Kinetics and mechanism of the cobalt(II) electroreduction on a dropping mercury electrode in oxalate solutions containing an excess of the supportingelectrolyte

The kinetics and mechanism of electroreduction of oxalate complexes of Co(I I) are studied by de polarography. It is found that the electroreduction pr ocess is irreversible and the current is limited by diffusion. According to polarograms, the total two-electron process includes the slow stage of tra nsfer of the first electron which occurs via an outer-sphere mechanism. It is shown that, in the region of the total oxalate ion concentration 10(-4) to 10(-2) M, the slow electrochemical stage involves aquacomplexes Co2+. Th ese form in a preceding reversible chemical reaction from mono or dioxalate complexes of Co(II) that are predominant in solution. The effect of the el ectrical double layer structure on the rate constant of electroreduction of Co(II) aquacomplexes is discussed. The dependence of the half-wave potenti al on the concentration of oxalate ions is used to calculate stepwise stabi lity constants for the CoC2O4 and Co(C2O4)(2)(2-) complexes, specifically, K-1 = 1.3 x 10(-4) M-1 and K-2 = 4.0 x 10(-2) M-1.