THE CHRONOPOTENTIOMETRIC METHOD FOR MEASURING THE KINETICS OF METAL DISSOLUTION AND CEMENTATION REACTIONS

The validity of the chronopotentiometric method, which has been used f or studying the kinetics of metal dissolution and cementation (displac ement) reactions was examined. Using the dissolution of electrodeposit ed zinc films as an example, results are presented for the dissolution in a triiodide solution, in an acid solution and in an acidified trii odide solution. The effect of the substrate electrode, on which the zi nc film is deposited, was found to be negligible in the case of the tr iiodide dissolution reaction which is known to be diffusion controlled . However, the substrate electrode can have a profound effect on the d issolution in acid, a reaction which is known to be chemically control led. In addition, the apparent dissolution rate in acidified triiodide solutions can be greater than the sum of the separate dissolution rat es. Explanations for these findings, based on the catalytic effect of the substrate electrodes, are presented. The applicability of the chro nopotentiometric method to the cementation of copper (II) on to electr odeposited zinc films was also investigated. Similar difficulties to t hose found with the zinc/acidified triiodide system were encountered. As a result of the understanding of the pitfalls of the chronopotentio metric method developed in this work, it was possible to review critic ally previous measurements of this type, which have been reported in t he literature. It was concluded that, while the majority of previous s tudies are correct, a serious error has been made in one paper.