Electrophoretic investigations of the complexing of cadmium and zinc with EDTA

Abstract The formation of EDTA complexes of 109 Cd and 65 Zn in KNO 3 solutions has been investigated by high-voltage paper electrophoresis. The concentration of EDTA was varied from 1 μ M to 10 m M , the concentrations of inactive cadmium in the systems were 0, 0.1 m M and 1 m M , the concentrations of inactive zinc were 0, 1 μ M , 10 μ M and 0.1 m M , and the pH of the system was 8.1. The behaviour of cadmium and zinc was examined by measuring the dependence of their electrophoretic mobilities on the EDTA concentration and on the age of the system up to 5 days. From the results obtained, it is evident that in 0.1 M KNO 3 both ions have similar cationic mobilities. At very low and at very high concentrations of EDTA, cadmium and zinc give well-defined cationic and anionic zones, respectively. In the intermediate range of EDTA concentration, i.e. , in the transition region, 109 Cd gives well-defined zones with continuous change in electrophoretic mobilities, but 65 Zn gives very pronounced multi-zonal tailing. This indicates that EDTA reacts rapidly with cadmium, but rather slowly with zinc. From the results, the variation in effective stability constants of the cadmium-EDTA and zinc-EDTA complexes with the age of the system was calculated, and the values are compared with the effective stability constants obtained for both ions in sea water and 0.5 M NaCl systems at pH 8.1. The stability constants have also been calculated from the electrophoretic data using methods devised by other authors; these methods are discussed on the basis of the results obtained.