Ring–disc electrodes. Part 23.—Studies of proton fluxes at a thionine-coated electrode

Abstract

A ring–disc electrode with a bismuth oxide ring has been used to measure the flux of H⁺ or OH^– entering or leaving a thionine-coated electrode as the coat is either reduced or oxidised. Experiments have been carried out in the pH range 4–9. Comparison of the proton and electron fluxes allows the determination of the dissociation constants of the leucothionine species in the coat. The current transients are very similar for reduction and oxidation and do not vary significantly with pH, whereas the proton fluxes exhibit very different behaviour depending on the direction of change and on the bulk pH. This suggests that electron transfer, as opposed to counterion diffusion, is the rate-limiting process in charge transfer in the layer. This conclusion is further supported by the fact that on oxidising leucothionine three protons and two electrons are released simultaneously . If counterion diffusion were rate limiting, then there would be a fast two-proton two-electron change followed by a slower release of a third proton and counterion. This is not observed.