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Osmium and ruthenium can be selectively isolated from a mixture of the noble metals by distillation in the form of 0504 and Ru04. In hydrochloric (or sulfuric) acid media and in the presence of iodide, 0504 is selectively reduced to osmium(IV) and the 05162-complex is formed. Quantitative conversion of Os(VIII) into this complex takes place in 0.8 M U solutions and in 0.04 M HCl (or 0.1 M H2SO4) within 5 (or 10 min), respectively. The 05162-complex reacts with trioctylamine ((C8HI,)3N) and the ion associate ((C8HI,)3NH+)2OsI62-is formed. The ion associate can be isolated from aqueous solutions by extraction into carbon tetrachloride. Both the OsI62-complex and the ion pair with trioctylamine can be used for spectrophotometric determination of osmium directly in aqueous solutions or after extraction into the organic solvent. Ruthenium does not interfere with the determination of osmium in the form of the 0sI62-complex. KeywordsNoble metals analysis, osmium, iodide complex, spectrophotometryDistillation of ruthenium and osmium in the form of volatile tetroxides, Ru04 and 0504, is generally a preliminary step of analysis of multicomponent samples of the noble metals.1'2 Most often hydrochloric acid is used as the absorbing solution for both the tetroxides. Ru(VIII) is easily reduced to lower oxidation states and chloride complexes are formed.2.4 In 6 -10 M hydrochloric acid, the RuCl62-complex is the main reduction product. Reduction of Os(VIII) in chloride solutions proceeds slowly. Quantitative conversion of osmium into the stable OsCl62-complex requires heating the solution for a minimum of 20 min at 90 -100° C.4,5 Ruthenium interferes with the determination of osmium in chloride solutions. Separation of osmium from ruthenium is generally carried out before the determination.1,2 It has recently been established that osmium can be determined in the presence of ruthenium in chloride solutions by derivative spectrophotometry after numerical calculation of absorption spectra of the OsCl62-and RuC162-complexes.4 The method is time consuming and requires a suitable modern instrument.The reduction of 0SO4 by iodide in hydrochloric (or sulfuric) acid medium has been studied in this work. It has been found that osmium(VIII) is rapidly reduced to osmium(IV) by I-anions at room temperature and the 05162-complex is formed. The complex is stable in aqueous solution. It can be a basis of spectrophotometric determination of osmium. It can also be used for preconcentration of osmium by extraction after the reaction with high molecular weight amines. The optimum conditions for quantitative extraction of osmium from aqueous solutions with trioctylamine ((CRH17)3N) into carbon tetrachloride have been established. Ru(VIII) is not reduced in iodide solutions under the conditions of osmium reduction and does not interfere with the determination of osmium. The rapid quantitative conversion of osmium into the 05162-complex in iodide solutions and the selective extraction of osmium with high molecular weight amines may be competitive to...