A new cathodic stripping voltammetric method for determining organic copper complexation in seawater

Abstract

Different analytical methods may determine copper-complexing organic ligands of different strengths and concentrations in seawater, both because a variety of natural ligands may exist and because different methods may have different detection windows. Measurement of organic complexation by any single method at any single detection window may give only partial information on the true concentration of organically complexed and other forms of copper in seawater. Differential pulse cathodic stripping voltammetry (DPCSV) methods for studying dissolved trace metal complexation in seawater rely upon establishment of a competing equilibrium for the metal between the complexing ligands naturally present and a competing ligand added to the sample, followed by detection of the metal-added ligand complexes by DPCSV. The detection window can be varied by varying the concentration of the added competing ligand, or by adding a competing ligand which forms complexes of different strength with the metal of interest. We have developed a new DPCSV method for determination of organically complexed copper using tropolone as the added competing ligand because tropolone forms copper complexes that have different strength from those formed with catechol, the competing ligand added in a similar method. (The tropolone-copper complexes are weaker.) Thus, the tropolone-DPCSV method allows determination of ligands that form weaker copper complexes but that may exist at a higher concentration than those determined using the catechol-DPCSV method. In addition, tropolone is relatively stable and can be equilibrated with copper and natural ligands in a sample for extended periods (at least 12–15 h). We have demonstrated that this method can accurately determine total dissolved copper concentrations in standard seawater reference materials. Then, after calibrating the strength of the interaction between copper and tropolone in seawater of various salinities, we determined the concentration of natural copper-complexing organic ligands and the conditional stability constants of their copper complexes in surface water samples from the Indian Ocean and the North Sea. Over the range of copper concentrations employed in the titrations, we detected one copper-complexing organic ligand class in each sample. For the Indian Ocean sample, we determined a ligand concentration of 4.13 ± 0.05 nM, and copper complexes formed with this ligand had a conditional stability constant of 10 12.6 ± 0.5. For the North Sea sample, we determined an average ligand concentration of 16.2 ± 0.8 nM and an average conditional stability constant of 10 12.4 ± 0.8. Comparison of these results for the North Sea sample with those using the catechol-DPCSV method indicated that, as a result of the weaker copper complexes formed with tropolone relative to those with catechol, the tropolone-DPCSV technique determined copper-complexing organic ligands that exist at a higher concentration but form weaker copper complexes than those determined using the catechol-DPCSV technique. These results indicate that greater than 99.7% (in the Indian Ocean sample) and essentially 100% (in the North Sea sample) of the dissolved copper in these samples was organically complexed.