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 tetraalkyla mmonium ions on current efficiencies for deposition of cadmium and zin c. Benzyl alcohol, which blocked hydrogen evolution and lowered the in terfacial dielectric constant, increased current efficiencies for the deposition of zinc and cadmium. Tetraalkylammonium salts, which had be en shown to behave consistently as ion pairing catalysts for depositio n of cadmium from chloride systems, were found to be inconsistent cata lysts for deposition of zinc from chloride media. This difference is a ttributed to the fact that formation constants of the cadmium chloride complexes are much larger than formation constants for the zinc compl ex. Because zinc-cyanide formation constants are higher than zinc-chlo ride formation constants, the ion pairing catalysis of the tetramethyl ammonium ion in the presence of cyanide, with and without benzyl alcoh ol, was higher in the cyanide system, leading to higher current effici encies. In the presence of benzyl alcohol, when tetrabutylammonium ion s were used they did not catalyze deposition of the metal as well as t etramethylammonium ions. This difference in rate is ascribed to the pr esence at the cathode of a benzyl alcohol film with pores through whic h the smaller tetramethylammonium ion-pair can pass more easily than c an the larger tetrabutylammonium ion-pair.