Differential electrolytic potentiometry with periodic polarisation. Part XXIII. The effect of bias and distortion on periodic differential electrolytic potentiometry, the d.c. output produced and time-biassed differential electrolytic potentiometry in oxidation-reduction titrations

Abstract

Any departure from the pure, symmetrical, bias-free input waveform; external d.c. bias; internal d.c. offset, distortion, amplitude or mark-to-space (time) bias; produces a deterioration in the periodic differential electrolytic potentiometric titration curve. The peak potential is decreased, the peak becomes less sharp, the discrimination becomes worse, errors are introduced and the electrodes more quickly become deactivated when any bias or distortion is present in the input current waveform, and this effect is manifest with 2 per cent. contamination of the waveform and destructive at 5 per cent. A d.c. bias causes the peak to split into two peaks. At the same time, the electrodes produce a d.c. output; the symmetrical polarisation shows no d.c. component in the output; and for d.c. offset and amplitude bias, this d.c. output is the same as for classical d.c. differential electrolytic potentiometry. With a time-biassed periodic input of any waveform, a d.c. output of unique properties is produced. This output has the same forms as the classical d.c. differential electrolytic potentiometric curves, but the end-points are sharper, the discrimination is better, the end-points are error-free with dichromate and cerium(IV) titrants, the d.c. potential stabilises very quickly and remains drift-free, even for type II (b) curves, the high-quality end-point persists to very low concentrations, the electrodes retain their activity for a long time and the process is independent of frequency. Such titrations can be performed as fast as titrations with visual indicators. The positive errors in classical d.c. differential electrolytic potentiometric titrations with cerium(IV), chromium(VI) and in zero-current potentiometric titrations with vanadium(V) are explained.