Retention behaviour of thorium(IV) and uranyl on a reversed-phase column with glycolate and mandelate as eluents

Thorium(IV) and uranyl were separated using a C18 reversed-phase column with eluents containing glycolic acid or mandelic acid. Detection was achieved using postcolumn reaction with Arsenazo III, followed by spectrophotometric detection. The chromatographic characteristics of this system were studied in detail with a view to determining the mechanism by which these metal ions are retained. The percentage of methanol in the eluent was found to influence retention in typical reversed-phase fashion. The results suggest that thorium(IV) and uranyl were eluted as either neutral or weakly charged complexes under the chromatographic conditions utilized. Examination of species distribution diagrams showed that the observed elution behaviour could not be explained from the predicted forms of the complexes present. However, consideration of the susceptibility of these metals to form mixed ligand complexes with hydroxide ions occupying one or more positions in the coordination sphere of the metal led to the suggestion that mixed anionic complexes were present. This hypothesis is used to explain the elution behaviour of thorium(IV) and uranyl with a range of complexing eluents.