On the theory of ionic adsorption from binary electrolyte solutions at electrodes

Abstract

The Grahame-Parsons, Alekseev-Popov-Kolotyrkin and Vorotyntsev models of ionic adsorption in 1:1 binary electrolyte solutions are compared with a computer experiment assuming independence of the inner-layer capacity on the specific adsorption of ions. It is demonstrated that the Grahame-Parsons and Alekseev-Popov-Kolotyrkin models are formally equivalent for the solutions analysed and that their adsorption parameters can be recalculated by the subsequent equations. For infinitely dilute solutions at a given electrode charge, all three models require determination of a limiting value of specific adsorption, X lim For the choice of an adequate adsorption isotherm and double-layer model, the specific adsorption of anions in binary solutions of KCl, KBr and KI on Hg and Bi electrodes was studied. The theoretical analysis showed that the most informative systems were the KBr solutions at the Bi/H 2O interface, where the transition from undercharged to overcharged surface conditions occurred in the middle of the logarithmic concentration scale. For these systems, the best coincidence was obtained by use of the modified virial isotherm considering the discreteness effect, due to the specifically adsorbed ions, both in the inner and the outer Helmholtz plane.