Cyclic voltammetry at microelectrodes in the absence of added electrolyte using a platinum quasi-reference electrode

The cyclic voltammetric behaviour of the ferrocene/ferricinium (Fc/Fc⁺) couple has been examined in acetonitrile without deliberately added electrolyte using scan rates over the range 5 mV/s to 50 V/s. Platinum (0.5 μm, 2.5 μm and 25 μm radius) and gold (5 μm radius) microelectrodes were used as working electrodes, with platinum wire quasi-reference electrodes to minimize contamination. At slow scan rates (5 to 500 mV/s) sigmoidal shaped steady state voltammograms were generally observed on the forward (oxidative) scan as is the case with electrolyte. Reverse (reductive) scans were not strictly sigmoidal and exhibited small peaks. This phenomenon is not observed in the presence of electrolyte and is attributed to ionic migration of the Fc⁺ cation. With a two electrode configuration, employing a platinum wire quasi-reference electrode, the forward and reverse scans of cyclic voltammograms were not superimposed at low scan rates unless small radii and low ferrocene concentrations are used. This distortion may be attributed to polarization of the reference electrode. Use of a three electrode platinum configuration, in a potentiostatic model while increasing the noise level, decreases this problem. At fast scan rates in excess of 1 V/s, planar diffusion terms are apparent. Additionally, iR (ohmic) distortions are considerable and are enhanced as the electrode radius and ferrocene concentration increase. Despite this problem, which mitigates against obtaining reliable thermodynamic data or assessment of electrochemical reversibility, the important diagnostic criterion of the presence of chemical reversibility or otherwise of an electrode process can still be clearly ascertained at fast scan rates, since the reduction current arising from the presence of the Fc⁺ ion in the reverse scan is observed, as is the case in the presence of electrolyte.