Abstract
Disk electrodes consisting of Graphite-azobenzene'-mixtures were used. The total height of both waves, one due to hypoiodite formation and the other to iodate formation, is determined by the diffusion of hydroxyl ion to the electrode, if the solution is not buffered and if the hydroxyl ion concentration lies within a certain range. It was thus possible to determine the diffusion coefficient the hydroxyl ion as a function of ionic strength. An analysis of the potential/current- density diagrams of iodate formation shows that the electron-transfer reaction is first order with respect to iodide, 0·23 order with respect to OH − ion in the pH range 6–10·6 and zero order at higher pH values. This and other observations lead to the conclusion that iodide ions discharge on those portions where hydroxyl ions are adsorbed on the surface, the adsorption being governed by an adsorption isotherm of the Freundlich type. Some further qualitative observations support the mechanism OH aq− ⇌(OH) ad− I − + (OH −) ad− →(JOH −) ad + e. The dependence of half-wave potential on rate of rotation and pH are in quantitative agreement with the mechanism proposed. The height of the diffusion-controlled limiting current of iodate formation in the presence varying amounts of phosphoric acid shows that increasing phosphoric acid content of the solution decreases the iodide diffusion coefficient.
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