Comparison of voltammetric methods and partial ultrafiltration for the analysis of copper complexation by fulvic acids in seawater

Copper (Cu) speciation in seawater is frequently measured using competitive ligand exchange cathodic stripping voltammetry (CSV) or anodic stripping voltammetry (ASV), but no comprehensive comparison of the results obtained by both techniques have been done to date. In this study, Cu complexation by the model organic matter Suwannee River fulvic acid (SRFA) was studied using these two techniques. The complexation capacity was similar for both techniques, but the conditional stability constants differed markedly, with log K' Cu´ values of 7.65 ± 0.17 for ASV, and up to 11.0 ± 0.2 for CSV. As a result, Cu speciation at 10 nM Cu and 5 mg L −1 SRFA, is predicted as 90.00% or 99.99% organically complexed Cu, respectively, for ASV and CSV, resulting in 100 times different inorganic Cu concentrations. The complexation of Cu by SRFA was also studied using partial ultrafiltration, and the observed degree of complexation was closer to that predicted by ASV. A thorough analysis of possible analytical bias that could have influenced the results was done, as well as a literature review of studies that have compared ASV or CSV with another speciation technique. On this basis, it was inferred that equilibrium methods based on ligand competition (such as CSV and competitive equilibration with MnO 2 ) predict much higher Cu complexation by organic ligands than any other methods, which may be explained by kinetic limitations of these techniques. Other methods, such as ASV or cation-exchange resins, may underestimate complexation but, in turn, provide biologically meaningful results according to the literature.