The effect of blocking additives on the electrodeposition of cadmium

Voltammetric and current efficiency studies were made on the effect of benzyl and substituted benzyl alcohols on the competitive deposition of cadmium and evolution of hydrogen. The adsorbed alcohol additives acted as general blocking agents blocking the evolution of hydrogen. In the weakly complexing sulfate system these additives formed complexes with the Cd(II), and in the chloride system they caused a cathodic shift of the cadmium reduction potential, indicating a shift in the distribution of the chloride complex ions to a higher concentration of the more highly complexed species. Electrogenerated hydroxyl ions precipitated cadmium hydroxide in longer chain alcohol films. It was shown that in the presence of these alcohol additives in the chloride system in which a portion of the Cd-bearing ions are negatively charged, the addition of quaternary ammonium ions, which are known phase-transfer catalysts, accelerated the rate of reduction of cadmium(II), indicating that ion pairing occurred. On the other hand, these additives either inhibited or did not affect the rate of reduction of cadmium(II) in the sulfate systems in which the cadmium ions are present as positively or neutrally charged species. The fact that the ion pairing and acceleration occurs with the negative chloride complex, even though this complex is present in very small concentrations in the bulk of the solution, indicates that the presence of the low-dielectric-constant alcohol at the electrode interface has increased the concentration of these negative complexes in the neighborhood of the electrode.