Determination of ambient dissolved metal ligand complexation parameters via kinetics and pseudo-voltammetry experiments

Abstract

Dissolved chemical speciation of metals in natural waters encompasses a wide range of inorganic and organic compounds including metal organic ligand complexes, ML. Because of the different filters used, “dissolved” speciation can range from simple metal-ligand complexes with an average size of about 0.66 nm (mass of <3 k-daltons) to nanoparticles of 1 to 100 nm to colloidal forms that are 10 to 200–400 nm in size. Strong metal-ligand, ML, complexes are normally considered to be in <1 nm size fraction. Over the last 3 decades, competitive ligand exchange – cathodic stripping voltammetry (CLE-CSV) titrations have been the method of choice to study complexation. These titrations primarily give information on the excess ligands in the sample rather than the actual ligand in MLunknown complexes because they require adding metal to the sample. Thus, metal-ligand CLE-CSV titrations do not provide much information on the actual ligand present in MLunknown. However, pseudovoltammetry provides the thermodynamic stability constant, Ktherm, for Zn, Cu, Cd and Pb as the MLunknown complex is destroyed by reduction at the Hg electrode to form metal(Hg). Pseudovoltammetry does not require the addition of any reagents to the sample, but cannot be performed for ions such as Fe(III) [and Mn(III)] because reduction of the ion results in the reduction of the metal ion in the complex without destroying MLunknown. For these ions, kinetic experiments to recover the metal in the ML complex can provide information on the MLunknown dissociation rate constant, kd, and the conditional equilibrium constant, KcondML′. In these kinetic experiments, a competitive ligand (Lcomp) is added to the sample, and over time the MLcomp complex is measured by CSV. If all the metal in MLunknown is recovered, kd of MLunknown can be determined. If only a portion of the metal in MLunknown is recovered, equilibrium is achieved and KcondML′ as well as kd can be determined in a single experiment; kf can then be calculated. We describe how these methods can be used to determine information on the actual MLunknown complex. We show that 7 thermodynamic, kinetic and speciation parameters (Ktherm, KcondML′, KcondM′L′, kf, kd, αM′, αL′) for MLunknown complexes can be derived from a combination of two of these experiments. The approaches described here are useful to determine these parameters for known ML complexes once a ligand has been isolated by advanced separation methods (e.g., LC-MS) and reacted with a metal of interest.