1911 Encyclopædia Britannica/Osmium

​OSMIUM [symbol Os., atomic weight 190.9 (O=16)], inchemistry, a metallic element, found in platinum ore in smallparticles, consisting essentially of an alloy of osmium andiridium and known as osmiridium. It was first obtained in1803 by Smithson Tennant (Phil. Trans., 1804, 94, p. 411). Itmay be prepared from osmiridium by fusing the alloy withzinc, the zinc being afterwards removed by distillation. Theresidue so obtained is then powdered and ignited with bariumnitrate, which converts the iridium into its oxide and the osmiuminto barium osmiate. The barium salt is extracted by waterand boiled with nitric acid, when the osmium volatilizes in theform of its tetroxide. As an alternative the osmiridium is fusedwith zinc, the regulus treated with hydrochloric acid, and thenheated with barium nitrate and barium peroxide. After fusion,the mass is finely powdered and treated with cold dilute hydrochloricacid; and when action has finished, nitric and sulphuricacids are added, the precipitated barium sulphate removed,the liquid distilled and the osmium precipitated as sulphide.The sulphide is converted into sodium osmichloride by fusionwith salt, in a current of chlorine, the sodium salt transformedinto ammonium salt by precipitation with ammonium chloride,and the ammonium salt finally heated strongly (H. Sainte-Claire-Devilleand H. J. Debray, An. min., 1859 [5], 16, 74;see also C. E. Claus, Jour. prakt. Chem., 1862, 85, p. 142; F.Wöhler, Pogg. 31, p. 161; E. Leidie and L. Quenessen, Bull.soc. chim., 1903 (8), 29, p. 801). The tetroxide, OsO₄, can beeasily reduced to the metal by dissolving it in hydrochloricacid and adding zinc, mercury, or an alkaline formate to theliquid, or by passing its vapour, mixed with carbon dioxideand monoxide, through a red-hot porcelain tube. The metalhas a blue-grey colour, and may be obtained in the crystallinestate by solution in tin. Its specific gravity is 21.3-22.48(Deville and Debray) and its specific heat is 0.03113 (Regnault).It can be distilled in the electric furnace. In the massive stateit is insoluble in all acids, but when freshly precipitated fromsolutions it dissolves in fuming nitric acid. On fusion withcaustic potash it yields potassium osmiate. It combines withfluorine at 100° C, and when heated with chlorine it formsa mixture of chlorides. A colloidal variety was obtained byA. Gutbier and G. Hofmeier (Jour. prakt. Chem., 1905 (2), 71,p. 452) by reducing osmium compounds with hydrazine hydratein the presence of gum arable.

Several oxides of osmium are known. The protoxide, OsO, isobtained as a dark grey insoluble powder when osmium sulphite isheated with sodium carbonate in a current of carbon dioxide. Thesesquioxide, Os₂O₃, results on heating osmium with an excess of thetetroxide. The dioxide, OsO₂, is formed when potassium osmichlorideis heated with sodium carbonate in a current of carbondioxide, or by electrolysis of a solution of the tetroxide in thepresence of alkali. It is insoluble in acids and exists in severalhydrated forms. The osmiates, corresponding to the unknowntrioxide OsO₃, are red or green coloured salts; the solutions areonly stable in the presence of excess of caustic alkali; on boiling anaqueous solution of the potassium salt it decomposes readily, forminga black precipitate of osmic acid, H₂OsO₄. Potassium osmiate,K₂OsO₄2H₂O, formed when an alkaline solution of the tetroxide isdecomposed by alcohol, or by potassium nitrite, crystallizes in redoctahedral. It is stable in dry air, but in moist air rapidly decomposes.The tetroxide, OsO₄, is formed when osmium compounds areheated in air, or with aqua regia, or fused with caustic alkali andnitre. It is obtained as a yellowish coloured mass and can besublimed in the form of needles which melt at 40° C. It possessesan unpleasant smell and its vapour is extremely poisonous. Itdissolves slowly in water, and the aqueous solution is reduced bymost metals with precipitation of osmium. It acts as an oxidizingagent, liberating iodine from potassium iodide, converting alcoholinto acetaldehyde, &c.

Osmium dichloride, OsCl₂, is obtained as a dark coloured powderwhen the metal is heated in a current of chlorine. Its solutionin water is deep blue in colour, but the colour changes rapidly togreen and yellow. The trichloride, OsCl₃, is only known in solutionand is formed by the reducing action of mercury on ammonia calsolutions of the tetroxide. A hydrated form of compositionOsCl₃·3H₂O has been described. The tetrachloride, OsCl₄, is obtainedas a dark red sublimate (mixed with the dichloride) when osmium isheated in dry chlorine. It is soluble in water, but the dilute solutionreadily decomposes on standing. It combines with the chlorides ofthe alkali metals to form characteristic double salts of the typeOsCl₄·2MCl (osmichlorides). Potassium osmichloride, K₂OsCl₆, isformed when a mixture of osmium and potassium chloride is heatedin a current of chlorine, or on adding potassium chloride and alcoholto a solution of the tetroxide in hydrochloric acid. It crystallizesin dark red octahedral which are almost insoluble in cold water.The aqueous solution decomposes rapidly on boiling. Iodine has noaction on osmium, but on warming the tetro.xide with a mixtureof potassium iodide and hydrochloric acid a deep emerald green,colour is produced, due to the formation of a compound OsI₂·2HI;this reaction is a delicate test for osmium (E. Pinerua Alvarez,Comptes rendus, 1905, 140, p. 1254). Osmium disulphide, OsS₂, isobtained as a dark brown precipitate, insoluble in water, by passingsulphuretted hydrogen into a solution of an osmichloride. Thetetrasulphide, OsS₄, is similarly prepared when sulphuretted hydrogenis passed into acid solutions of the tetroxide. It is a brownish blacksolid, insoluble in solutions of the alkaline sulphides. The atomic,weight of the metal has been determined by K. Seubert (Ber., 1888,21, p. 1839) from the analysis of potassium and ammonium osmichlorides,the values obtained being approximately 191.