On the behaviour of cysteine as ligand of cadmium(II)

Abstract

The ability of cysteine to form complexes with cadmium(II) in aqueous solutions has been investigated at 25°C and in constant ionic medium NaCl at two different concentrations, 1.00 and 3.00 mol l −1. The presence of chloride ions was necessary to avoid the precipitation of cadmium(II). Two kinds of measurements were carried out. The electromotive force of galvanic cells containing glass and cadmium amalgam electrodes was measured as a function of cadmium and hydrogen ion concentrations in acid or moderately alkaline solutions in order to obtain the free concentration of cadmium(II) and hydrogen ions. The experimental data obtained in 1.00 mol l −1 NaCl were explained by assuming the presence of CdHL and CdH 2L 2, while those obtained in 3.00 mol l −1 NaCl were accounted for with the formation of CdHL, CdH 2L 2, CdH 3L 3 and CdH 2L 3. Moreover, polarographic measurements were carried out under the same experimental conditions but in alkaline solutions, and the formation of CdL 2 and CdL 3 was assumed from the shift of E(1/2) of cadmium(II) with an excess of cysteine. The stability constants of the assumed species were determined. Protonation constants of cysteine in 1.00 and 3.00 mol l −1 NaCl have been also determined. A comparison with the behaviour of serine and α-aminopropanoate towards cadmium(II) is proposed.