Co(III) ammine electroreduction reactions as kinetic probes of double layer structure: The reduction of fluoropentaammine cobalt(III) at varying ionic strengths

The electroreduction rate of fluoropentaammine cobalt(III) was studied in a variety of single electrolytes of varying ionic strengths at the mercury-aqueous interface in order to assess the experimental double layer effects in the presence of anion specific adsorption in comparison with the predictions of the coupled Gouy-Chapman-Stern-Frumkin (GCSF) theory. The net charge densities in the inner layer region determined from the experimental rate data using the GCSF model were usually in good agreement with the corresponding literature values that were determined from equilibrium double layer data over a range of ionic strengths (μ=0.01 to 1.0 M) and electrode charge densities (q^m～0–15μC cm^?2) in NaF, KPF₆, KCl, NaN₃, KNO₃ and NaClO₄ electrolytes. Large discrepancies between these kinetic and equilibrium results were observed in concentrated Na₂SO₄ electrolytes which were ascribed to the effects of ion-pairing between Co(NH₃)₅F²⁺ and SO₄^2?. The relative success of the simple GCSF model for this and other Co(III) ammine reduction reactions is compared and contrasted with the corresponding behavior of other electrode reactions that have been studied previously, and possible reasons for the behavioral simplicity of the present systems are suggested. The suitability of Co(III) ammine electroreduction reactions as kinetic probes of the double layer structure at solid electrode-aqueous interfaces is noted.