Speciation of cadmium mixed ligand complexes in salt water lakes

Abstract

Amalgam voltammetry has been used to study heavy metal interaction in model lake water in KNO₃ at 23 ^oC at concentration levels of genuine lake water. The hanging drop amalgam electrode was prepared in situ before exchanging the medium for the sample solution. Half-wave potentials at two metal ion concentrations were measured, one at the actual concentration in the lake while the other at a much lower one. The experimentally determined shifts in half-wave potentials are used to compute several formation constants.

At the natural [CO₃^2-] of 0.5 M in the lake, the main contributor to the speciation of cadmium is [Cd(CO₃Cl₂)]^2-. At high [Cd²⁺], the DPASV detects the presence of free Cd²⁺ ions, hence, potential polluting effect, while the amalgam reports [Cd(CO₃)₂Cl)]^3- to be dominant above [CO₃^2-] = 0.8 M. There is a variation in the number of complexes detected, their stabilities and percentage distribution in the two methods. Cd²⁺ ion concentration also affects the number of complexes formed and their stabilities.

KEY WORDS: Heavy metals, Hanging drop electrode, Amalgam voltammetry, Speciation, Cadmium mixed ligand complexes

Bull. Chem. Soc. Ethiop. 2003, 17(1), 85-94.