Speciation of Zinc mixed ligand complexes in salt water systems

Amalgam voltammetric method has been used to study heavy metal interaction in model lake water in KNO3 at 23¼C at concentration levels of genuine lake water of total ionic strength 1.44 M. Hanging drop amalgam electrode was prepared in situ before exchanging the medium containing Zn2+ ions for the sample solution with ligands of known concentrations. Half-wave potentials at the actual metal ion concentration in the lake and at a much lower one (10-2 times) were experimentally determined. Shifts in half-wave potentials are used to compute metal complex formation constants. The study reveals the existence of various species and a prediction is made of the distribution of the major labile complex species of Zn2+ present in the lake water. The concentration level of Zn2+ ions in the lake seems to play a role in determining the number of complexes formed and their stabilities. The results show that hydroxo/chloro and hydroxo/fluoro mixed ligand complexes have free Zn2+ ions as predominant species at the natural pH of the lake water and these systems are, therefore, potentially polluting. However, the contribution of OH- ions to the speciation of Zn is of no consequential effect because in its normal state, the [OH-] of the lake water is quite low and the aquatic life is, therefore, not threatened. The CO3 2- ions are the major contributors to the speciation of zinc in the lake water because the natural [CO3 2-] of 0.5 M is high. The normal lake water is thus, dominated by the carbonato complexes [ZnCO3Cl4]4-, [ZnCO3Cl2]2-, [ZnCO3Cl]- and to a less extent, ZnCO3. Keywords : Heavy metals, hanging drop electrode, amalgam voltammetry, speciation. Discovery and Innovation Vol. 19 (3) 2007: pp. 188-194