Double-layer effects in the electroreduction of cobalt(III) trioxalate complexes to cobalt(0) atoms on a dropping mercury electrode

A polarographic investigation of solutions containing trioxalate complexes of Co(III) reveals that the overall electrode process Co(III) --> Co(0) is accompanied by slow formation of ionic associates between aquacomplexes of Co(II) and trioxalate complexes of Co(III). Use of a relatively high concen tration of oxalate ions (0.001 M) quells this association. Under these cond itions, it is possible to divide the measured current into two constituents related to the primary electroreduction Co(III) --> Co(II) and the subsequ ent process Co(II) --> Co(0). The earlier data on the rate constant and app arent transfer coefficient for the process Co(II) --> Co(0) are used for ca lculating corrected Tafel plots for the electrode reaction Co(III) --> Co(I I). The plots are practically linear and independent of the concentration o f the supporting-electrolyte cation in a wide range of potential drops (sim ilar to1 V) across the dense layer. A theoretically expected parallel shift of the plots is observed in solutions with a variable concentration of dis charging species. This observation and similar data obtained for a mixed po tassium-sodium supporting electrolyte bear testimony to good agreement betw een the obtained results and the theory.