Multivariate curve resolution as a tool to minimize the effects of electrodic adsorption in normal pulse voltammetry

Abstract

Multivariate curve resolution by alternating least squares (MCR-ALS) was applied to normal pulse voltammograms (NPV) of heavy metal complexes that adsorb on the electrode. Adsorption usually causes: (i) a maximum superimposed on NPV waves, (ii) a decrease of the limiting current and (iii) a shift of half-wave potentials. Together, these effects result in excessively high stability constants when electrochemical methods of equilibrium calculation are applied. As an example of this behaviour, the Cd(II)-polymethacrylate (PMA) system was chosen. Despite the non-linearity of electrodic adsorption, MCR-ALS allows the decomposition of currents into three ca. linear contributions: (i) reduction of the free metal ion diffusing from the bulk solution, (ii) reduction of the metal complex also diffusing from the bulk and (iii) reduction of the metal ion accumulated by adsorption onto the electrode surface prior to pulse application. This almost ideal situation is achieved at high pulse times (which ensures that the adsorptive contribution is not much higher than the diffusive ones) and especially when NPV signals are analyzed in derivative form. Under these conditions, reasonable lacks of fit are obtained, as well as stability constants coincident with these determined by reverse pulse voltammetry (RPV), a technique especially unaffected by adsorption.