A comparison of the effects of ion pairing and blocking additives on electrodeposition of zinc and cadmium

A comparison was made of the effects of benzyl alcohol and tetraalkylammonium ions on current efficiencies for deposition of cadmium and zinc. Benzyl alcohol, which blocked hydrogen evolution and lowered the interfacial dielectric constant, increased current efficiencies for the deposition of zinc and cadmium. Tetraalkylammonium salts, which had been shown to behave consistently as ion pairing catalysts for deposition of cadmium from chloride systems, were found to be inconsistent catalysts for deposition of zinc from chloride media. This difference is attributed to the fact that formation constants of the cadmium chloride complexes are much larger than formation constants for the zinc complex. Because zinc-cyanide formation constants are higher than zinc-chloride formation constants, the ion pairing catalysis of the tetramethylammonium ion in the presence of cyanide, with and without benzyl alcohol, was higher in the cyanide system, leading to higher current efficiencies. In the presence of benzyl alcohol, when tetrabutylammonium ions were used they did not catalyze deposition of the metal as well as tetramethylammonium ions. This difference in rate is ascribed to the presence at the cathode of a benzyl alcohol film with pores through which the smaller tetramethylammonium ion-pair can pass more easily than can the larger tetrabutylammonium ion-pair.