Speciation studies by capillary electrophoresis – distribution of rhodium(III) complexed forms in acidic media

Abstract

The feasibility of capillary electrophoresis for distinguishing between the rhodium(III) species occurring in different acidic environments has been demonstrated. The separation was optimum under acidic electrolyte conditions in which the complexed Rh species were at their most stable and the electroosmotic flow approached zero, thereby aiding resolution. Identification of the forms of Rh and estimation of their relative equilibrium content were accomplished by use of a diode-array detector. The distribution of the metal complexes was highly dependent on the nature and concentration of the acid and the age of the rhodium stock solutions. On dilution Rh(III) tends to be readily hydrolyzed, giving rise to a wider variety (and a varied distribution) of complexed forms. In 0.1 mol L^–1 HCl, four differently charged chloro complexes – RhCl₄(H₂O)₂ ^–, RhCl₃(OH)(H₂O)₂ ^–, RhCl₃(H₂O)₃, and RhCl₂(H₂O)₄ ⁺ –  were separated and identified. When a stock solution in 11 mol L^–1 HCl was run, Rh produced a major peak ascribed to RhCl₆ ^3–and two slowly migrating peaks from ions in which one or two of the chloride ligands were probably replaced by water and hydroxyl ion, as a result of hydrolysis. The aquatic cationic species were found to be predominant in HClO₄ and HNO₃ solutions, whereas only negatively charged forms of Rh(III) occurred in sulfuric acid. This speciation information opens also new possibilities of assessing the catalytic activity of Rh in kinetic reactions.