Effect of the formation of a charged intermediate on the electroreduction of cobalt(III) trioxalate complexes to cobalt(0) atoms

Kinetic relationships for the recharge of a cobalt(III) trioxalate complex on a mercury electrode are derived for electrode potentials of 0 to -1.5 V vs. point of zero charge (PZC) of mercury. This wide range includes high ne gative potentials at which the recharge of Co(C2O4)(3)(3-) an discharge of Co2+ occur simultaneously. The contribution of the reaction Co(II)/Co(0) (w hose kinetic parameters are known) into the overall reduction of Co(C2O4)(3 )(3-) to metallic cobalt is calculated. Migration fluxes of species present in solution are shown to be insignificant; hence, these can be neglected i n the calculations. Relationships, which permit the determination of a part ial polarization curve for the Co(III)/Co(II) recharge from the overall rat e of the Co(III)/Co(0) process, are found. A quantitative evidence for the appearance of a secondary current drop at potentials near -0.7 V vs. PZC is obtained. The drop is caused by variations in the outer Helmholtz plane po tential because of the commence of discharge of Co2+.