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 speciati on in lake water was developed. The technique involves ligand exchange of added EDTA with natural organic ligands and measurement by differe ntial pulse anodic stripping voltammetry (DPASV); pretreatment of samp les and separation are not necessary. The validity of the ligand excha nge approach was demonstrated in model waters containing glycine and C DTA (or EDTA), as representatives of weak and strong organic ligands. The ligand exchange and DPASV measurements were applied to water sampl es from Lake Greifen, a small eutrophic lake in Switzerland. The avera ge fractions of dissolved zinc species in this lake were 7.8% free zin c ions, 33.5% weak organic complexes and 50.5% strong organic complexe s. 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 ex tent 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 ra tios of total Zn concentration relative to organic ligands in the lake .