EPR and ENDOR characterization of Cu-Chelex, a comparison study with Cu(Gly)₂ and Cu(IDA)₂

Chelex chelating resin is classed among the weak acid cation exchange resins by virtue of its carboxylic acid groups, but it differs from ordinary exchangers in its high selectivity for metal ions and its much greater bond strength. Chelex chelating resin operates in basic, neutral, and weakly acidic solutions at pH=4 or higher. Another feature of the Chelex 100 resin is that its volume changes when its ionic form is altered. Because of their importance in analytical techniques, iminodiacetate (IDA) chelating resins have been widely studied. Absorption kinetics, pH absorption, and selectivity order toward metal ions have been defined. Actual selectivity values for any particular system depend on pH, ionic strength, and the presence of other complex-forming species. Several studies of coordination of the IDA ligand in solution and in the crystal state have been carried out recently, but no structural information is available for an IDA group included in a polymeric matrix (chelating resins). Several IDA group coordination modes could be hypothesized. IDA could use all three of its donor atoms, as free ligands do, or it could use only one or two atoms. It may be assumed that different species/complexes can be obtained. Structural information, as in many other areas of materials science, is essential to gain an understanding of the samples physical behavior. The major difficulties in determining the coordination structure of a metal ion with a chelating resin lie in its highly complex amorphous structure. Here we use EPR and ENDOR spectroscopy to derive the structure of the metal ion-binding site of the different complexes.