Uranium Hexafluoride, UF₆

Uranium Hexafluoride, Uranic Fluoride, UF₆, is the only known compound of hexavalent uranium (with the possible exception of the boride in which the condition of the uranium is not established) which does not contain oxygen. It was first prepared by Ruff and Heinzelmann by the action of fluorine on uranium pentachloride at -40° C. The action proceeds as already described (see equation above), and the volatile hexafluoride is distilled off from the tetrafluoride. The pentachloride, when acted upon by dry hydrogen fluoride, yields a compound, UF₅.xHF, which breaks up on distillation into the tetra- and hexa- fluorides, but this method of preparation is less convenient than the preceding one owing to the difficulty of separating the hexafluoride from hydrogen fluoride. Uranium carbide reacts with fluorine in presence of a little chlorine at -70°C., with formation of the hexafluoride.

Uranium hexafluoride yields glistening, colourless or pale yellow, monoclinic crystals, which fume in the air and sublime under reduced pressure at ordinary temperature. It boils at 56.2° C., and the calculated mean latent heat of evaporation between 42° and 57° C. is 29.4 calories per gram ( = 10360 calories per gram molecule). The variation of the boiling-point with the vapour pressure is as follows:

Temperature, ° C	56.2	48	45	41	37
Pressure, mm	764.6	521.2	410.1	406.1	298.2

The crystals melt at 69.2° C. at which temperature the vapour pressure is (by extrapolation) 1490 mm. or about 2 atmospheres. The vapour density at 448° C. is 11.7 (air = l), corresponding to a molecular weight of 338 (the theoretical value being 12.16).

The crystals have density at 20.7° C., 4.68. They are very hygroscopic and soluble in water. Chemically, uranium hexafluoride is highly reactive, vigorously attacking alcohol, ether, or benzene, in the last case depositing carbon. It reacts more slowly with carbon disulphide, paraffin, chloroform, and nitrobenzene. It dissolves readily in tetrachlorethane. It attacks glass in presence of a trace of moisture, forming silicon tetrafluoride and uranium oxyfluoride. It is decomposed by nitric acid. Ammonia forms ammonium uranate and fluoride. It is reduced by most non-metals and metals, except gold and platinum. Sulphur forms the disulphide, US₂, and uranous fluoride, and a gas is evolved which appears to be a new fluoride of sulphur. It is stable in presence of dry air, oxygen, nitrogen, carbon dioxide, chlorine, bromine, or iodine.