S I , Mohan and Rechnitz (249)suggested:
+
E,, = Elo S,ln(a, + k,,
a,SI’Si)
(2)
Rather than plotting a family of k,,(app.) values for each a,, one can readily correct k,, (app.) to k, through Ink,, = In k,,(app.)
+ (1- S,/S,)ln a,
(3) The method has been used by Hakoila, Lukkari, and Mannonen (145). The basis for well known activity standards for the more common ISE’s, previously published by Bates and Alfenaar (17), is the assumption that the hydration number of an ion is an unalterable property of the species. In contrast, the single ion activity coefficient depends upon the nature of the other ion in simple salt solutions. Bates (18) has reported single ion activities for NaC1, KC1, KI and CaClz to 1 or 2 M, using this assumption. A new pH buffer for the ANALYTICAL CHEMISTRY, VOL. 48, NO.
5, APRIL 1976
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