Electrochemical methods have been used to provide a thermodynamic description of the adsorption of citric acid (HA) and dihydrogen citrate (H(2)A(-)) on a Au(111) surface. We compare the adsorption of citric acid species from an electrolyte (pH 1) corresponding to the fully-protonated tri-acid species and an electrolyte (pH 3) corresponding to an approximately equimolar mixture of H(3)A and H(2)A(-). Our results indicate a stronger adsorption of the monovalent ion compared to the neutral citric acid although both sets of experiments reveal similar charge numbers per adsorbed ion species. The latter values are dependent on the electrical variable (electrode potential or charge density) and approach a value of -3 at the most positive potentials (charge densities) studied for both sets of data (pH 1 and pH 3). This indicates that the adsorbed species is the fully de-protonated citrate ion, A(-3). From our chronocoulometry data we determine the outer Helmholtz potential for the citrate covered gold surface which is significantly larger for the system at pH 3 compared to pH 1. The results of this study on a two-dimensional gold surface can be extended to three-dimensional citrate-stabilized gold colloids, and rationalize the pH induced coagulation of colloidal sols. (c) 2005 Elsevier B.V. All rights reserved.