The kinetics of electron transfer in the thionine-coated electrode

Transient studies using potential jump and optical absorbance have been carried out on thionine-coated electrodes. Both types of transient are shown to fit a simple diffusion model. At pH 1.3 the characteristic diffusion time varies with the size of the anion present in the electrolyte, showing that diffusion of the counter ions is the rate-limiting process. At pH 5 faster, more uniform, rates are found which are attributed to rate-limiting electron hopping. The rate constant for this process is in rough agreement with the rate of the conproportionation reaction of thionine and leucothionine. The electron-hopping process also limits the maximum current that can be passed through the modified electrode in the steady state.