Short Communication The electrosorption valency and partial charge transfer Robert de Levie * Department of Chemistry, Bowdoin College, Brunswick, ME 04011, USA Received 11 February 2003; received in revised form 8 August 2003; accepted 27 August 2003 Abstract The electrosorption valency as usually defined is an extra-thermodynamic and self-contradictory concept. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Thermodynamics; Electrosorption valency; Charge; Charge transfer When ions adsorb at the electrode solution interface, partial charge transfer is possible, and sometimes even plausible. Grahame [1] had already indicated the close parallel between halide adsorption on mercury and (covalent) chemical bonding. The related concepts of formal partial charge transfer and electrosorption va- lency were subsequently taken up by Lorenz and Sali e [2] and by Schultze and Koppe [3], and their present status was reviewed recently by Schultze and Rolle [4]. These are concepts that properly belong in the domain of quantum mechanics [5]. However, the electrosorption valency c is often suggested to be a thermodynamic property. In this short communication we will consider its thermodynamic validity. In order to do so we will first recall two well-studied cases, viz. those of the ideally polarizable and the non- polarizable interface. In order to keep the mathematics simple we will in all cases consider the interface between mercury (identified as phase I) and an aqueous solution of a single strong z, z-electrolyte (phase II), with a ref- erence electrode assembly (containing the metal phase III plus, possibly, its own solution phase II 0 together with a barrier membrane selectively permeable to the necessary ion) that responds thermodynamically to ei- ther cation or anion. We will not take into account the metal wires connecting the cell with the measuring equipment, because the contact potentials between the various metals are constant at constant temperature, and we will not be concerned here with the absolute values of the cell potential E ¼ / I  / III , but only with its changes dE ¼ dð/ I  / III Þ¼ d/ I  d/ III , where / is the inner potential. Such changes dE are unaffected by constant contact potentials. Under these conditions, the Gibbs adsorption equa- tion will take the form dr ¼ C þ d ~ l þ þ C  d ~ l  þ C e d ~ l e;I þ C w dl w ; ð1Þ where r is the interfacial tension (as distinct from the electrosorption valency c), l denotes the chemical po- tential, and ~ l the electrochemical potential, while the subscripts +, ), e, and w indicate cations, anions, metal electrons, and water molecules, respectively. Where necessary to avoid possible ambiguity, roman numerals are used to identify the phase involved. 1. The ideally polarizable electrode In the absence of charge transfer we convert Eq. (1) into dr ¼ C þ dl s þ q e dE  þ C w dl w ð2aÞ or dr ¼ C  dl s þ q e dE þ þ C w dl w ð2bÞ depending on whether we use a reference electrode re- sponding to the solution anions or cations, respectively. In order to go from Eq. (1) to (2a), (2b) or (3) we use the following relations: Journal of Electroanalytical Chemistry 562 (2004) 273–276 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry * Tel.: +1-207-725-3028; fax: +1-207-725-3017. E-mail address: rdelevie@bowdoin.edu (R. de Levie). 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.08.027