Coupled radiotracer and voltammetric study of the adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid on polycrystalline gold

Coupled application of a version of the in-situ radiotracer 'foil' method a nd voltammetry provided information on the time-, potential-, concentration - and pH-dependent adsorption of l-hydroxy-ethane-l, l-diphosphonic acid (H EDP) on a polycrystalline gold electrode, and on the effect of Zn2+ ions on the adsorption phenomena. Adsorption processes on the oxide-free surface o f gold were observed to be potential-dependent in the potential range 0.05- 1.00 V (versus RHE), while formation and irreversible accumulation of oxida tion products of HEDP could be detected at E > 1.00 V. The relative adsorpt ion strength of HEDP (its dissociation and/or oxidation products) was found to be higher on an oxide-free gold surface than on an oxide-covered one. T he surface excess of HEDP increased with increasing pH. Addition of Zn2+ io ns to the solution exerted a substantial effect on the HEDP accumulation. N amely, significant differences in the surface coverage, as well as in the k inetics and mechanism of HEDP adsorption could be detected in the potential regions below and above E = 0.2 V. Reduction of Zn(II) species at E less t han or equal to 0.1 V is probably coupled with the induced adsorption of HE DP on an Au electrode, leading to the formation of a polymolecular HEDP-Zn surface complex layer. (C) 2000 Elsevier Science S.A. All rights reserved.