The Kinetics of the Iodine Redox Process at Platinum Electrodes

Abstract

Three paths for the iodine redox process at plat inum electrodes are dis-cussed with reference to stationary current/potential curves obtained for the cathodic reduction of the tri- iodide ion in potassium iodide solutions. It is shown that the process follows the path originally proposed by Vetter, the slow stage being the transfer of ~n electron from the plat inum to an adsorbed iodine atom, and that the degree of reversibil ity of the process increases as the con-centration ratio of tri-iodide to iodide is decreased. The apparent energy of activation for the over-all process is 5.3 kcal 'mole- ~ for solutions containing only potassium iodide and tri-iodide. The results, and those of other workers, in-dicate that the electrode surface is nonuniform. Comparat ive ly l itt le work has been publ ished on the kinetics of the iodine redox process at p la t inum electrodes. Vetter (1) has commented on work pub-l ished prior to 1952, and has studied both the anodic and cathodic directions using 1N sulfuric acid as support ing electrolyte. Using the d-c stat ionary cur-rent /potent ia l curve method, he found the reac-t ion to be fast such that the nature of the redox process was largely obscured by the t ransport of iodine to and from the electrode. For this reason, he turned to an a-c method (2), and, from the deter-mined dependence of the exchange current density on the concentrat ions of the iodide and tr i- iodide ions, deduced a mechanism which may be symbolized as follows, where S denotes a surface site, the nature of which is to be established.