The electropolishing of rotating copper‐disk electrodes in concentrated phosphoric acid is studied by steady‐state measurements, ac‐impedance spectroscopy, and flow‐modulation spectroscopy. The effects of applied potential, rotation speed, and temperature on the response of the system are investigated at the mass‐transfer‐limited plateau. Both the Arrhenius behavior of the limiting current and the ac‐impedance data suggest that the rate of reaction is controlled by mass transfer of an acceptor to the copper surface and not by precipitation of a salt film. It is not possible to determine from steady‐state measurements the diffusion coefficient of the acceptor because its identity and bulk concentration are not known with certainty. Therefore, the steady‐state measurements are complemented with flow‐modulation spectra which allow an estimation of the diffusion coefficient of the acceptor without knowledge of its bulk concentration.
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Vidal et al. (1995) studied this question.