Square‐wave voltammetry at spherical and disk microelectrodes as a function of electrode radius and frequency

Square‐wave voltammograms obtained with small spherical electrodes and influenced by electrode kinetics have been analyzed theoretically as a function of electrode radius and frequency for a reduction reaction involving a soluble reactant and product. If the redox reaction is reversible, or totally irreversible, dimensionless peak currents depend linearly on the parameter y = (D₀/ƒ)^1/2/r, where r is the electrode radius, D₀ the diffusion coefficient of the oxidized species, and ƒ the square‐wave frequency. The variation of electrode size or frequency enables the determination of the charge transfer coefficient α or the kinetic parameter k_s/D^1/2 of an irreversible reaction (k_s = standard heterogeneous charge transfer rate constant) if its reversible peak potential is known. Differences between square‐wave voltammograms at spherical and inlaid disk microelectrodes are considered theoretically and shown to be small under most conditions. As expected, in the limiting case where steady‐state mass transfer conditions prevail at microelectrodes, both the square‐wave current and peak potential become independent of the frequency. Steady‐state voltammetry, which is readily achieved at a slow d.c. scan rate of potential, may be more suitable than square‐wave voltammetry for kinetic measurements at microelectrodes, although the latter is likely to be the more sensitive technique in the analytical sense. This is in contrast to conventionally sized electrodes where the square‐wave method is likely to be superior in all contexts. Criteria of reversibility are proposed and confirmed by square‐wave voltammetric measurements on the reversible oxidation of ferrocene and quasi‐reversible reduction of ferricyanide at an inlaid gold microdisk electrode.