Temperature dependence of the transfer coefficient. The reduction of iodate in alkaline media

Abstract

The reduction of iodate on a dropping mercury electrode was studied over a wide range of temperatures, and compared with earlier studies on the reduction of bromate under nearly identical conditions. A fully computerized experimental setup was used. This allows collection of a large number of experimental points, for proper statistical analysis of the data. The transfer coefficient was found to be a function of temperature, decreasing by about 10% with increasing temperature over 80°C. This behavior is very different from that found earlier for the reduction of bromate, where α is strictly independent of temperature. The reduction of iodate occurs at less negative potentials than that of bromate, far from interference by solvent decomposition. This allows very accurate determination of the Tafel parameters, but makes diffuse double layer correction more difficult, since specific adsorption cannot be excluded. We have tested the effect of specific adsorption of I − by changing the concentration of iodate and by adding as much as 10 mM of NaI. The effect of the chloride was tested by replacing it with fluoride as the anion of the supporting electrolyte. The effect of surface active agents was tested by adding Triton X-100. Small variations in the transfer coefficients were observed between these experiments, but the temperature dependence of α persisted in all cases. Correction for the diffuse double layer effect changes the value of the transfer coefficient, but does not eliminate its temperature dependence. It must be concluded that the transfer coefficient for the reduction of iodate is temperature dependent, and this dependence cannot be attributed to experimental inaccuracy or inadequate correction for the effect of the diffuse double layer resulting from specific adsorption. This is in contrast to our findings for the reduction of bromate, hydroxylamine and the H 3O + ion, for all of which α is found to be strictly independent of temperature.