Detection of a strong ligand for copper in sea water and determination of its stability constant

Abstract

A method for the detection and characterization of an organic ligand with a high conditional stability constant but a low concentration in natural waters is proposed. The method is based on a combination of a procedure that involves the ligand-exchange reaction between natural ligands and chelating ragents and the analysis of the experimental results on metal speciation by a simple equilibrium model. Application of this method to samples of sea water from different marine environments suggested the presence of a strong ligand for copper in most samples of sea water examined. The lower limits of the concentration of the strong ligand in oceanic water were evaluated to be < 0.01–0.20 nM. The conditional stability constant for the copper complex with this ligand was evaluated to be higher than 1013.8–1014.3 1 mol−1 at an ionic strength lower than 10−5 M at pH 5.71 and 4°C. In coastal water, the concentration of the strong ligand was higher than in oceanic water, but the conditional stability constant was the same. The conditional stability constant for the strong ligand, evaluated by the present method, is equal to or one order of magnitude greater than that of ethylenediaminetetraacetic acid under the same conditions.