Potentiodynamic, RRDE, chronopotentiometric and chronoamperometric methods were used to investigate the mechanism of the anodic oxidation of iodide-mediated sulfur dioxide solution. The properties of catalytic current in chronoamperometry are derived and the rate constant of the reaction between iodine and sulfur dioxide in sulfuric acid is determined from the experimental catalytic current.
The catalytic depolarization effects of potassium iodide are between 0.25V and 0.5V, depending on electrode materials and current densities. The anodic oxidation behaviors are different at the platinum and graphite electrodes and are affected by the ensuing chemical reactions. At low electrode potential iodide ions are initially oxidized to iodine and this is followed by the chemical reaction of iodine with sulfur dioxide. At high electrode potential the anodic oxidation of iodide progresses to iodate and this is followed by the chemical reaction of iodate with sulfur dioxide. The iodide ions regenerated from the chemical reaction near the electrode surface are available again for oxidation without a supply of iodide ion from the bulk solution. The anodic current increases with the increase of potassium iodide concentration, sulfur dioxide concentration, and rotation speed of electrode.
The intermediate product, i.e. iodine, was detected by the RRDE measurement. The ring current generated by the electrochemical reduction of iodine increases with the increase of potassium iodide concentration and rotation speed of electrode. The transition time in chronopotentiometry for the anodic oxidation of iodide-mediated sulfur dioxide solution is much longer than that for the electrochemical oxidation of iodide solution by the regeneration of iodide near the electrode surface. The RDE, RRDE and chronopotentiometric results demonstrate almost no electrochemical oxidation of sulfur dioxide.
The reaction rate constant of iodine and sulfur dioxide at 22℃ in 0.5M sulfuric acid is $1.2×10^4(1.mol^{-1}.sec^{-1})$. the reaction layer thickness in which the chemical reaction between electrogenerated iodine and sulfur dioxide occurs and the diffusion layer thickness for the case of pure diffusion are discussed. The reaction layer thickness decreases with the increase of sulfur dioxide concentration and is in the range of $6.3×10^{-5}$ to $4.0×10^{-4}cm$ for 5.0-200mM $SO_2$ at the steady state.
The depolarization effects of iodide-mediated sulfur dioxide solution in comparison with the conventional water electrolysis are 1.4V for the graphite electrode and 1.65V for the platinum electrode.
