# 1911 Encyclopædia Britannica/Osmium

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​OSMIUM [symbol Os., atomic weight 190·9 (O＝16)], in chemistry, a metallic element, found in platinum ore in small particles, consisting essentially of an alloy of osmium and iridium and known as osmiridium. It was first obtained in 1803 by Smithson Tennant (Phil. Trans., 1804, 94, p. 411). It may be prepared from osmiridium by fusing the alloy with zinc, the zinc being afterwards removed by distillation. The residue so obtained is then powdered and ignited with barium nitrate, which converts the iridium into its oxide and the osmium into barium osmiate. The barium salt is extracted by water and boiled with nitric acid, when the osmium volatilizes in the form of its tetroxide. As an alternative the osmiridium is fused with zinc, the regulus treated with hydrochloric acid, and then heated with barium nitrate and barium peroxide. After fusion, the mass is finely powdered and treated with cold dilute hydrochloric acid; and when action has finished, nitric and sulphuric acids are added, the precipitated barium sulphate removed, the liquid distilled and the osmium precipitated as sulphide. The sulphide is converted into sodium osmichloride by fusion with salt, in a current of chlorine, the sodium salt transformed into ammonium salt by precipitation with ammonium chloride, and the ammonium salt finally heated strongly (H. Sainte-Claire-Deville and 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, OsO$_4$, can be easily reduced to the metal by dissolving it in hydrochloric acid and adding zinc, mercury, or an alkaline formate to the liquid, or by passing its vapour, mixed with carbon dioxide and monoxide, through a red-hot porcelain tube. The metal has a blue-grey colour, and may be obtained in the crystalline state 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 state it is insoluble in all acids, but when freshly precipitated from solutions it dissolves in fuming nitric acid. On fusion with caustic potash it yields potassium osmiate. It combines with fluorine at 100° C, and when heated with chlorine it forms a mixture of chlorides. A colloidal variety was obtained by A. Gutbier and G. Hofmeier (Jour. prakt. Chem., 1905 (2), 71, p. 452) by reducing osmium compounds with hydrazine hydrate in the presence of gum arabic.
Several oxides of osmium are known. The protoxide, OsO, is obtained as a dark grey insoluble powder when osmium sulphite is heated with sodium carbonate in a current of carbon dioxide. The sesquioxide, Os$_2$O$_3$, results on heating osmium with an excess of the tetroxide. The dioxide, OsO$_2$, is formed when potassium osmichloride is heated with sodium carbonate in a current of carbon dioxide, or by electrolysis of a solution of the tetroxide in the presence of alkali. It is insoluble in acids and exists in several hydrated forms. The osmiates, corresponding to the unknown trioxide OsO$_3$, are red or green coloured salts; the solutions are only stable in the presence of excess of caustic alkali; on boiling an aqueous solution of the potassium salt it decomposes readily, forming a black precipitate of osmic acid, H$_2$OsO$_4$. Potassium osmiate, K$_2$OsO$_4$2H$_2$O, formed when an alkaline solution of the tetroxide is decomposed by alcohol, or by potassium nitrite, crystallizes in red octahedra. It is stable in dry air, but in moist air rapidly decomposes. The tetroxide, OsO$_4$, is formed when osmium compounds are heated in air, or with aqua regia, or fused with caustic alkali and nitre. It is obtained as a yellowish coloured mass and can be sublimed in the form of needles which melt at 40° C. It possesses an unpleasant smell and its vapour is extremely poisonous. It dissolves slowly in water, and the aqueous solution is reduced by most metals with precipitation of osmium. It acts as an oxidizing agent, liberating iodine from potassium iodide, converting alcohol into acetaldehyde, &c.
Osmium dichloride, OsCl$_2$, is obtained as a dark coloured powder when the metal is heated in a current of chlorine. Its solution in water is deep blue in colour, but the colour changes rapidly to green and yellow. The trichloride, OsCl$_3$, is only known in solution and is formed by the reducing action of mercury on ammoniacal solutions of the tetroxide. A hydrated form of composition OsCl$_3$.3H$_2$O has been described. The tetrachloride, OsCl$_4$, is obtained as a dark red sublimate (mixed with the dichloride) when osmium is heated in dry chlorine. It is soluble in water, but the dilute solution readily decomposes on standing. It combines with the chlorides of the alkali metals to form characteristic double salts of the type OsCl$_4$.2MCl (osmichlorides). Potassium osmichloride, K$_2$OsCl$_6$, is formed when a mixture of osmium and potassium chloride is heated in a current of chlorine, or on adding potassium chloride and alcohol to a solution of the tetroxide in hydrochloric acid. It crystallizes in dark red octahedra which are almost insoluble in cold water. The aqueous solution decomposes rapidly on boiling. Iodine has no action on osmium, but on warming the tetroxide with a mixture of potassium iodide and hydrochloric acid a deep emerald green, colour is produced, due to the formation of a compound OsI$_2$.2HI; this reaction is a delicate test for osmium (E. Pinerua Alvarez, Comptes rendus, 1905, 140, p. 1254). Osmium disulphide, OsS$_2$, is obtained as a dark brown precipitate, insoluble in water, by passing sulphuretted hydrogen into a solution of an osmichloride. The tetrasulphide, OsS$_4$, is similarly prepared when sulphuretted hydrogen is passed into acid solutions of the tetroxide. It is a brownish black solid, 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.