RETENTION MODEL FOR THE SEPARATION OF ANIONIC METAL - EDTA COMPLEXES IN ION CHROMATOGRAPHY

A retention model is derived for complex anions eluted from an anion-exchange column with multiple ionic eluents containing hydrogencarbonate, carbonate, and hydroxyl species and the sample solution, containing transition metals, anions and complexing ligand. The theory is based on the generalized ion-exchange equilibrium, protonation and complex-formation equilibria. The unknown parameters of chromatographic ion-exchange equilibrium constants for sample and eluent species are determined from the experimental retention data by iterative minimization, using a non-linear regression algorithm. The model was utilized to predict the retention behaviour of CdEDTA2-, CoEDTA2-, MnEDTA2- and NiEDTA2- ions. The capacity factors of complex ions were determined for wide ranges of pH values and eluent concentrations. Good agreement was obtained between the observed and predicted retentions.