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The reversible reduction of [S2Mo18O62]4- to [S2Mo18O62]5- and [S2Mo18O62]6- at a glassy carbon macrodisk electrode has been studied by cyclic voltammetry in acetonitrile as a function of the concentration of [(C6H13)4N]4[S2Mo18O62] in the absence and presence of [(C6H13)4N]ClO4 as the added supporting electrolyte. Consideration is given to the influence of scan rate, reference-working electrode distance, [(C6H13)4N]4[S2Mo18O62], and electrolyte concentrations. Experimental data confirm theoretical predictions that cyclic voltammetry at a macrodisk electrode is a viable technique for studies of multiply charged electroactive species without added electrolyte, provided the influence of enhanced complexities associated with effects of increased solution resistance, the mass transport contribution from migration, and convection arising from enhanced density gradients are considered. Enhanced density gradients present in the absence of added supporting electrolyte give rise to a more marked dependence of voltammograms on the angle of the electrode and hence lead to significant distortion of wave shapes at low scan rates. The summation of all these obstacles implies that quantitative evaluation of cyclic voltammograms of multiply charged species requires significantly greater care in the absence than in the presence of added supporting electrolyte.

The meaning and significance of uncompensated resistance are carefully explained. Many factors influence the uncompensated resistance and several of these are explored in this article, using idealized models of an electrochemical cell. Among the factors whose roles have been elucidated are the shape and size of the cell, the location of the reference electrode, the shape of the working electrode, and the size and position of the counter electrode. Worthwhile compensation is shown to be impossible with microelectrodes.

There are circumstances in which it is useful to know the concentration, or its gradient, of an electroactive product or reactant at some distance from an electrode. Here it is demonstrated that this information is available, in principle, under conditions of semiinfinite planar diffusion, provided that the surface concentration and/or the current are known as functions of time. Several methods of calculation are explored, using the example of linear sweep voltammetry. Some of these methods rely on analytical formulations; others are purely numerical. Concordance of the results of the various methods provides validation of all. A method based on the convolution of the current is especially versatile.

An exact treatment derives the steady-state limiting current of a one-electron reduction for the N ⇌ O(+) + A(-) mixture at a hemispherical microelectrode. Either or both of the neutral N and cationic O(+) species may be electroactive. A supporting salt is present at any concentration, including zero or excess; its ions are electropassive and do not interact with the other solutes or each other. The various species are treated as having distinct diffusivities, linked to their mobilities through the Nernst-Einstein relationship. Universal electroneutrality is assumed. The predictions of the model are compared with published experimental data on the reduction of aqueous weak acids; agreement is excellent at intermediate, but poor at low, support ratios. Analysis of the unsupported case shows that the neutral N species dissociates in a narrow zone close to the electrode, and the injection of ions there serves to increase the electric field in the outer region of the transport zone. This enhances cationic migration enormously, leading to an unsupported limiting current that is much more than double the supported value. However, the limiting current is drastically diminished by traces of foreign electrolyte. Curiously, the limiting current with full support adopts the same value when equilibration is fast as when it is very slow, although the mechanisms are totally different.

An exact treatment is developed to predict the steady-state limiting voltammetric current, I(L), for a system in which the reaction O(+) + A(-) ⇆ N occurs reversibly in solution, in the presence of supporting electrolyte C(+)A(-). Either or both O(+) and N undergo a one-electron reduction at the hemispherical microelectrode. The dependence of I(L) on the formation constant of the equilibration reaction, the electrolyte concentration, and the support ratio is derived for any trio of values of these parameters and for any combination of diffusivities of the O(+), N, and A(-) species.