ELECTROCHEMICAL-BEHAVIOR OF SOME CHELATE AMINOCARBOXYLATE COMPLEXES OF COBALT IN AQUEOUS-SOLUTIONS

Electrochemical behavior of cobalt iminodiacetate, nitrilotriacetate, and (ethylenediamine)tetraacetate (Coida . 2H2O, NaConta . 2H2O, and N aCoedta . 2H2O, respectively) on Hg, Au(Hg), Au, Ag, Pt, Pt(Hg), and v itreous carbon was studied by polarography, voltammetry, and rotating disk electrode techniques. It was shown that electrooxidation of [Coid a]0 at the mercury electrode was complicated by mercury dissolution an d that the processes with [Coida]0 and [Conta]- participation occurred with pronounced overpotential at Au-, Pt-, Ag-, and vitreous carbon e lectrodes and were due to electrochemical transformations of the ligan d. The transfer coefficient alpha and the rate constant k(alpha = 0.5, logk = -3.17 +/- 0.02) were determined for [Coedta]- electroreduction at Au(Hg). It was found that solution pH did not affect the [Coedta]- electroreduction rate at Au(Hg) and Hg over the entire range of reage nt stability. The electrostatic retardation of this process was not ob served at the negative surface charges at Hg. The retardation of this process was observed for the Au and Pt electrodes in a wide potential range. This retardation appears to be a result of ligand adsorption, w hich accumulates near the surface due to the difference in the stabili ty constants of the reagent [Coedta]- and the product [Coedta]2-. The analysis of corrected Tafel dependence of [Coedta]- electroreduction o n Au(Hg) allows us to make the conclusion that the center of dischargi ng species [Coedta]- in transition state is localized at the outer Hel mholz plane.