Voltammetric studies of zinc speciation in lake Elementaita

Abstract

The complexes of zinc with simple ligands F-, Cl-, OH-, HCO3-, CO3-2 were studied mainly to determine their stability constants in relation to their different physiochemical forms and the complex species distribution. The determination was done in two media, aqueous solution of the same ionic strength as the lake water and in actual lake water, which was obtained from lake Elementaita. In determining the complexes of CO3-2 and HCO3-, NH4NO3 was used to suppress zinc precipitation.

Polarographic techniques i.e. differential pulse polarography (DPP) and differential pulse anodic stripping voltammetry (DPASV) were used to determine the shifts in peak potentials, which were used to obtain the stability constants and the distribution of the complexes. Two levels of zinc concentration were used; 3.06 x 10-4 M and 6.12 x 10-7 M zinc, and DPASV was used at both zinc concentrations while the former method was used only at higher concentration.

In both lake water and aqueous media, one Cl- complex {ZnCl+}, two HCO3- complexes {ZnHCO3+, Zn(HCO3)20}, one F- complex {ZnF+}, four OH- complexes {ZnOH+, Zn(OH2+0, Zn(OH)3-, Zn(OH)4-2}one F- complex {ZnF+}, four OH- complexes {ZnOH+, Zn(HCO3)20, Zn(OH)3, Zn(OH)4-2} and three CO3-2 complexes {ZnCO30, Zn(CO3)2-2, Zn(CO3)3-4} were identified. In aqueous media at 6.12 x 10-7 M zinc, six OH- complexes {ZnOH+, Zn(OH)20, Zn(OH)3-, Zn(OH)4-2, Zn(OH)5-3, Zn(OH)6-4} were identified, while in the lake water only the first four of these complexes were detected. Also at 6.12 x 10-7 M zinc, two Cl- complexes (ZnCl+, ZnCl20) were identified but only the first one was detected in the lake water. A number of complexes not yet reported in the literature have been shown to exist. These are Zn(OH)3-, Zn(OH)4-2, ZnHCO3-, Zn(HCO3)20, Zn(CO3)2-2 and Zn(CO3)3-4.

The contribution of the Cl-, F-, and OH- ligands to the speciation of zinc in this lake was found to be low, because Cl- and F- form weak complexes. The OH- is at a very low concentration in the lake water. Therefore speciation of zinc in the lake water depends on the CO3-2 and HCO3- ligands.