The anodic behavior of iodide at platinum in the presence of an iodine film under potentiostatic steady-state and hydrodynamic modulation conditions

Abstract

The oxidation behavior of iodide on platinum covered by an iodine film was investigated using rotating ring-disk electrode under potentiostatic steady-state conditions at constant rotation speed and using hydrodynamic modulation techniques. The ring electrode response shows that there is no net accumulation of iodine species at the disk, and that the disk current is controlled by mass transport coupled to the equilibrium I− +I2(s) 6jI3−. The steady-state film thickness depends on the flux of iodide at the film/solution interface. Iodide transport through the film probably occurs through a Grotthuss chain-transfer type mechanism, with a diffusion coefficient estimated to be 3×10−8 cm2/s. The modulation response for iodide oxidation is only about 15–20% of that predicted for convective-diffusion control, indicating that film relaxation processes are slower than the relaxation of the diffusion layer. The transient response to a step, change in rotation speed is the result of iodine film dissolution being limited by mass transfer of iodide ion to form triiodide at the iodine film/solution interface.