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

The feasibility of capillary electrophoresis for distinguishing between the rhodium(III) species occurring in different acidic environments has been d emonstrated. The separation was optimum under acidic electrolyte conditions in which the complexed Rh species were at their most stable and the electr oosmotic 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 meta l complexes was highly dependent on the nature and concentration of the aci d 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 distrib ution) of complexed forms. In 0.1 mol L-1 HCl, four differently charged chl oro complexes - RhCl4(H2O)(2)(-), RhCl3(OH)(H2O)(2)(-), RhCl3(H2O)(3), and RhCl2(H2O)(4)(+)- were separated and identified. When a stock solution in 1 1 mol L-1 HCl was run, Rh produced a major peak ascribed to RhCl63- and two slowly migrating peaks from ions in which one or two of the chloride ligan ds were probably replaced by water and hydroxyl ion, as a result of hydroly sis. 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.