ON THE STRUCTURE OF RHODIUM COMPLEXES IN AQUEOUS-SOLUTIONS OF RHODIUM(III) NITRATE AND CHLORIDE AND THE ADSORPTION ON GAMMA-ALUMINA

Chemical species in aqueous solutions of rhodium(III) nitrate and chloride were characterized by UV-VIS spectrometry and the adsorption of rhodium complexes on gamma-alumina in acidic solutions was studied. The UV-VIS absorption spectra confirmed that a cationic complex, [Rh(H2O)(6)](3+) exists in the commercially available ''rhodium nitrate solution''. The hexaaqua complex was not found in the aqueous solution of Rh(NO3)(3) . 2 H2O and only a strong absorption spectrum was observed in the UV region. In the aqueous solution of rhodium chloride, a neutral complex, [RhCl3(H2O)(3)] was found and this complex was accompanied by a small amount of hydroxo complexes. It was shown that the hexaaqua complex can not adsorb on gamma-alumina at room temperature under strong acidic condition as pH<1, but the uptake of Rh by alumina occurrs during high temperature impregnation with increasing dissolution of alumina. The hydroxo complex [Rh(OH)(H2O)(5)](2+) seems to react with the alumina surface by ligand exchange reaction. The uptakes of the neutral and anionic complexes by alumina from the rhodium(III) chloride solution were also presumed to proceed by the ligand exchange reaction mechanism, because the addition of chloride ion to the impregnating solution showed no inhibition effect on the adsorption of the rhodium(III) complexes.