Direct Measurement of E^r_1/2 with Reversible and Ec [Electrochemical] Electrode Processes by Second Harmonic Alternating Current Polarography and Voltammetry

Certain characteristics of the second harmonic ac polarographic response, which have been well-established in the literature, suggest that this form of electrochemical readout has excellent possibilities for precise, direct determination of the thermodynamlcally-slgnificant reversible half wave potential, E^r_1/2. Specifically, the dc potential corresponding to the second harmonic polarographic wave's minimum (absolute value readout) or zero current point (phase-selective readout) appears to represent a superior index of E^r_1/2 than the commonly-employed dc polarographic halfwave potential or the fundamental harmonic peak potential. Theoretical arguments are discussed and experimental data are presented in support of the foregoing concept. Data with reversible redox couples at the dropping mercury electrode and a variety of stationary electrodes clearly show that higher precision is provided by the second harmonic approach. With several systems undergoing reductive EC mechanisms, the second harmonic approach is found to yield an excellent appraisal of E^r _1/2 under conditions where the dc and fundamental harmonic ac polarographic estimates contain significant systematic error due to kinetic influence of the follow-up chemical reaction.