Investigation of the ability constants of cadmuim complexes in salt lake media by polarographic methods; Lake Elmentaita

Abstract

The stability constrants of the complexes of cadmium with the liands; chloride (Cl-), fluoride (F-), carbonate (CO32-, bicarbonate (HCO-3) and hydroxide (OH-) have been determined. These have been determined in two types of media; in actual lakewater which was obtained from lake Elmentaita and in aqueous solutions of the same ionic strength as the lake water. The ioninc strength in aqueous media was maintained with potassium nitrate. In determining the complexes of CO32-, HCO3- and OH-, 0.02 molar ammonium nitrate was used to suppress cadium precipitation. Two methods, namely differential pulse polarography (DPP) and differential pulse anodic sstripping voltammetry (DPASV) werre used in dtermination of stability constrants was done at two different concentrations of cadmium. These were 4 x 10-4M and 8 x 10-7. DPASV was used at both cadmium concetrations while the former method was used only at the higher concentration.

In both lakewater and aqueous media, four chloro complexes (CdCl+,CdCl20, CdCl-3, CdCl42-), three carbonato complexes (CdCO30, Cd(CO3)22-, Cd(CO3)34-, and four hydroxo complexes (CdOH+, Cd(OH)20, Cd(OH)2- and Cd(OH)42-)were identified. In aqueous media, three fluoro complexes (CdF+, CdF20 and CdF3-) were identified, while in lakewqter only the first two of these complexes were detected. Three bicarbonato complexes (CdHCO3+, Cd(HCO3)20 and Cd(HCO3)3-) were identified in aqueous media. In lakewater only the first two of these complexes were detected.

It was found that chloro complexes would contribute a great deal to the speciation of cadmium at low or neutral PH values. The low PH value is not realised in the lakewater. The contribution of fluoro complexes in lakewater was found to be negligible. However, at the naturally high PH value of this lake, cadmium was found to exist mainly as teh carbonato and bicarbonato complexes. The contribution of the hydroxide ligand to the speciation of cadmium in this lake was found to be low because its concentration in the lakewater was quite low.