The speciation of rhodium(III) in hydrochloric acid media by capillary zone electrophoresis

A fast and effective method to study the aquation of rhodium(III) chlorocomplex in hydrochloric solutions using capillary zone electrophoresis (CZE) is developed. At least five species, some of which seem to be oligomeric, are formed in solution during the aquation process at pH>1. The fast hydration of RhCl63− makes this species impossible to detect. The first species detected in the optimised conditions is RhCl5(H2O)2− although RhCl4(H2O)2− is the main species during the first stage of the aquation process. When equilibrium is reached either RhCl3(H2O)3 or a cationic complex, RhCl2(H2O)4+, is formed as the main species. Matrix Assisted Laser Desorption Ionisation Time of Flight Mass Spectrometry (MALDI-TOF MS) is used as a novel technique to elucidate the structure of the rhodium aqua/chloro complexes formed in solution. Results obtained by CZE are confirmed by spectrophotometry.