USE OF AC 2ND HARMONIC PHASE-ANGLE VOLTAMMOGRAMS TO MEASURE HETEROGENEOUS KINETIC-PARAMETERS

The use of second harmonic phase angle voltammograms to measure heterogeneous kinetic parameters has two main advantages. (1) The IR drop effects are negligible for small amounts of uncompensated resistance (12-15 OMEGA with concentration 1 mM, area of the electrode 0.025 cm2, amplitude of sine wave 10 mV, frequency from 100 to 700 Hz). (2) Distortions caused by potential-control errors, resulting from the use of positive-feedback IR compensation, are eliminated. In the method described here the kinetic parameters are obtained by fitting the phase angle voltammograms with theoretical curves. The data analysis program finds the best fit by optimizing the correlation coefficient using zig-zag optimization. Kinetic parameters measured both in aqueous solution (reduction of K3Cr(CN)6 in 1 M KCN, k(s) = 0.35 +/- 0.06 cm s-1, alpha = 0.55 +/- 0.02) and non-aqueous solution (reduction of [Ru(bpy)3]2+ in both DMF and acetonitrile, k(s) = 0.24 cm s-1; alpha = 0.500) agree well with literature values. The key feature of the technique is its ability to obtain reliable kinetic parameters of "fast" electrode processes from data obtained at fairly low frequencies. Also, a study of the effects of uncompensated resistance R(u) on the second harmonic phase angle reveals that when R(u) exceeds approximately 15 OMEGA, contamination of the perturbation signal with components at the second harmonic frequency is significant.