Ring–disc electrodes. Part 21.—pH measurement with the ring

The development of a new ring–disc electrode technique is described in which a bismuth ring electrode is used as a potentiometric detector of changes in pH produced by reactions taking place on the disc electrode. The convective-diffusion equation for the system is solved and theoretical results are presented for the ‘detection efficiency’, which compares the change in pH at the ring electrode with that at the disc-electrode surface. The effects of buffers are considered. Provided that CO2 is excluded, experimental results are found that are in excellent agreement with the theoretical predictions. Results are presented for the generation of H+ and OH– on nickel, gold and thionine-modified electrodes. The technique is shown to be very sensitive in that fluxes as low as 10–12 mol cm–2 s–1 and changes in coverage of fractions of a monolayer can be measured.