Revised Ion Exchange Method for Estimation of Conditional Stability Constants of Metal-Dendrimer Complexes

This study reports a revised method for determination of the conditional stability constant (Kc) and the equilibrium metal-to-ligand molar ratio (n) for various metal-dendrimer complexes. The method employs a system containing an ion-exchange resin, a dendrimer, and a metal (Cu2+ or Pb2+) solution. It considers the multi-dentate nature of dendrimers and employs a nonlinear (Langmuir) isotherm model to interpret the ion exchange isotherm. For its minimal dendrimer uptake, a weak acid cation-exchange resin, IRC-50, was found most suitable for the study. For complexes of Cu2+ with a homologue of dendrimers with NH2 terminal groups Kc increased from 1016.2 to 1036.6, and n increased from 1 to 13 as the generation number rose from 0.0 to 4.0. Kc for complexes of Pb2+ with a generation-1 dendrimer with NH2 terminal groups was 1017.2 at pH 5, compared to 1020.9 for complexes of Pb2+ with a generation-1.5 dendrimer with carboxylate terminal groups.