The effect of tin(II) chloride adsorption on the electrochemical oxidation of tin(II) and other reactions at gold and platinum electrodes

During a study of the electrooxidation of tin(II) chloride at gold and platinum electrodes by cyclic voltammetry, a remarkable hysteresis was associated with the oxidation of tin(II) chloride that is attributed to the adsorption of tin(II) chloride on the gold electrode. The adsorbed tin(II) chloride inhibits the oxidation of the bulk tin(II) chloride and therefore shifts the potential for oxidation to more positive values. However, once the adsorbed tin(II) chloride is oxidized, the dissolved tin(II) chloride is oxidizable at less positive potentials. This inhibition by adsorbed tin(II) chloride also affects other inner-sphere electrode reactions (e.g., oxidation of I−), but not outer-sphere reactions. This inhibition is explained in terms of the need of a bridging ligand to facilitate electron transfer in inner sphere reactions. This inhibition is even more pronounced on Pt electrodes, where no oxidation wave for tin(II) chloride is observed. However, adsorption of chloride ions, 2-mercaptoethanol, or iodide causes desorption of the tin(II) chloride and the appearance of an irreversible wave for oxidation of the tin(II) in solution.