Influence of the adsorption phenomena on the NPP and RPP limiting currents for labile metal-macromolecule systems

Abstract

Abstract The impact of adsorption effects on limiting currents obtained in reverse pulse (RPP) and normal pulse polarography (NPP) is analysed critically for induced reactant adsorption systems with any ligand to metal ratio. Qualitative explanations in terms of the concentration profiles are provided. We demonstrate that, for labile systems, no influence on the RPP limiting current can be found from the complex adsorption. However, ligand adsorption increases the RPP limiting current, this influence decreasing as the total metal concentration decreases. So, RPP normalised limiting current values ( φ ) are suggested to improve the fitting of the stability constant at very low total metal concentrations. If the total ligand concentration is chosen for maximum sensitivity, the remaining effect of the ligand adsorption leads to a bias in the stability constant of less than 14%, provided that the diffusion coefficient of the complex and ligand is 20-fold lower than the diffusion coefficient of the metal ion. Because the limiting NPP and RPP currents are equal for any ligand to metal ratio and without adsorption, approximate expressions derived for NPP facilitate the determination of the stability constant from the φ versus total metal concentration plot. This procedure is applied to some experimental results of Zn and Cd complexed with poly(methacrylic) acid.