Abstract
We discuss the voltammetric methods that are used to assess metal–organic complexation in seawater. These consist of titration methods using anodic stripping voltammetry (ASV) and cathodic stripping voltammetry competitive ligand experiments (CSV-CLE). These approaches and a kinetic approach using CSV-CLE give similar information on the amount of excess ligand to metal in a sample and the conditional metal ligand stability constant for the excess ligand bound to the metal. CSV-CLE data using different ligands to measure Fe(III) organic complexes are similar. All these methods give conditional stability constants for which the side reaction coefficient for the metal can be corrected but not that for the ligand. Another approach, pseudovoltammetry, provides information on the actual metal–ligand complex(es) in a sample by doing ASV experiments where the deposition potential is varied more negatively in order to destroy the metal–ligand complex. This latter approach gives concentration information on each actual ligand bound to the metal as well as the thermodynamic stability constant of each complex in solution when compared to known metal–ligand complexes. In this case the side reaction coefficients for the metal and ligand are corrected. Thus, this method may not give identical information to the titration methods because the excess ligand in the sample may not be identical to some of the actual ligands binding the metal in the sample.
Highlights
In the last two decades, our knowledge of trace metal speciation has grown tremendously
The titration experiments include (1) anodic stripping voltammetry[2] (ASV), which is useful for metals that react at the electrode directly (Cu2z, Zn2z, Cd2z, Pb2z), and (2) cathodic stripping voltammetry/competitive ligand exchange[3,8,9] (CSV-CLE) which is useful for metals that do not react at the electrode directly but have a metal–ligand complex that does (Fe3z, Co2z)
Voltammetry can provide information on a ligand binding a metal because many metal–ligand complexes give a discrete peak or half-wave potential
Summary
In the last two decades, our knowledge of trace metal speciation has grown tremendously. The second type of voltammetry method involves the breakdown of the actual complex in situ and is termed pseudovoltammetry,[45,46,47,48] which is useful for metals that react at the electrode directly This method gives information on the amount of ligand binding to a specific complex with a thermodynamic constant, Ktherm, that differs from Kcond ML. Appendix 2 shows types of strong ligands (functional groups are circled) that bind to Fe(III) and which may bind to other metals
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