Double-layer effects on simple electrode reactions II. The effects of specifically adsorbed anions and ion-pairing on the reduction of Eu3+ and Cr3+

The kinetics of the reduction of Eu³⁺ and Cr³⁺ at mercury electrodes have been studied in various mixed perchlorate electrolytes containing iodide, bromide, and p-toluene-sulfonate as adsorbing anions. Specific adsorption data were obtained by means of the constant ionic strength approach due to Hurwitz and Parsons. The rate enhancements observed in the presence of the first two (monatomic) anions were in good agreement with the predictions of the simple Frumkin model only when the experimental, rather than calculated diffuse-layer capacitances were employed in the data analysis. It was demonstrated that the effects of ion-pairing and ligand-bridging of the reactants with the adsorbing anions were negligible under the experimental conditions chosen. From experiments with these systems and also with weakly adsorbing chloride electrolytes it was concluded that ion-pairing did not enhance the electrochemical reactivity of either Eu³⁺ or Cr³⁺. The value of the analyses described in separating various contributions to the catalytic effects of adsorbing anions is emphasized.