Electrochemical behaviour of iodide at a rotating platinum disk electrode in methanol

The oxidation of iodide at a rotaling platinum disk electrode in methanol has been studied. At low concentrations only one oxidation wave is seen, but at higher concentrations two steps are discerned. This is in accordance with the theory, assuming that the equilibrium between iodide, ttiiodide and iodine is reached fast and the oxidation of iodide to iodine occurs reversibly. only in approximately neutral and moderately alkaline solutions iodine can be. oxidized to iodonium methoxide. The oxidation occurs irreversibly. Neutralization titrations confirm the formation of iodonium methoxide. The diffusion coefficients of the iodine species and the stability constant of the triiodide ion were determined. INTRODUCTION For several reasons the voltammetry of iodide in various solvents is of interest. Though the behaviour of iodide in aqueous solutions has been studied extensively[l], just as in many organic solvents, as acetic acid[2], acetonitrile[3], dimethylformamide[4], dimethylsulfoxide[5], nitromethane[6] and pyridine[‘I], remarkably little has been published on the voltammetry of iodide in methanol. In the context of research on the coulometric Karl-Fischer titration reaction, which is very often performed in methanol, we have investigated the electrochemical behaviour ofiodide in this solvent. We have confined ourselves to iodide species with oxidation numbers 1, l/3,0 and + 1, since higher oxidation numbers than + 1 are not of importance in the context of our investigations on the Karl-Fischer titration. Oxidation of iodide to iodine In dilute methanolic solutions up to ca 1 mh4, iodide shows one oxidation wave to iodine, while in more concentrated solutions it is clearly oxidized in two steps, via the formation of triiodide : 21til, + 2e(1) I, + I-*1; . (2)