Electrolyte solutions in liquid ammonia—x. A cyclic voltammetric examination of electron transfer between solvated ruthenium(III) and ruthenium(II) cations, and anodic passivation of platinum electrodes

Abstract

Single electron transfer between the hexammine cations of Ru(III) and Ru(II) at a Pt metal electrode in liquid ammonia closely follows Nernstian behaviour during the first voltammetric cycle if the voltage sweep rate is low. The redox electrode based on the Ru(III)/Ru(II) couple at a Pt metal surface deviates from voltammetric reversibility as: (i) the voltage ramp rate, (ii) the concentration of Ru(NH 3) 3+ 6 cation, and (iii) the number of applied voltammetric cycles, are increased. X-ray photoelectron spectra of the surface layer of Pt metal after exposure to anodic potentials in excess of 0.3 V reveal the Pt metal surface is oxidized in liquid ammonia (electrolytically and in voltammetric cycles) to a gold-coloured compound containing equimolar amounts of Pt(II) and Pt(IV). These results are discussed in the light of previous contentious reports about the use of H 2 gas on Pt as a primary standard electrode for work with liquid ammonia solutions.