Copper complexation profiles in the Atlantic Ocean: A comparative study using electrochemical and ion exchange techniques

Abstract

The dissolved copper concentrations in several profiles of the water column in the Atlantic Ocean were between 1 and 6 nM, and indicate copper removal from the surface layer and mixing of water masses at greater depth. The ligand concentrations ( C L) decreased from a maximum of 1800 nM in surface waters to 6–20 nM in deeper waters, with values for log K′ CuL (conditional stability constants) between 9.4 and 11, as indicated by the MnO 2 technique. Ligand concentrations obtained with the DPASV (differential pulse anodic stripping voltammetry) technique showed more variability and were generally higher, whereas the log K′ CuL values were smaller at 7.5–10.3. Such differences in results obtained with different techniques may be due to certain operational parameters of each technique, which to some extent govern the detection window. Speciation calculations showed that the organic copper fraction at all depths amounts to 89–99.8% using data from the MnO 2 method, and 98.8–99.4% using data from the DPCSV (differential pulse cathodic stripping voltammetry) method. Comparison of the techniques used showed that the stronger ligands, or sites, detected by the DPCSV, and to a smaller extent MnO 2 procedures were of most relevance to the study of metal speciation, since these ligands form much more stable copper complexes than those detected by DPASV. The strong complexing ligands are not detected by DPASV due to lack of sensitivity. For reasons of convenience and sensitivity DPCSV is to be preferred over the MnO 2 method.