Electroanalysis and Coulometric Analysis Allen 1. Bard, Department of Chemistry, The University of Texas, Austin, Texas 7871 2 HIS PAPER surveys the literature Kuwana and Coworkers (150, 151) have servation of radiation reflected from or Tand developments during 1966 employed both commercially available scattered by the electrode surface. through December 1967; papers pub- and laboratory prepared electrodes coil- Ellipsometric methods, which involve lished before 1966 which have not ap- sisting of a thin layer of seniiconduct- precise measurements on elliptically peared in previous reviews in this series ing tin oxide on a glass substrate. They polarized light to obtain information have also been included. reported successful results for the oxida- about the propertieb and thicknesses of tion of o-tolidine (which has apparently films formed on the electiode surface, become a standard test system for have been widely used, bee e.g. (82, 148, evaluating optical methods), although 242) and reference> contained theiein. BOOKS AND REVIEW ARTICLES the unique nature of the electrode sur- Direct nieasurements of the reflectance Several new books and journals con- face causes some difficulties. Mark aiid of a polished platinum electrode surface taining material of electroanalytical coworkers (236, 237) prepared tram- have been used to study oxide (137) and and electrochemical interest have ap- parent electrodes by depositing thin films iodine (85) film formation. Perhaps peared. The newest edition of “Stan- of platinum on glass using a commercially the most unique experiments employing dard Methods of Chemical Analysis” available “liquid platinum” preparation. optical methods are tho-e described by (329) includes chapters discussing the The platinum films thus produced were Walkel (326). In these, a continuous fundamentals of electrogravimetric and found to adhere well to the glass aid to specular reflection technique wab em- coulometric methods. Zozulya has be relatively inert to chemical attack. ployed; light from a laser \vas directed written a brief monograph on coulo- An electrode which probably comes at the inside of a cylindrical silver metric analysis (341) and coulometric closest to the conventional electro- cathode in such as niaiiiier that multiple titrations in nonaqueous solvents have chemical working electrode, recently reflections occurred before the final re- been discussed (146, 250). h new se- described by Murray and coworkers flection of the beam to the detector. ries of monographs, “Electroanalytical (199), consists of a gold minigrid elec- Absorbance changes obqer ved have been Chemistry--A Series of Advances,” trode sandwiched between glass plates. ascribed to the presence of hydrated (26) has also appeared. Two recent Because the spacing between the glass electrons at the electrode-solution iiiter- journals containing papers of interest in plates is about 80 microns, the cell also face. The identification of hydrated electrochemistry are the Russian journal behaves as a thin-layer electrochemical electrons tewlting from the electro- Elektrokhimiya (available in an English cell. chemical reduction of protons surely translation as Soviet Electrochemistry) To obtain higher sensitivities and ob- will be of great importaiice in e\tplaiiiing and Analytical Letters. serve only the layer of solution clo-e to the data of the myriad paper.; coiicerned The “Encyclopedia of Industrial the electrode surface, interrial reflection with the hydrogen electrode reaction. Chemical Analysis” contains sections on techniques [e.g., attenuated total re- Laier. have also been employed in re- coulometry (111, 187) and electrode- flectance (ATR) or frustrated multiple cent interferometric (211) ant1 holo- position (67). Gross and Murray have internal reflectance (FMIR) spectrob- graphic interferometric (133) experi- also reviewed several electroanalytical copy] have also been employed. Prob- ments. The major contributions of methods in an encyclopedia article ably the most complete discussion of optical techniques to electrochemistry (93),and the application of coulometry ATR at a tin oxide electrode-solution probably lie in the future. However, (308) and electrodeposition (190) tech- interface, employing visible radiation, interest in these topics is demonstrated niques to trace analysis has been dis- has been given by Hansen, Eiuwma, by talks delivered on theqe techniques at cussed. A number of review articles and Osteryoung (96, 97). Those au- the Winter, 1968, Gordon Re.earch Con- on coulometric titrations (37, 75, 221, thors discussed factors contributing to ference in Electrochemistry by Gcnshaw 261, 295, 301, 338) and controlled po- absorbance changes during electrolysis, and Xuller on ellipsometry, O’Brieu on tential coulometry (144, 220, 258) have and correlated these changes with elec- interferometric methods, 1lcIntyre on appeared. trochemical parameters in different reflectance studies, aiid Kuwana and techniques. .Although the method ap- Iiruger on other optical methods. pears useful, interpretation of the ab- Digital Electrochemical Instrumen- sorbance changes is quite complicated, tation. Electroanalytical instrumen- NEW TECHNIQUES see e.g. (281). Internal reflection spec- tation during the decade 1938-67 was Optical Methods of Observing Elec- troscopy in the visible region has also characterized by eutensive use of trode-Solution Interface. While spec- been reported on the thin platinum film analog computer devices (e g ~ opera- troscopic investigations of solutions electrodes (236, 237). Mark and Pons tional amplifiers). -1lthough digital following electrolysis have been car- (180) reported preliminary result.; on techniques weie used from time to ried out for many years, interest has internal reflection techniques (FMIR) time, for example, in the constiuction turned recently to direct observa- at a germanium electrode under in- of coiilonieters based 011 analog-to- tion of the electrode surface or the frared radiation. Reed and Yeager digital (-ID) converters or in digital de- solution at the electrode surface dur- (24s) have mentioned similar experi- termiliation of transition or coulometric ing the electrolysis. One approach ments in an attempt to determine the titiation times, the last years have involves the use of transparent elec- extent of adsorption of organic species shown increa+ig application of digital trodes so that radiation passes through from aqueous solutions. data acquiyition methods aiid even di- the layer of solution at the electrode h different type of a spectroscopic- rect digital control of electrochemical surface as well as the bulk solution. electrochemical experiment involves ob- proce‘ses. Since digital data acquisition 64 R ANALYTICAL CHEMISTRY methods have been used to record volt- noble metals and metals resistant to HC1 proven effective in the separation of age signals obtained from transducers in solutions; rates of solution for Cd, Cu, lithium isotopes by electrolysis using many different fields, with commercially Au, In, Ir, Pd, Pt, Rh, Ta, and W were lithium amalgam electrodes and di- available systems or modules, the ex- studied (43). Electrographic stripping methylformamide (14, 16) or n-propyl- tension of these methods to electro- of Au and Ag from different alloys has amine (32) solutions of lithium salts. A chemical experiments is quite straight- been reported (270). The anodic solu- Pb-TI mixture has been suggested as forward. Geiierally this involves the tion method for the determination of useful in testing the performance of a use of mi AD converter and a multi- hydrogen in steel and iron coatings has column (15). In a similar experiment, plexer, so that several channels may be been discussed; errors occur in the de- enrichment of a fused PbCI2 solution in scanned, with output on paper tape termination because of loss of hydrogen chlorine-37 by electrolysis in a tube (45) or punched cards (47). in side reactions and other factors (175, filled with powdered glass employing a For rapidly varying signals, where 263). An electrolytic method of de- liquid lead cathode has been described these methods niay be too slow, direct termination of Si3Nain silicon steel has (265). Electrolytic separation in capil- recording iiito the core storage of a also been described (333). Buechel and lary tubes by a related method has also multichannel analyzer niay be employed Effenkammer (48) have suggested that been discussed (325). (160). The advantages of these meth- the dissolution of heterogeneous alloys Other Separations. .I method of ods are that more 1)recise measurements, at constant potential has advantages metal ion removal by deposition on a not limited by reading an oscilloscope over the constant current dissolution powdered metal cathode for the screen or recorder chart, can be made, met hod. purpose of solution purification has that the worker is relieved of analyzing Countercurrent-Exchange Elec- been patented (196). A method of recorded data, and that the data are trolysis. This technique consists of separating electroactive gases such as obtained in a form which can be put into electrolysis at a liquid metal elec- oxygen and hydrogen at catalytically a digital computer without further trode (mercury or an amalgam) with activated electrodes has been described preparation. The procedure can be the electrolyte phase and electrode (IO). Zebreva and Kozlovskii (339) carried OIIC step farther by using digital transported in countercurrent direc- have discussed, in iome detail, the