Resolving of Voltammetric Data for the Ni–Glycine and Cu–Glycine Complexation Systems with Reversible and Irreversible Electrochemical Response Using MCR-ALS

Abstract

The Ni–glycine and Cu–glycine systems, which form successive complexes, were studied by differential pulse polarography. The Ni–glycine complexes were inert while the Cu–glycine system showed labile behavior. An irreversible electrochemical response was found for the Cu–glycine system. The data were analyzed both by conventional methods (hard modeling) and the multivariate curve resolution method with alternating least-squares optimization (MCR-ALS, soft modeling approach). Resolving titration data for Cu2+ and Ni2+ with glycine as the complexing ligand shows that this method is a powerful tool for the determination of stability constants of metal complexes, especially when the conventional methods involve complicated and time consuming approaches such as for systems with an irreversible reduction process. Although for labile complexes the current is not always linear with respect to the concentrations, the results obtained by the two approaches are similar, so the variation from linearity of the signals is small and can be neglected. Also, the analysis of the labile complex data by a Gaussian peak adjustment (GPA) algorithm showed that this system has a small deviation from bilinearity. Thus, the MCR method provided good estimations of the complexation parameters, and is suitable for use as a complementary tool for studying systems with successive labile or inert complexes with both reversible and irreversible responses. Differential pulse polarograms were measured at room temperature for buffer solutions with pH = 7.5 and 0.1 mol⋅L−1 ionic strength.