3". Electroanal. Chem., 143 (1983) 361-374 361 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands ADSORPTION OF TRIPHENYLARSINE OXIDE AT THE MERCURY ELECTRODE FROM METHANOL AND WATER-METHANOLIC MIXTURES P. NIKITAS, A. ANASTOPOULOS and D. JANNAKOUDAKIS Laboratory of Physical Chemistry, University of Thessaloniki, Thessaloniki (Greece) (Received 14th January 1982; in final form 25th June 1982) ABSTRACT The adsorption of triphenylarsine oxide (TASO) on the Hg electrode from methanolic and water-methanolic solutions of LiC10 4, is investigated by means of electrocapillary measurements. An analysis is carried out in terms of congruence of the experimental adsorption isotherm with respect to potential and electrode charge. At negative changes a satisfactory fit of Frumkin's isotherm with the experimental results is obtained when a negative interaction parameter, equal to -0.8, is used. At positive charges the increasing depression of the electrocapillary curves is attributed to changes within the adsorption layer, mainly involving the introduction of As 8+ =0 ~- groups into the double layer after the reorientation of TASO molecules and to a minor degree *r-electron interactions. At these polarizations, although the field congruence of the adsorption isotherms seems to remain effective, it is shown that this congruence is only of an apparent nature. Finally, the various adsorption parameters are calculated and discussed. INTRODUCTION Arsinoxides, when compared to phosphinoxides, present an almost equally strong inhibitory action on electrochemical reactions in various solvent systems [1,2]. However, the adsorption of phosphinoxides has received considerable attention [3-6], while there exists a complete lack of a similar study for arsinoxides. A study is presented here for the adsorption of TASO at the Hg electrode from methanolic and water-methanolic solutions of LiC104 by means of electrocapillary measurements. From the 3' vs. E curves of the systems investigated, our work is directed towards a quantitative analysis, yielding information about the potential and charge of maximum adsorption, the maximum surface excess, the area occupied and the orientation of the adsorbed TASO molecules. The dependence of the adsorption isotherm on the electric field is also examined and a fit of the experimen- tal data to a Frumkin isotherm is attempted. The increasing depression of the electrocapillary curves at positive charges makes possible an examination of the behaviour of the adsorbed TASO molecules under such polarizations. The change of the solvent system permits the study of the influence of water on the adsorption model of TASO. The effect of the solvent is 0022-0728/0000-0000/$03.00 © 1983 Elsevier Sequoia S.A.