The specific gravity of osmium in the pulverulent form is about 10; but after having been heated to the fusing point of rhodium in the oxyhydrogen jet, it acquires a density of 21·4, and in the crystalline state it has a sp. gr. of 22·477. Osmium appears to be the least fusible of all the metals.
There are five known oxides of osmium:
1. Osmium protoxide. (OsO). The anhydrous protoxide is of a greyish-black colour. It is insoluble in acids. Its bluish-black hydrate, which dissolves in hydrochloric acid, forms a solution of osmium dichloride of a deep indigo blue colour. The solution absorbs oxygen readily, and becomes converted into the tetrachloride (OsCl4).
2. Osmium sesquioxide. (Os2O3.) This has never been isolated. Its salts, which are uncrystallisable, are of a rose-red colour.
3. Osmium dioxide. (OsO2.) This is black.
4. Osmium trioxide. (OsO3.) This possesses feebly acid properties. It has never been obtained in a separate form; with potassium it forms a crystalline, sparingly soluble compound, having the composition K2OsO4, 2H2O, a dipotassic osmite.
5. Osmium tetroxide. Syn. Osmic Acid (OsO4). This oxide may be obtained by operating, according to Fremy’s process, on the ores of platinum, as already described.
It is also formed when metallic osmium is heated with potassic nitrate, or roasted in air. It crystallises in colourless, transparent, flexible needles, which fuse easily, and dissolve readily in water. Its aqueous solution, however, does not redden litmus. Tetroxide of osmium is converted into vapour at about 100°C. The fumes are excessively irritating and deleterious, and have an odour somewhat like that of chlorine. This oxide unites with alkalies, but not with acids. It is given off as tetroxide when the alkaline solution which contains it is boiled. If applied to the skin, this oxide becomes partially reduced, and imparts a permanent black colour to the skin, due to the deposition of metallic osmium. With tincture of galls its solutions give a distinctive blue precipitate.
There are four chlorides of osmium, the best known of which are the dichloride and the tetrachloride.
1. Osmium dichloride. (OsCl2). Syn. Osmious dichloride, Osmium protochloride. This is green, and sublimes in green needles. It may be obtained by heating metallic osmium in a current of dry chlorine gas. It forms double salts, which are of a green colour.