Must ac voltammetry employ small signals?

This paper revisits reversible ac voltammetry from a contemporary viewpoint, stressing aspects that are not confined to small applied voltages. Exact new formulas are derived that give the heights of the ac voltammograms for both the fundamental and harmonic frequencies when the ac signal Vcos{ωt} is applied across the faradaic element of a reversible working electrode, together with a slowly ramped dc voltage. It is demonstrated that, at low values of V, the magnitude of the peak ac current at the fundamental frequency is proportional to V, whereas this magnitude becomes independent of V when the latter is large. The phase shift is 45° under all conditions. This algebraically predicted behaviour is supported by the results of Fourier transform calculations of the shapes of the fundamental and harmonic ac waves. A new result, with exciting analytical implications, is that the area under an ac voltammogram is accurately proportional to V irrespective of V’s magnitude. A novel, and more useful, interpretation of faradaic admittance is suggested. A careful examination of the roles played by uncompensated resistance and double-layer capacitance shows that the effects of these extraneous elements differ between the fundamental and harmonic frequencies. Formulas are presented for correcting for the presence of these circuit elements.