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 μg/l Zn(II) and increasing amounts of ligand, 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)4−3, ZnHCO3+, Zn(HCO3)2 and ZnSO4 were found to occur at ligand concentrations ≤ 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 ≥ 0.1% of the total amount of metal were: Zn(H2O)2+6 (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.