Alternating Current Linear Sweep and Cyclic Voltammetry at a Dropping Mercury Electrode with Phase-Selective Fundamental and Second Harmonic Detection

The analytical application of phase-selective ac linear sweep voltammetry (fundamental and second harmonic) with scan rate synchronized to a dropping mercury electrode is considered. Commercially available instrumentation was adapted to provide the technique, and the theoretical response was obtained over a wide range of operating conditions for both the faradaic and charging current components. The techniques combine the advantages of fast scan rates (up to 200 mV/sec used in this work), extremely high reproducibility (better than 1% at the 10^-6M level), and linear calibration curves over a wide concentration range linear calibration curves over a wide concentration range. With the second harmonic method, flat base lines were obtained despite the growth of the mercury drop during the scan duration, and this would appear to be the preferred technique. At the high frequencies necessitated by the condition ΔEωt ≫ vt, slight non-ideality leads to sloping base lines in the fundamental mode. Comparison with the dc method shows considerable advantage of ac techniques with respect to resolution. Cyclic ac voltammograms can also be obtained at the dropping mercury electrode with the same instrumentation.