Pseudopolarography of trace metals. Part III. Determination of stability constants of labile metal complexes

Suitability of the pseudopolarographic approach for determination of the stability constants of labile metal complexes at low metal concentrations is explored. It is shown that the methodology based on the DeFord-Hume principle can be successfully applied to treat data obtained from pseudopolarograms. For reversible electrochemical systems, such as thallium(I)-chloride or cadmium(II)-chloride, the half-wave potential of a pseudopolarogram, or the corresponding "critical" potential, can be used to calculate the stability constants. A "formal" potential, evaluated by manual fitting of experimental and simulated (pseudo)polarograms, or a potential in a range where all curves yield a reversible slope, can be used as the "critical" potential. For systems in which reversibility of the electrochemical reaction changes (zinc(II)-oxalate and bismuth(Ill)-chloride), the shift of the half-wave potential depends both on the complexation parameters and on reversibility of the electrochemical reaction. Therefore, in such cases only the "critical" potential can be used for determination of stability constants. All calculated stability constants presented in the paper are in good agreement with the literature values. The benefit of using pseudopolarographic approach lies in the minimization of various effects (such as precipitation, adsorption, ligand excess, etc.), which can influence determination of stability constants at higher metal concentrations.