TEMPERATURE-DEPENDENCE OF THE TRANSFER-COEFFICIENT - THE REDUCTION OFIODATE IN ALKALINE MEDIA

The reduction of iodate on a dropping mercury electrode was studied ov er a wide range of temperatures, and compared with earlier studies on the reduction of bromate under nearly identical conditions. A fully co mputerized experimental setup was used. This allows collection of a la rge number of experimental points, for proper statistical analysis of the data. The transfer coefficient was found to be a function of tempe rature, decreasing by about 10% with increasing temperature over 80 de grees C. This behavior is very different from that found earlier for t he reduction of bromate, where alpha is strictly independent of temper ature. 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 adsorp tion cannot be excluded. We have tested the effect of specific adsorpt ion of I- by changing the concentration of iodate and by adding as muc h 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 effec t of surface active agents was tested by adding Triton X-100. Small va riations in the transfer coefficients were observed between these expe riments, but the temperature dependence of alpha persisted in all case s, Correction for the diffuse double layer effect changes the value of the transfer coefficient, but does not eliminate its temperature depe ndence. It must be concluded that the transfer coefficient for the red uction 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 adsorpt ion. This is in contrast to our findings for the reduction of bromate, hydroxylamine and the H3O+ ion, for all of which alpha is found to be strictly independent of temperature.