Formation of Thick Layers of Iodine During the Anodic Oxidation of Iodide on a RDE

The open-circuit behavior of iodine layers formed on a RDE during the anodic oxidation of iodide was investigated. A plateau in potential was observed. Its value was found to depend on the concentration of free iodine in solution, and its length on the amount of iodine on the surface. It was shown that the sudden change in the nature of the iodine layer associated with the sharp aecrease in current is not due to a phase transformation from supercooled liquid to solid iodine. The rate of decay of potential on open circuit from steady state can be used to estimate the average diffusion coefficient of NaI in the iodine layer. The experimental observations are consistent with a model in which I(-) ions migrate or diffuse through the layer of I(2) and are oxidized at the metal/I(2) interface, while iodine is being dissolved at the I(2)/solution interface. Steady state is reached when the rates of these two processes become equal.