Evaluation of kinetic parameters from steady-state voltammograms at ultramicrodisk electrodes.

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Platinum ultramicrodisk electrodes 0.8 μm in radius were constructed by anodic electropolishing of a platinum wire followed by sealing it with soft glass. Voltammograms for oxidation of [Mo(CN)8]4−, Fe2−, and [Fe(CN)6]4−, being of a sigmoidal form, were measured at these electrodes and microdisk electrodes with 7.5 μm in radius under the essentially steady-state condition. A voltammogram of Fe2+ at 7.5 μm electrode was drawn out, exhibiting a halfwave potential shifted from the formal potential. This trend was more distinctive for a wave at 0.8 μm electrode. Radii of the electrodes and diffusion coefficients of the electroactive species were determined consistently from the limiting currents. The conventional log-plot showed that electrode reactions of Fe2+ and [Fe(CN)6]4− were, respectively, totally irreversible and almost reversible at these electrodes. Application of the modified log-plot to the voltammograms yielded straight line, of which slope allowed us to evaluate the transfer coefficient and of which intercept gave values of the charge-transfer rate constant. Simulated voltammograms at these values were in good agreement with experimental ones.
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