PSEUDOPOLAROGRAPHIC DETERMINATION OF STABILITY-CONSTANTS OF LABILE ZINC-COMPLEXES IN FRESH-WATER

The formation and distribution of labile zinc species in fresh water were investigated by using a pseudopolarographic approach. Pseudopolarograms were obtained by plotting the anodic stripping voltammetric peak current versus the deposition potential for a series of solutions containing 100 mu g/l Zn(II) and increasing amounts of Iigand, at the pH of a typical river water sample. Stoichiometry and stability constants of the complexes were evaluated from the shift of the half-wave potential of the pseudopolarogram as a function of the ligand concentration. Four major inorganic ligands occurring in river waters were examined: chloride, hydrogenphosphate, hydrogencarbonate and sulphate, but no shift was obtained for chloride at the concentration levels tested. The species ZnHPO4, Zn(HPO4)(3)(4-) ZnHCO3+, Zn(HCO3)(2) and ZnSO4 were found to occur at ligand concentrations less than or equal to 0.25 M, and their stability constants calculated. The distribution diagram of Zn in the river water investigated showed that Zn species present in concentration 2 0.1% of the total amount of metal were: Zn(H2O)(2) (aquo-ion), ZnHPO4, ZnHCO3+, Zn(HCO3)(2), ZnSO4, ZnOH+ and Zn(OH)(2). Species of higher coordination index only appear at a ligand concentration superior to that present in fresh water.