Ring–disc electrodes. Part 22.—Theory of the measurement of proton fluxes at the disc

Abstract

Proton fluxes at the disc of a ring–disc electrode can be measured by potentiometric measurement with a bismuth oxide ring electrode. When the bulk pH is close to 7, fluxes as low as 10^–12 mol s^–1 cm^–2 can be measured. Theory is developed to describe the relation between the disc flux and the ring pH, allowing for the fact that the diffusion layer may be divided into an H⁺ region and an OH^– region with a titration reaction taking place on the interface between the regions. Analytical expressions, allowing for the different diffusion coefficients of H⁺ and OH^–, are derived for four separate cases. A case diagram is constructed to show the interrelation between the different cases and to show the relation between the disc flux, the bulk pH and the pH at the ring.