Kinetics and mechanism of deposition of zinc from zincate in concentrated alkali hydroxide solutions

Abstract

Galvanostatic investigation has been carried out of the kinetics of the reaction on zinc amalgam hanging mercury drop immersed in alkaline zincate solutions at KOH concentrations ranging from 1 to 14 normal. Three kinds of data have been extracted: (i) the pseudo-capacitance as a function of overpotential from the portion of the charging curves prior to plateaux, (ii) the Tafel plots from quasi-steady state values at the plateaux and (iii) the Sand's products as function of current density from transition times. In (i), the pseudo-capacitance was found to increase with overpotential, in (ii), two slopes (60 mV dec-1 and 120 mV dec-1) have been found in the cathodic direction and a limiting current in the anodic one; in (iii), the Sand's product was found to decrease with increasing (i). On the basis of the above findings the reaction mechanism has been suggested to be of the cece type, with the chemical dissociation of the intermediate univalent zinc complex in between the two elctrochemical steps as rate-determining. A modification of the method of evaluating the reaction orders was introduced to account for a simultaneous change of more than one activity of reactants in concentrated solutions. Using this, reaction order was found to be 2 with respect to both OH- ions and water, suggesting that the electroactive species is not only reduced in the number of ligands but also dehydrated with respect to the prevailing species. The anodic limiting current appears to be due to the accumulation of Zn(OH) species to cover all the surface free of adsorbed Zn(OH)2. Exchange cd and rate constants of all the four steps of the reaction have been estimated.