Electrochemical Periodicities

Abstract

IN the course of experiments on the anodic polarisation of platinum electrodes in dilute sulphuric acid solutions saturated with hydrogen, we have observed, at small current densities, some striking periodicities. The change of potential difference with time in a typical experiment is shown in Fig. 1. In this case the electrode potential falls continuously with a marked break at E_H = + 0.4 volt, to about + 1.0 volt, and then begins to rise slowly. At + 0.9 volt it suddenly jumps back to its initial value. This process appears to repeat itself indefinitely. In one experiment, with a current density of 62 × 10^-7 amp./cm.², we observed eighteen such periodicities in about 18,000 seconds, and the phenomenon showed no signs of coming to an end. With greater current densities the return to the initial value, which is near the reversible hydrogen potential, takes place at about + 0.5 volt, and only a limited number of periodicities is observed. Fig. 2 (IV) is an example, obtained with a current density of 150 × 10^-7 amp./cm.². The number of peaks is indefinite and varies from experiment to experiment. At high current densities no periodicities are observed, but we have photographic records showing small peaks with a period of 0.2 sec. using a current density of 3500 × 10^-7 amp./cm.².