Kinetic limitations in measuring stabilities of metal complexes by competitive ligand exchange-adsorptive stripping voltammetry (CLE-AdSV)

Abstract

The kinetic limitations of Competitive Ligand Exchange-Adsorptive Stripping Voltammetry, CLE-AdSV, for the determination of very stable metal complexes are explained in detail. For a given type of metal, from a certain lower limit of the complex stability constant, K, the usual simple equilibrium interpretation of CLE-AdSV measurements is not generally valid. By critical assessment of data for natural waters we show that in many cases the reported stability constants appearto derive from nonequilibrium conditions in the bulk sample and hence overestimate the real values. Fe(II) is a special case due to the particular kinetic features of hydroxide as a ligand. Our results call for validation of such data by analysis on the basis of the kinetics involved and/or by independent kinetic-free experimental approaches. Earlier speculations from CLE-AdSV results on very strong ligands and derived features such as the potential bioavailability of trace metals in natural waters require reconsideration.