Determination of the complexing capacity and conditional stability constants of complexes of copper(II) with natural organic ligands in seawater by cathodic stripping voltammetry of copper-catechol complex ions

A new method is proposed for the determination of complexing capacities and conditional stability constants for complexes of copper(II) with dissolved organic ligands in seawater. This method is based on ligand competition by the added ligand catechol for free metal ions. The concentration of copper-catechol complex ions is measured with great sensitivity by cathodic stripping voltammetry. The concentration of the free copper ion is calculated from the concentration of copper-catechol complex ions. Ligand concentrations and conditional stability constants are obtained from a titration of the ligands with copper. Two techniques for treatment of the data are compared. A seawater sample, originating from open oceanic conditions, is analysed and two complexing ligands were detected, having concentrations of 1.1 × 10^?8 and 3.3 × 10^?8 M, and conditional stability constants (log K′_CuL) of 12.2 and 10.2, respectively.