Determination of copper complexation with natural organic ligands in seawater by equilibration with MnO 2 II. Experimental procedures and application to surface seawater

Abstract

The MnO 2 adsorption method combined with voltammetry is proposed for the direct determination of metal complexation in seawater of various salinities as a more satisfactory alternative to direct voltammetric measurements and bioassay methods. A small quantity of MnO 2 is equilibrated with copper ions in filtered seawater. Natural organic ligands in the seawater compete for copper with the MnO 2. Total dissolved copper is measured by differential pulse anodic stripping voltammetry after filtration and acidification of the sample. Preconcentration of natural water samples is unnecessary and measurement is performed at the natural equilibrium pH of the aerated sample. The analytical limit of detection of the method depends on contamination from the filtration step, and for copper complexation a ligand concentration of 5 × 10 −8 M was obtained. The sensitivity can be increased by use of radioisotopes as tracers. The method is very versatile in that complexation of various metals may be determined by any analytical method that measures total dissolved metal concentrations. Neither organic ligands nor their complexes with copper adsorb on the MnO 2 at pH8, but at pH 1.8 MnO 2 is an efficient scavenger for electroactive organic material. Samples of surface water from the Irish Sea and the Atlantic Ocean were found to contain ligand concentrations of 1.7 × 10 −7 and 1.1 × 10 −7 M, with conditional stability constants (log values) of 9.84 ± 0.13 and 9.86 ± 0.23, respectively, at pH 8.0.