Cycllc voltammetry has been used to measure the extent of superoxlde Ion solvatlon (and consequent shift In the redox potentlal for the 02/02-couple) In acetonltrlle, dlmethylformamlde, pyridine, and dlmethyl sulfoxide. A parallel study of the reverslblllty of the 02/01 couple as a function of electrode materlal has been made for platinum, gold, mercury, and glassy carbon In acetonltrlle. The results Indicate that surface Interactions between superoxide Ion and metal-electrode surfaces occur. Additional studies of the reductlon of oxygenIn the presence of protic substrates and metal salts confirm that Lewis aclds promote the disproportkmatlon of superoxide Ion. I n the presence of electrophlles (e.g., CCI, and esters) the reduction current for dioxygen Is enhanced because of rapid nucleophlllc reactlons by 4-, which result In regeneration of 02.Although the electrochemical reduction of dioxygen to superoxide ion in aprotic solvents has been known since 1965 (1-5), the effects of media and electrode material have not been addressed in any systematic study. An early study (6) established that activated metal electrodes in aqueous media catalyze the reduction of O2 to water at a reduction potential characteristic of the particular metal. In contrast, at passivated metal electrodes the reduction of 02, which is independent of electrode material and pH, yields H202 via a one-electron mechanism with facile disproportionation of the 0,-intermediate (7). Three recent reviews discuss the redox chemistry and electrochemistry of dioxygen (8-10).The general view has been that the electrochemistry of O2 in aprotic media also is independent of media and electrode materials, and that only protic contaminants promote the rapid disproportionation of the 02-product to H202 and 0 2 . For a test of this conclusion the present study has been undertaken to compare the effects of various media and electrode materials on the reduction potential, reversibility, and overall electron stoichiometry for the cyclic voltammetry of dioxygen. EXPERIMENTAL SECTIONInstrumentation. The cyclic voltammetric measurements were made with either a Princeton Applied Research Model 173/ 175/179 potentiostat/universal programmer/electronic integrator or a universal potentiostat/amperostat that was constructed from operational amplifiers (11). The electrochemical cell (Brinkman) included (a) a platinum-foil auxiliary electrode that was isolated from the bulk solution by a glass tube with a medium frit, (b) a Ag/AgCl reference electrode in a glass tube closed with a cracked sofbglaas tip that contained aqueous Me4NCl (concentration was adjusted to obtain a potential of 0.000 V vs. SCE) (12), (c) a luggin capillary that contained the bulk electrolyte solution and the reference electrode (the capillary tip was placed within 1 mm of the working electrode surface), (c) inlet/outlet tubes for gas dispersion in the solution or over its surface, and (d) the working electrode. The latter was either a Beckman Model 39273 platinum-inlay electrode (area, 0.23 cm2) or on...

The proton-induced disproportionation of superoxide ion (Of) in dimethylformamide and in acetonitrile has been studied by stopped-flow spectrophotometry, cyclic voltammetry, and controlled-potential voltammetry at a rotating-disk electrode. For strongly acidic substrates (HC104, HC1, and NH4C104) the disproprotionation reaction is second order with respect to Of, but too rapid to measure accurately, k > 107 M'1 s'1. With less protic substrates (ascorbic acid, phenols, catechols, alcohols, and water) the rate of disproportionation is first order with respect to Of and with respect to substrate; proton transfer is the rate-limiting step. In dimethylformamide the second-order rate constants range from k = 2 X 104 M"1 s"1 for ascorbic acid to k = 1 X 10'3 for water. On the basis of the kinetic data, a self-consistent mechanism for superoxide disproportionation by acidic substrates in nonaqueous solvents is proposed.

No abstract

No abstract

No abstract