Voltammetric measurements without ohmic and other forms of distortion in aromatic hydrocarbon solvents

Abstract

Aromatic hydrocarbons such as benzene, toluene and xylene represent a unique class of solvents of considerable interest for electrochemical research and technology. Unfortunately, voltammetry in these high resistance aromatic hydrocarbons is complicated by ohmic iR drop and other problems when using conventional sized electrodes or even microelectrodes (radius > 1 μm). The use of hexylammonium perchlorate as an electrolyte and ultramicroelectrodes (radius < 1 μm) enables the reversible diffusion controlled theoretical response to be observed for oxidation of ferrocene in benzene, toluene and xylene under steady state conditions with slow scan rates ( < 100 mV s −1). Ambient temperatures can be used with benzene and toluene. However, slightly elevated temperatures (45°C) are required to increase the solubility of the electrolyte in xylene. With larger radii microelectrodes or at faster scan rates, a peak-shaped rather than sigmoidal-shaped reduction response is commonly observed in benzene and toluene on the reverse scan of cyclic voltammograms. Tentatively, this non-steady state response is attributed to nucleation and precipitation of insoluble ferricinium perchlorate onto the electrode surface. Under conditions of completely spherical diffusion, which is the case at ultramicroelectrodes, these precipitation reactions are believed to occur too far away from the electrode surface to influence the voltammetric response. The importance of electrode size, scan rate and electrolyte concentration as factors in obtaining distortion free voltammetric data in aromatic hydrocarbons are described in detail.