Implementation of a chemical equilibrium constraint in the multivariate curve resolution of voltammograms from systems with successive metal complexes

A multivariate curve resolution (MCR) method, using a constrained alternating least squares (ALS) procedure with a new chemical equilibrium constraint, was applied to differential-pulse polarograms of successive metal complexes. This new restriction imposes the fulfilment of a chemical model defined by a set of stability constants that are optimised along the iterative ALS procedure. The reliability of the method was tested with simulated data and with polarograms measured for the systems Zn(II) + glutathione and Cd(II) + 1,10-phenanthroline. These systems respectively yield two and three successive and electroactive complexes, which are inert from the electrochemical point of view, that is, the complexes virtually do not dissociate during the measurement. Although the presence of electrode adsorption could induce overestimation of some concentrations and losses of linearity between concentrations and signals, the results showed that the proposed method can yield satisfactory estimations of the stability constants in this kind of system. The performance of the new method is compared with the performances obtained using MCR-ALS without the equilibrium constraint and using traditional curve fitting least-squares approaches.