Polarographic determination of half-wave potentials for copper-organic complexes in seawater

A methodology has been developed to determine the half-wave potentials of organic Cu chelators in seawater and algal culture media from pseudopolarograms obtained by anodic stripping voltammetry. A linear relationship between half-wave potentials determined from the pseudopolarogram, and thermodynamic stability constants was observed for a collection of model ligands that spanned a wide range of stability constants (log K=12–26.5) and included many functional groups likely to be important in copper complexation in seawater. Results suggested that it is possible to determine thermodynamic stability constants from half-wave potentials, as reported previously for Zn and Fe. Pseudopolarograms from culture media of three Cu stressed phytoplankton species showed very different thermodynamic stability constants, indicating the presence of chemically distinct ligands. The cyanobacterium Synechococcus produced at least two strong Cu chelators. One had a thermodynamic stability constant of log K=36.7±3.0 (obtained by extrapolation of the scale) and complexed from 30%–50% of the total Cu. The second was electrochemically inert, even at −1.6 V. The eukaryotic species produced several chelators with stability constants ranging from log K=22.6–39.1. Seawater samples collected from coastal waters showed similar characteristics. These thermodynamic constants are very high relative to model ligands in the literature, which could reflect coordination by specialized biomolecules. Alternatively, the large negative half-wave potentials could be due to Cu(I) complexes (with log K=10–20) or reflect reductive ligand decomposition of kinetically inert Cu(II) complexes.