In Part I the phenomena observed in the polarisation of platinum electrodes in hydrogen and oxygen saturated solutions were explained in terms of a theory of electrolytic depolarisation. In order to elucidate these effects and obtain further support for the equations put forward it seemed desirable to investigate the kinetics of processes in which depolarisers were present in the solution in known variable amounts. We have therefore studied the cathodic reduction of methylene blue solutions and the cathodic and anodic behaviour of solutions of quinhydrone at platinum electrodes. In the investigation of electrolytic reductions and oxidations a transition phenomenon has frequently been observed. In such cases the current voltage curve consists of three parts: (1) at low current densities electrolysis takes place at only a small displacement from the initial potential; (2) at higher current densities there is a transition region in which the potential changes continuously until a much greater displacement of the potential has been reached; (3) beyond this transition region a second current voltage curve is obtained which is not far removed from that obtained for the liberation of hydrogen at the cathode, or of oxygen at the anode. It was observed by Wilson and Youtz in the case of the oxidation of ferrous salts that the transitional current density depended on the concentration of the oxidisable substance, and Ellingham has recently made a similar observation for the cathodic process in nitric acid solutions.
The kinetics of electrode processes. Part II.―Reversible reduction and oxidation processes
