Uranium Tetrafluoride, UF₄

Uranium Tetrafluoride, Uranous Fluoride, UF₄, is the chief product obtained when the metal is acted upon by fluorine. It may be prepared by the action of hydrogen fluoride on urano-uranic oxide or on uranous oxide; or by reduction of a solution of uranyl fluoride with stannous chloride. It is also formed with uranium hexafluoride when the pentachloride is acted upon by fluorine at -40° C. thus:

2UCl₅ + 5F₂ = UF₄ + UF₆ + 5Cl₂.

The hexafluoride may be removed by distillation. Dry hydrogen fluoride acts in a similar manner. Uranium carbide, UC₂, reacts with pure fluorine to yield the tetrafluoride only.

Uranous fluoride is a green amorphous powder, insoluble in water, hardly attacked by dilute acids and only dissolved with difficulty by concentrated acids, except in the case of nitric acid which readily decomposes it. When heated in the air it loses fluorine and leaves a residue of urano-uranic oxide. At ordinary temperatures it is gradually oxidised to the uranyl compound, UO₂F₂. If heated in absence of air it melts at about 1000° C.

The insolubility of uranous fluoride affords a method of estimating the metal by precipitation with hydrofluoric acid.

The monohydrate, UF₄.H₂O, is obtained by drying at 100° C. the voluminous green precipitate produced by adding hydrofluoric acid to a solution of uranous chloride.

Double fluorides have been obtained by reduction of complex alkali uranyl fluoride solutions by means of oxalic or formic acid under the influence of sunlight. The potassium salt, KUF₅, is a green powder, insoluble in water.