Kinetic features of metal complexes with polysaccharide colloids: Impact of ionic strength

The dynamic features of metal binding by a gel-like polysaccharide, carboxymethyldextran (CMD), are investigated by stripping chronopotentiometry (SCP). This technique measures the diffusive flux properties of the metallic species in the ligand dispersion as defined by their concentration, mobility, and lability. Cadmium(II) forms only 1:1 complexes with CMD, the lability of which is well described by Eigen mechanism principles, that is, the removal of a water molecule from the inner hydration sphere of the metal ion is limiting the complex formation rate. Lead(II) and copper(II), however, also form intramolecular bidentate complexes with CMD, which requires a conformational reorganization of the polymeric chain. The reorganization process appears to be the rate-limiting step of the overall complexation reaction, which takes place on a time scale of hours. The influence of ionic strength on the rate of bidentate complex formation is insignificant. In contrast, its impact on the stability of the monodentate complex follows the corresponding Donnan potential of the soft CMD particle.