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

The kinetics and mechanism of electroreduction of oxalate complexes of Co(II) are studied by de polarography. It is found that the electroreduction process is irreversible and the current is limited by diffusion. According to polarograms, the total two-electron process includes the slow stage of transfer 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+. These form in a preceding reversible chemical reaction from mono or dioxalate complexes of Co(II) that are predominant in solution. The effect of the electrical double layer structure on the rate constant of electroreduction of Co(II) aquacomplexes is discussed. The dependence of the half-wave potential on the concentration of oxalate ions is used to calculate stepwise stability 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.