Apparent anomaly during rotating disk voltammetry in ionic liquids

An apparent anomaly is described in which maxima on rotating disk voltammograms for ferrocene oxidation in an ionic liquid were found. The ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, bmimPF(6), is a neoteric solvent of current interest. The maxima are shown to be due to the very high viscosity of the solvent, 3.26 poise, which yields an unusually high Schmidt number (Sc) of 3.5 x 10(7), compared to a value of about 10(3) for aqueous solutions. It is estimated that over 100 revolutions of the disk are needed to achieve the Levich limiting current following application of a voltage step to the disk. The current transient resulting from application of a voltage step to the rotating disk is found to be in agreement with previously developed theory, even for these highly viscous systems. Analysis of the transient permits a determination of Sc values without measurement of the viscosity and density of the solvent, or diffusion coefficient of the reacting species. (C) 2002 The Electrochemical Society.