Study of the iodide—tri-iodide redox electrode in dimethylsulphoxide

Abstract

The electrochemistry of solutions of sodium iodide and sodium iodide with added iodine dissolved in dimethyl sulphoxide has been investigated in the temperature range from 25 to 45°C. The current voltage curves were determined using a platinum rotating disk electrode working at 300 up to 6000 rpm. Both anodic and cathodic processes were considered, in particular when solutions with added iodine were employed. The anodic current/voltage curves are characterized by two well defined steps, the total limiting current being proportional to the iodide-ion concentration, while the cathodic current/voltage curve involves only one step, the limiting current density being linearly related to the tri-iodide-ion concentration. From the diffusion-controlled processes, the diffusion coefficients of the diffusing species in both the cathodic and anodic processes were evaluated. The ratio between the diffusion coefficients of tri-iodide and those of the iodide ions is in agreement with the predictable values based on the results of the corresponding aqueous systems. The current/voltage curves in the region before the limiting current densities are appreciably irreversible. Those curves were obtained by the method of Frumkin and Tedoradse for intermediate kinetics, and the kinetic parameters of the reactions, including the reaction orders, have been calculated. The kinetics of the reactions are discussed in terms of three possible reaction schemes which comprise the initial participation of iodide or tri-iodide ions. The experimental transfer coefficients for the cathodic and anodic processes add up to one.