Informal Discussion of EIectroanaIyticaI Chemistry
Departing again from the traditional lead-off article, we are sharing with our readers some notes on an informal discussion of electroanalytical chemistry which took place at the Boston meeting of the American Chemical Society. The discussion was sponsored by the Division of Analytical Chemistry, and the report published here was prepared through the joint efforts of W. H. Reinmuth, C. N. Reilley, and L. B. Rogers.
T
session was divided into three parts, each of which was limited to one hour: electrochemical techniques, role of the electrode surface, and processes in the solution. A speaker briefly introduced each topic and then presided over the remaining 45 minutes of discussion from the floor. I n each case, discussion was active and promised to continue far beyond the allowed time. HE
ELECTROCHEMICAL TECHNIQUES
W. H. Reinmuth (Columbia University) pointed out some ambiguities of nomenclature in the current literature. He proposed that the term “voltammetry” be used to cover the entire field of current-potential-time studies a t electrodes and that t h e terms “polarography” and “galvanometry” be reserved for the cases in which potential nnd current, respectively, are controlled variables. The methods may be classified in five categories according to the types of signals observed: 1. Direct techniques in which the result is a conventional polarographic curve (chronopotentiometry and largeamplitude square-wave polarography) 2. Derivative techniques in R-hich the response is first-derivative or pseudodeiivative of the conventional wave (small sine- and square-wave polarography, hanging-drop polarography) 3. Second-derivative techniques [Barker’s radio-frequency polarography ( 6 ) and second-harmonic small sinewave polarography] 4. Integral techniques in v hich the integral of current is measured over some lengths of time (anodic stripping, controlled-potential, and controlledcurrent coulometry) 5. Techniques in which a steady state is attained a t the electrode or results are extrapolated to zero time (potentiometry, amperometry, galvanostatic and potentiostatic techniques)
Deciding among such a large number 1450
ANALYTICAL CHEMISTRY
of techniques poses a major problem to the analyst or kineticist and raises the question as to fi*hich might be considered of general usefulness. Because investigators generally confine themselves to a single twhnique, insufficient attention has been devoted to an intercomparison of techniques. As an example, Reinmuth cited the startling disagreement in results obtained for the electron-transfer coefficient in the reduction of cadmium ion by different workers, each using a different technique. Until reasons for such discrepancies can be clearly ascertained, little progress can be made in deciding upon the relative merits of the various techniques. G. L. Booman (Phillips Petroleum Co., Idaho Falls) suggested that the present theories of a x . techniques may be predicated on an inaccurate model of electrode processes. A physically realizable model and a simple electrical analog involving a resistive indwtor, a perfect diode, and a transmission line were proposed. He also suggested that a single instrument, comparable in cost to a conventional recording polarograph might be designed to incorporate a.c., square-wave, radio-frequency, and pulse techniques. Such an instrument would greatly expand the analytical information available from measurements of electrode currents. W. D. Cooke (Cornel1 University) noted that American workers seem to have overlooked small-amplitude sinewave polarography despite its advantages over polarography in resolving waves having similar half-wave potentials and in detecting small concentrations of one component in the presence of large amounts of others. By adjusting the frequency of the a.c. component and the composition of the supporting electrolyte, it is often possible to eliminate interferences. Reinmuth suggested that the fact that all substances which
give conventional polarograms do not give a.c. polarograms could lead to unpleasant surprises for the unwary worker. Cooke observed that metal ions, in general, give very good a.c. responses except those present in oxygenated forms. R. N. Adams (University of Kansas) pointed out that though one generally associates irreversible behavior in conventional polarograms with absence of waves in the a x . technique, this is not always the case in organic systems a t solid electrodes. When irreversibility is due to rapid decomposition of the product of a reversible oxidation, the a.c. may be sufficiently rapid so that decomposition does not occur in a single cycle and a reversible ax. Kave is obtained. He cited mercaptans and p-hydroxydiphenylamine as examples. Reinmuth noted that the tensammetric waves of Breyer (ti’), not because of electrochemical reaction but because of adsorption and desorption, also offer possibilities for determining species with a x . polarography, even though they are not reactive. He went on to point out that a difficulty of small-amplitude a.c. methods (ca. 10 mv.) centers on the restrictions that the reaction must reverse on each half cycle in order for a response to be observed. If the large amplitude (ca. 500 mv.) square-wave technique of Kalousek (11) is used, this becomes a much less restrictive condition. Response can be observed with a larger number of compounds, although the technique is no longer a derivative one and thereby sacrifices resolution. D . T. Sawyer (University of California, Riverside) pointed out that this technique requires more complicated instrumentation than the smallamplitude sine wave. Cooke raised the point that the hesitancy of analysts to use newer techniques may result from the lack of commwrially available equipment.
ildarns noted that most of the tecliniques require only simple modifications of availnble equipment 'ri hich can be done rdiile retaining the former functions. Thus, it is possible to have wide versatility in a single instrument. Irving Shain (Gniversity of 11-1sconsin) mentioned that differential techniques deserve re-evaluation in terms of never methods. He pointed out that sensitivity in hanging-drop po1:trography could be increased tenfold by compensating differi~ntiallyfor residual current using a sccond elcctrode in pure supporting electrol>tc. €It- also rioted the possibility of avoiding tlie necessity of deaeration and of balancing out large amounts (up to 500-fold excess) of one species to determine smaller amounts of another (16). 11. T. Iielle? (Oak Ridge Sational Laboratory) noted that the differential tcclinique of Semerano and Riccoboni ( f ? ) , nhich uses two dropping electrodes, incurred difficulties which centered around synchronization of the two capillarirs These problems can be avoided by using electronic filtering techniques. I). H. Geshe (Harvard University) drew attention to the recent work of Austen, Given, Ingram. and Peover (5) in 1% hicli electron-spin resonance was used to characterize free radicals of hydrocarbons and quinones produced by controlled-potential electrolysis and stabilized a t liquid-nitrogen teniperature. Geske, nith A. 1%.Mahi, is placing a n electrode assembly directly in the resonance cavity (see Figure I), so that even short-liwd radicals can be observed. I-sing acetonitrile as solvent. they have observed about 0.1 p i o l e of a chlorine-containing radical n-hen 0.1M lithium perrhlorate is oxidized using a 3-ma. currmt and a platinum electrode. T h y are also extending the technique to mercury electrodes. ELECTRODE SURFACE 17. C. Anson (Culifornia Institute of Technology) pointed out that it is not surprising that changes in the character of the surface of metal electrodes produce very marked changes in the electrometric behavior of the electrodcs. because a reaction a t an electrode is a heterogeneous process in which the surface of tlic electrode is bound to play a n iniportant part. K h e t h e r the adherent film of PtO and PtOz ( 4 ) is formed chemically or electrochemically, steady potentials are obtained which may be much more oxidizing than the reversible potential of the Pt-PtO or PtO-PtOn couples. This indicates that the oxide film protects the platinum from further oxidation but does not prevent electron exchange from occurring between the oxidized electrode and tlie solution. Furthermore, oxidized platinum elec-
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Magnet
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Figure 1 .
Pt electrode
Electrode assembly
trodes frequently behave very differently from unoxidizcd electrodes in electro-oxidations and reductions. For instance, the reductions of ferric to ferrous iron in sulfuric acid solutions (12) and iodate to iodide ( 2 ) occur much more reversibly a t previously oxidized electrodes. On the other hand, oxidations of iodine to iodate ion ( 3 ) and of oxalic acid to carbon dioxide (15) proceed much less reversibly a t oxide-coated plntinum electrodes. Anson emphasized that no explanation of the mechanism of action of the oxide films that n ill satisfactorily account for even the few CLises cited is available. There is some evidence that the nicre presence of the oxide film may have less influence on the reactions occurring a t the electrode than do the oxidation and reduction processes a t the elcctrode which form and remove the film. J. J. Lingane (Harvard University) remarked that the oxide film could also be insulating the electrode from electron transfer. He suggested that the potential might be established through platinum-platinum ion couples n-hich n ere in chemical equilibrium with the oxidants and reductmits in the solution. L. B. Rogers (llassachusetts Institute of Technology) pointed out that the surface film niay act as a semipermeable membrane. He cited corrosion literature n hich indicated that in some cases metal ions moved out to the oside surface to form oxide; in other cases 0x1 gen diffused through the oxide to meet the metal atom a t the inner surface He proposed that studies be made of reactions on ovide surfaces for n hich the method of transfer nas knon-n and that coniparisons be made betneen different surfaces in order to gain a better understanding of the role of oxide films. Adams pointed out that in nonaqueous solvents oxide films often do not form, and this behavior
offers the possibility of circumventing film effects when they are undesirable. He t h m pointed out that niercury(1) chloride f h s (13, 15) did not allow inorganic rcnctioiis to tahe place, whereas they did not interfere with organic reactions such as oxidations of amines. Reinmuth questioned TI lietlier the structure and composition of halide and oxide films might be elianging with potential and thus contributing to t h e complication in intcqircting their effect on reactions. H c cited experiments with halide films on hanging mercury-drop electrodes 11-hicli intlicntetl that, in the potential region aiound +O.O volt us. S.C.E., internnl stresses on thc film are often so great as to cause it to erupt. Adams noted that in the oxidation of colored organic compounds variations in color intcneity nere noted over the electrode surface :md cautioned that. in vieiv of these ol)scrv'itions, the surface layer seemed to he of unevcn structure. Therefore, it is not a valid procedure to speak in terms of over-all structure. Lingane nientioried recent results in his laboratory, n liich indicated the formation of halide films on platinum electrodes in chloride media. This is surprising in view of the solubility of the chloroplatinum complexes. He went on to state that, on intuitive grounds. he feels it less likely that the mechanism of reaction films involves electron tranqfer rather than atom transfer. Adams suggested that one way to solve the filni problem n as to use che:nically inert substances. He has lieen stud>irig boron carbide electrodes and found that they have very lo^ residual currents over a voltage rnngc of about t 1 . 0 to -1.0 T-olt Z.S. S.C.E.Both oxidation and reduetion polarograms can be examined a t this electrode. Except in a very few inst:mces, the only cleaning procedure is nxdiing with distilled natcr. Another t j pe oq hlm is that formed by a reaction product. .Idaim reports that organic films as n-ell as oxide films can be avoided by preparing pastes of graphite with organic liquids n hich are not eleetroactive and are iinmiscible with water ( 1 ) . Tlie paste is used in a pool configuration. The useful range of a graphite-broiiionaplitliu~ene past
