Potassium Osmate, K₂OsO₄

Potassium Osmate, K₂OsO₄.2H₂O, results when a solution of the tetroxide in aqueous potassium hydroxide is reduced with alcohol. The solution acquires a red colour from which the salt separates. On slow recrystallisation, garnet-red to black octahedra5 are obtained, permanent in dry air, but decomposing in moist air as also in solution, yielding the tetroxide and lower oxides.

Potassium osmate may be obtained in a pure state by fusing osmium with potassium nitrate and hydroxide, dissolving the melt in water, and precipitating with alcohol. The potassium osmate thus obtained has a greyish violet colour, and is not quite pure. It is therefore heated to 120° C. with a mixture of sulphuric acid and chromium trioxide, whereby osmium tetroxide is produced. This volatilises and is collected in 10 per cent, potash solution, from which it is obtained in a pure, crystalline form on addition of alcohol.

The solution of the pure salt is not as unstable as was originally believed by earlier investigators, since it may be kept in the cold exposed to sunlight for many days without serious decomposition. On warming, decomposition is accelerated, and the odour of osmium tetroxide becomes pronounced.

At 200° C. the crystals become anhydrous in an inert atmosphere.

Osmates of Sodium and Barium have also been prepared.