The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in a series of NaH2PO4 solutions with an excess of inert electrolyte (0.1 M HClO4) at constant ionic strength and pH. Thermodynamic analysis using both electrode potential and charge density as independent electrical variables is described. The Gibbs excess, Gibbs energy of adsorption and charge numbers both at constant electrode potential and constant chemical potential for anion adsorption at the Pt(1 1 1) surface have been determined. The calculated electrosorption valencies and charge numbers at constant chemical potential are close to two electrons per adsorbed anion, suggesting that in the absence of co-adsorbed species, HPO42- is the predominant adsorbed species. The maximum Gibbs excess of adsorbed hydrogenphosphate attains a value of approximate to 3.2 x 10(14) ions cm(-2) which corresponds to a coverage of approximate to 0.22 ML. (C) 2009 Elsevier Ltd. All rights reserved.