Osmium Octafluoride, ОsF₈

Osmium Octafluoride, ОsF₈, being the most volatile of the products, is collected in a vessel cooled in a mixture of solid carbon dioxide and alcohol, under which conditions it yields a yellow, solid sublimate. It melts at 34.5° C. to a yellowish red liquid. Its vapour is colourless, has a characteristic odour and metallic taste: it attacks the mucous membranes of the nose and eyes. In moist air the vapour yields a white cloud.

The vapour pressures for different temperatures are as follow:

Temperature 0° C.	Pressure mm.	Temperature 0° C.	Pressure mm.
38.0	552.5	43.5	655.6
40.3	594.5	47.3	757.5
42.0	634.2	-	-

The mean vapour density is 177.5 (H = 1), that calculated for the formula ОsF₈ being 172.

When heated in a platinum tube, osmium octafluoride begins to decompose at 225° C., but the extent of decomposition only becomes appreciable at about 400° C.

Organic substances are violently attacked by osmium octafluoride, and the skin burnt. In aqueous solution the salt is colourless and partially hydrolysed, the solution having the odour of the tetroxide. In sodium hydroxide solution a yellowish red colour is obtained, characteristic of perosmates.

With alkali fluorides double salts are produced, the compositions of which are as yet unknown. They are less easily reduced, however, than the free octafluoride. Sodium hydroxide decomposes them with evolution of ozone.