The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a Pt(1 1 1) electrode in a series of solutions (0.1 - x) M HClO4 + x M KClO4 + 2.5 x 10(-3) M NaCl with a constant ionic strength and variable pH. The total charge densities were calculated by integration of cyclic voltammetry curves. The adsorption of chloride present in the electrolyte blocked hydrogen and OH adsorption at potentials 0.3 < E < 0.7 V (SHE). Consequently, the differential capacity, charge density and surface energy become pH independent in that potential range providing pH independent integration constants for the integration of CVs. For E < 0.3 V (SHE), a complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen were determined without a need to introduce any arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin-Markov coefficient) for adsorbed hydrogen were calculated using the Gibbs excess data. The charge numbers are equal to similar to 1, indicating that the adsorbed species is a totally discharged hydrogen atom. For 0.1 M HClO4 + 2.5 x 10(-3) M NaCl solution Gibbs excesses of both hydrogen and chloride were determined. In a narrow potential range 0.2 < E < 0.3 V (SHE) chloride and hydrogen atoms are simultaneously adsorbed at the electrode surface. We have demonstrated that their adsorption has a competitive character. (C) 2005 Elsevier B.V. All rights reserved.