Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition between disproportionation and stepwise electron transfer

The electrochemical oxidation of aqueous Cr(III) was examined using cyclic voltammetry with a polycrystalline Au electrode in KOH solutions of varying pH and Cr(III) concentration. The mechanism and kinetics for the oxidation of Cr(III) is a quasi-reversible diffusion-controlled reaction and is largely dependent on the solution pH. The reaction mechanism is initiated by an irreversible electrochemical electron transfer to form Cr(IV) which is the rate-determining step (RDS). Following the RDS, subsequent oxidation of Cr to its hexavalent state occurs by the disproportionation of Cr(IV) at low KOH concentrations and electron transfer at high KOH concentrations due to the involvement of OH⁻ in the disproportionation reaction. As the solution pH increases, the Cr(III) oxidation peak potential shifts negatively owing to the involvement of OH⁻ in the RDS. The competitive adsorption of OH⁻ and CrO₂⁻ on the electrode surface also plays an important role in the oxidation behavior.