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Electroanalysis and Coulometric Analysis Allen J Review of Fundamental Developments in Analysis Electroanalysis and Coulometric Analysis Allen J. Bard Deparfment of Chemisfry, The University of Texas, Austin 72, Texas RIS review mainly surveys the ods. The application of coulometric ing irreversibility with time to “aging” Tliterature during 1960 and 1961, (and other electrochemical) methods to of the oxide film. Both papers (113, and is patterned closely after previous analytical problems in atomic energy 170) assume that the reduction of oxy- reviews in this series. Papers published work has been rebiewed (136). The gen proceeds via a direct chemical re- before 1960 which have not appeared in reviews by Barendrecht (14) (66 refer- action between oxygen and platinum, previous reviews have also been in- ences) and Morris (137) (161 references) followed by electrochemical reduction of cluded. Research has increasingly been on coulometry, as well as one by the platinum oxide(s). Apparently the concerned with a study of the funda- Meites (130) on the application of con- rate or extent of the platinum-oxygen mental electrochemical principles upon trolled potential electrolysis to electro- reaction in an acidic solution is suffi- which electroanalytical procedures are gravimetry, coulometry, and synthesis, ciently small that a prereduced electrode based. The discussion of these theo- deserve mention. Other reviews on in an oxygen-saturated solution will retical electrochemical developments various phases of coulometry have also maintain the characteristics of a re- has been limited to areas which have the appeared (39, 110, 146, 183). duced surface. most direct bearing on electroanalysis. Anson has recently found that the As a general trend of the past two years, ELECTROCHEMICAL THEORY reversibility of both the reduction of coulometry continues to enjoy increas- iron(II1) and the oxidation of iron(I1) ing development and application, while Platinum Oxide Films. h topic is markedly decreased by the presence of classical electrogravimetry has mainly which continues to enjoy attention is an oxide film on the electrode (4). He been concerned with extension of pre- the effect of oxide films on electrode further showed that some effects hereto- viously developed methods. reactions occurring at platinum elec- fore ascribed to the presence of an oxide trodes. Previous work concerned film are actually caused by a film of BOOKS AND REVIEW ARTICLES with the evidence for oxide films platinized platinum formed when an on platinum electrodes has been oxidized electrode is reduced. It is The book of Charlot, Badoz-Lam- reviewed by Laitinen and Enke (108). this platinized platinum (rather than bling, and Tremillon (33)discusses elec- These authors also present new experi- platinum oxide) n-hich increases the trochemistry from the viewpoint of the mental work on the oxide films and reversibility of many electrode re- analytical chemist. Extensive use is offer possible mechanisms for their actions. Contrary to some reports, made of current-potential curves for formation. Khile the existence of Anson believes, as a general rule, that explaining the basis of the various oxide films is undisputed, the effect of oxidation of a platinum electrode pro- techniques. Chapters on coulometry, these films on electrode reactions re- duces a decrease in the reversibility of potentiometry, amperometry, and non- mains controversial. Davis (40) oxidations occurring at the rlectrode. aqueous solvents, as well as an extensive studied the variation of the rate con- The nature of the electrode surface also bibliography, are included. Reinmuth stant, k, and the transfer coefficicnt. CY, affects rates of organic electrode re- has written a chapter on the theory of of reactions occurring at a platinum actions (22). electrode processes for the new series electrode, with the extent of electrode Other factors may certainly affect the “Advances in Analytical Chemistry and oxidation. He concluded that a heavy behavior of platinum electrodes, and the Instrumentation” (160). The text of layer of oxide suppresses the rate of an absence of these effects must be assured Laitinen (IO?’) includes a chapter on electrode reaction, but a film about 1 before oxide film effects are invoked. electrolytic separations and electrolysis, monolayer thick increased the rate of Adsorption of surface active agents on which gives a brief, but particularly such electrode reactions as the oxidation an electrode is know to affect the rate lucid, discussion of the fundamental of arsenic(II1) or the reduction of of some polarographic reactions (160), principles of electrolysis. Controlled vanadium(V) , vanadium(VI), oxygen, and similar effects on platinum elec- potential electrolysis, as applied to or- and iodate ion. Anson previously had trodes must be expected. Formation ganic chemistry, has been reviewed by shown that iodate reduction proceeded of metal hydrous oxide films when an hIeites (131). Tetter has given a more reversibly at oyidized electrodes electrode is cathodized may aflect modern treatment of electrochemical (6). Linganc demonstrated that an electrode behavior. Even the orienta- kinetics in his recent book (207). A oxide film hinders the oddation of tion of the electrode cystallites can welcome addition to the growing list of oxalate ion (112), but is necessary for affect the rate of electrode reactions. chemical journals is the Journal of the reduction of oxygen (113). So Xikulin (149) recently showed that the I<lectroanalytical Chemistry. This chronopotentiometric reduction wave rate of oxygen reduction on a single journal contains an abstract section, for dissolved ouj-gcn ras found at a crystal of silver depended on the crystal- which provides up-to-date, extensive, platinum electrode n-hich TT as immersed lographic face upon which the reaction abstracts on many phases of electro- several days in an og-gen-saturated occurred. analytical and electrochemistry. sulfuric acid solution. This nas as- EPR Spectroscopy. The past try0 A number of recent review articles cribed to the dissolution of platinum years have witnessed a very rapid have dealt with coulometry. Reilley oxide in the acidic solution. Sawyer growth in the use of electron para- (169) emphasized the importance of the and Interrante (170) concur that the magnetic resonance (EPR) [also called appreciation of thermodynamic and reduction of oxygen occurred with less electron spin resonance (ESR)] spec- kinetic features of electrode processes in overpotential at a freshly oxidized troscopy as a means of identifying the development of coulometric meth- platinum electrode, but assign incrcas- free radical intermediates of electrode VOL 34, NO. 5, APRIL 1962 57 R processes. A knowledge of the elec- ELECTROSEPARATIONS (189); the separation of In from Fe trode reaction mechanism is funda- (172); the separation of Co, Si, and mental to the application of electro- Electrolytic separation of radioactive Cu from Pu (16); and the separation analytical techniques such as controlled isotopes by deposition on mercury or of Sb and Sn from large amounts of potential coulometry to organic deter- solid electrodes continues to enjoy Zn (9). minations. Many, if not most, organic application. Polonium was separated An interesting method for the deter- electrode reactions involve the addition from other metals on a platinum or mination of gases in metals is based upon or abstraction of a single electron as a tantalum cathode (1). An internal the liberation of a gas upon anodic primary step, causing the formation of electrolysis method for separation of dissolution of the metal (100). The a free radical. The presence of an polonium has also been described (204). determination of hydrogen in steel is unpaired electron on this radical sug- B study of the effect of current density described. The method was also con- gested the use of EPR as a means of and various solution conditions on the sidered applicable tu the determination identifying extremely low concentra- electrodeposition of tracer quantities of of nitrogen, but oxygen determinations tions (lop8 to 10-9M) of intermediate the actinide elements from an XH4C1- would probably require dissolution in a radicals, vihich usually continue to react HCI system n-as made by Mitchell (136). nonaqueous medium. in various secondary reactions. The A similar study lvas performed by Samartseva (168). Plutonium was sep- first applications of the electrogenera- ELECTROGRAVIMET RY tion of radicals inside the cavity of an arated by deposition on a tantalum disk EPR spectronieter were made by Maki cathode (214). The separation of ra- Reinmuth (161) has shown that a and Geske (118), who presented evi- dium and barium at a mercury cathode three-dimensional representation of the dence for the production of 'C104 by a constant current technique has reversible deposition of a metal on radical during the electro-oxidation of also been described (102). a solid electrode (plotting potential, lithium perchlorate in acetonitrile. In Further investigations of the elec- current, and percentage deposited) is a subsequent paper (56) they described trolytic separations of the rare earth useful in teaching the theory of elec- the electrogeneration and EPR spectra elements have also been made. Lan- trodeposition. of the nitrobenzene anion radical, a thanum-140 of 99.9% purity was pre- Controlled Potential Methods. The fairly stable radical, and resolved the pared by constant current electrolysis Japanese investigators continue to spectrum into the full 54 hyperfine of a Ba140-La140mixture
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