ZINC SPECIATION IN LAKE WATERS AND ITS DETERMINATION BY LIGAND-EXCHANGE WITH EDTA AND DIFFERENTIAL-PULSE ANODIC-STRIPPING VOLTAMMETRY

A technique to determine free zinc ion concentration and zinc speciation in lake water was developed. The technique involves ligand exchange of added EDTA with natural organic ligands and measurement by differential pulse anodic stripping voltammetry (DPASV); pretreatment of samples and separation are not necessary. The validity of the ligand exchange approach was demonstrated in model waters containing glycine and CDTA (or EDTA), as representatives of weak and strong organic ligands. The ligand exchange and DPASV measurements were applied to water samples from Lake Greifen, a small eutrophic lake in Switzerland. The average fractions of dissolved zinc species in this lake were 7.8% free zinc ions, 33.5% weak organic complexes and 50.5% strong organic complexes. Total dissolved Zn in the lake water column ranged between 18 to 25 nM, free zinc ion concentrations 1.3-1.8 nM, with an average pZn of 8.8 and an average ratio of total Zn to [Zn2+]approximate to 13. The extent of complexation of Zn in the lake was much less than in some sea waters (ratios 60-100 in the North Pacific), probably due to higher ratios of total Zn concentration relative to organic ligands in the lake.