More than 99% of copper ions in seawater are present in organic complexes which determine their bioavailability and toxicity. To characterise organic ligands, we group them based on similar affinities of the copper ion binding functional groups and copper complexing capacity parameters are determined (ligand group concentration Li, apparent stability constants logKi). In this study, we explored the effects of interferences such as the surface-activity of (1) specific organic molecules in model solutions and (2) natural organic matter in seawater samples, on the Cu peak detection in complexometric titration. We used differential pulse anodic stripping voltammetry on hanging mercury drop electrode. Fulvic acid, BSA protein and carbohydrates dextran, Na-alginate, chitosan and -carrageenan were model organic substances, while seawater samples were collected from the bottom and surface during the spring and summer seasons at the North Adriatic station 101. We investigated the possibility of reducing the interference of each adsorbed molecule on the electrode by applying the desorption potential Ed and adding T-X-100, the non-ionic, SDS, the anionic and TMAB, the cationic surfactant. The optimal conditions for natural samples were selected by analysing the results obtained for model solutions: the addition of 1 mg dm-3 T-X-100 together with Ed = -1.4 V for 1 s competitively inhibits the adsorption of surface- active natural organic matter on the electrode, greatly improving the shape and height of the Cu reoxidation peak. Two classes of ligands were determined in all samples at concentrations ranging from 12.0 to 45.3 nmol dm-3 for L1 and 22 to 71.5 nmol dm-3 for L2 with logK1 10.01 –11.17 and logK2 8.42 – 9.8, respectively. Ligand concentrations were highest in the surface layer in May and June, probably due to the spring activities of phytoplankton, which include exudation of surface-active organic compounds.
