Solvent extraction of osmium(IV) from sulfuric acid solutions in the presence of chloride ions

The effect of the chloride ion concentration on the composition of diverse aquachloro and aquachlorohydroxo osmium(IV) complexes, which are formed by the osmium tetroxide reduction in sulfuric acid solutions, is described. The respective osmium(IV) complexes can be separated by applying ion exchange chromatography. While at low chloride concentrations, cis-and trans-isomers of cationic [{OsCl2(OH)(H2O)(2)}(2)(mu-OH)](+) as well as anionic fac-[{OsCl3(OH)(H2O)}(2)(mu-OH)](-) and [OsCl5(H2O)](-) are present in solution, an increase of the chloride concentration leads to an increase in the number of different osmium-containing species. The chemical and physical behavior of cis-[OsCl2(OH)(H2O)(2)}(2)(mu-OH)](+) (1), trans-[{OsCl2(OH)(H2O)(2)}(2)(mu-OH)](+) (2) fac-[{OsCl3(OH)(H2O)}(2)(mu-OH)](-) (3), fac-[OsCl3(OH)(2)(H2O)](-) (4),fac-[{OsCl3(OH)(H2O)(2)(mu-O)](2) - (5), [OsCl5(H2O)](-) (6) (6) and [OsCl6](2-) (7) in regard to their solvent extraction with tri-n-octylamine (TOA) or tri-n-butyl phosphate (TBP) is presented. The extraction of [OsCl5(H2O)](-) (6) as a function of the concentration and type of the appropriate extractants as well as the nature of the diluent applied is studied. Stripping of 6 can successfully be carried out either from TOA extracts in ammoniac solutions or from TBP extracts in diluted sulfuric acid solutions. A possible mechanism is proposed for the chemical processes, which may take place during the absorption of osmium tetroxide in the gas cleaning system of nickel- or copper-producing plants. (C) 2003 Elsevier Science B.V. All rights reserved.