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 concentration of oxalate ions (0.001 M) quells this association. Under these conditions, it is possible to divide the measured current into two constituents related to the primary electroreduction Co(III) → Co(II) and the subsequent process Co(II) → Co(0). The earlier data on the rate constant and apparent transfer coefficient for the process Co(II) → Co(0) are used for calculating corrected Tafel plots for the electrode reaction Co(III) → Co(II). The plots are practically linear and independent of the concentration of the supporting-electrolyte cation in a wide range of potential drops (∼1 V) across the dense layer. A theoretically expected parallel shift of the plots is observed in solutions with a variable concentration of discharging species. This observation and similar data obtained for a mixed potassium–sodium supporting electrolyte bear testimony to good agreement between the obtained results and the theory.