ON THE BEHAVIOR OF CYSTEINE AS LIGAND OF CADMIUM(II)

The ability of cysteine to form complexes with cadmium(II) in aqueous solutions has been investigated at 25 degrees 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 elect romotive force of galvanic cells containing glass and cadmium amalgam electrodes was measured as a function of cadmium and hydrogen ion conc entrations in acid or moderately alkaline solutions in order to obtain the free concentration of cadmium(II) and hydrogen ions. The experime ntal data obtained in 1.00 mol l(-1) NaCl were explained by assuming t he presence of CdHL and CdH2L2, while those obtained in 3.00 mol l(-1) NaCl were accounted for with the formation of CdHL, CdH2L2, CdH3L3 an d CdH2L3. Moreover, polarographic measurements were carried out under the same experimental conditions but in alkaline solutions, and the fo rmation of CdL2 and CdL3 was assumed from the shift of E(1/2) of cadmi um(II) with an excess of cysteine. The stability constants of the assu med 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 alpha-aminopropanoate towards cadmium(II) is proposed. (C) 1997 Elsevier Science B.V.