Phase boundary potentials of absorbed films on metals.—Part I.—On the behaviour of oxygen on gold

Abstract

Whilst changes in interfacial potential differences owing to surface absorption can be measured both by thermionic and by photoelectric methods, these are not generally applicable to systems containing gases at relatively high pressures. That changes in metal gas potential differences occur when adsorption or surface reaction occurs was first demonstrated by Volta in measurements of contact potentials by a condenser method. His principle, with various modifications, has been employed by numerous subsequent workers, notably Fabroni, De la Rive, Pellat, Hughes, Henning, and especially Dubois. Ionization of the gap between two surfaces by means of the emission from radioactive materials was first employed by McLennan and Burton, and by Lord Blythswood and Allen, but their results are discordant, the cause indubitably lying, as Greimacher pointed out, in their use of radium salts. Reliable measurements of interfacial potential differences at liquid-gas interfaces were obtained by Guyot, Frumkin, Rideal and Schulman, when the relatively short range α-particles emitted by polonium were used as the source of ionization. This method has been extended to the examination of metal surfaces by Andauer and by Joffé and Lukirsky. On examination of the results obtained either by the various modifications of Volta’s method, or by that of Guyot for changes in the metal-gas potential differences as a result of oxidation of the surface, one is impressed by the smallness of the changes recorded. Andauer observed changes in the order of 0·1 volts when metal surfaces are reduced and oxidized, whilst the more recent publications of Dubois include the following values :—