Examination of the Zinc Cementation of Cadmium in Aqueous Solutions

The kinetics of the cementation of cadmium by zinc have been studied over a range of solution pH values and initial cadmium concentrations. The kinetics were interpreted in terms of a model postulating a transition from a migration controlled mechanism with consequent half order kinetics to a diffusion controlled regime following first order kinetics. The conditions under which each mechanism was found to apply were consistent with the model in which the immediate reductant was hydride ion generated by the interaction of hydrogen ions with the zinc. The reaction stoichiometry was remarkable in that conditions were observed under which more than the theoretical one mole of cadmium per mole of zinc could be reduced. This was ascribed to the electrolysis of water by the induced electrochemical system. A lag in both cadmium cementation and zinc dissolution was observed in many runs. The zinc dissolution was found to commence about 25% sooner than cadmium cementation, a fact ascribed to the establishment of the reducing electrochemical system.