Competitive equilibration techniques for determining transition metal speciation in natural waters: Evaluation using model data

Competitive ligand equilibration with metal titrations is one of the most important approaches currently available for determining the chemical speciation of trace metals in seawater. However, ambiguities in data interpretation still complicate the application of these methods. Here, we have used model data to explore and graphically demonstrate the effects of analytical limitations and protocols on the results obtained using competitive equilibration techniques. We have confirmed that such methods are best suited for identifying low concentrations of ligands forming strong metal complexes. Nevertheless, with care, the information obtained for weaker metal complexes and speciation in complicated ligand mixtures can give reasonable estimates of the in situ free metal ion concentration. In general, speciation results from competitive equilibration analyses cannot necessarily be extrapolated to high total metal concentrations at which complexes with unidentified weak ligands could become important.