Dissociation kinetics of nickel(II), zinc(II) and cadmium(II) complexes of 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N'-diacetic acid and 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N'-diacetic acid

Abstract

The dissociation kinetics of the complexes of nickel(II), zinc(II) and cadmium(II) of 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N'-diacetic acid (K21DA) and 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N'-diacetic Acid (K22DA) were studied in constant ionic strength aqueous medium with various [H+]-range, i.e., (0.88-53.9) x 10(-5) M and (0.5-7.5) x 10(-3) M. Copper(II) was used as the scavenger of free ligand and the rates of dissociation of these complexes have been found to be independent of [Cu2+]. All the complexes exhibit acid-independent and acid-dependent pathways. For NiK21DA, CdK21DA and CdK22DA complexes, the acid-dependent rates are linear functions of [H+]. For NiK22DA and ZnK21DA complexes, a saturation kinetics is observed, i.e., [H+]-dependence at low [H+] and [H+]-independent at high [H+]. The rationalization of such different observations is proposed to be due to difference in complex solution structures rather than the thermodynamic stabilities. Influence of acetate content in the buffer, temperature, and total electrolyte concentration on the late of dissociation has also been investigated and discussed.