COMPETITIVE ADSORPTION VERSUS SURFACE COMPLEXATION AS MODELS FOR THE SIMULTANEOUS ADSORPTION OF METAL-COMPLEXES AND FREE LIGANDS

A statistical thermodynamical approach to the study of anion-induced a dsorption of Cd(II) from halide solutions is presented. The simultaneo us adsorption of metal complex and ligand is introduced in the isother ms by considering two possible mechanisms - competitive adsorption and surface complexation. These isotherms have been tested for the system Cd(II) in KBr at several ionic strengths. The experimental surface ex cesses of Cd(II) calculated from single-step chronocoulometry can be s imulated, giving an explanation for the desorption of the metal comple x at positive potentials. Also, the change in ligand adsorption promot ed by the adsorption of the metal complex has been calculated. Both ap proaches lead to the conclusion that the anionic tricoordinate metal c omplex CdBr3- and the tetracoordinate CdBr42- are the adsorbed species on the electrode surface, with CdBr42- dominating at higher bromide c oncentrations.