Electric Modulated Reflectance at Mercury Electrodes: Transition between Electrostatic Adsorption and Chemisorption

Abstract

Reflectance changes at polished electrode surfaces can arise from changes of optical properties of the metal surface and from changes of local ionic concentrations in the double-layer. The significance of these two effects in the interpretation of overall reflectance changes with potential (“electro-reflectance” effects), in the absence of film formation due to oxidation of the metal, is examined for the case of a mercury electrode. A liquid Hg surface is, however, unsatisfactory as a mirror electrode due to electro-mechanical oscillations which cause spurious reflectance changes. A thin Hg film on top of an electrolytically amalgamated Pt surface gives excellent results. The transition between electrostatic double-layer adsorption effects and more specific chemisorption can be demonstrated optically in the reflectance behaviour associated with Cl– and I– adsorption, by examination of reflectance of perpendicular and parallel polarized light in conjunction with phase-sensitive detection measurements. The behaviour of adsorbed thiourea is compared with that of the two halide ions.