Uranium Tetrachloride, UCl₄

Uranium Tetrachloride, Uranous Chloride, UCl₄, may be prepared by heating uranium, uranium carbide, or a mixture of any oxide of uranium, or uranyl chloride, with carbon, in a stream of chlorine. A small quantity of the pentachloride is also formed, but this may be converted to the tetrachloride by heating at 235° C. in a current of carbon dioxide. Uranous chloride may also be obtained by the action of gaseous hydrogen chloride on uranous oxide; or by heating urano-uranic oxide in a current of carbon tetrachloride vapour, or in a mixture of chlorine and sulphur chloride. The product can be purified by sublimation in a vacuum, or by heating to redness in an atmosphere of dry chlorine. In the latter case, a hard glass tube, of about 4 cm. diameter, may be used; the uranous chloride is deposited close to the hot zone in the form of a green crystalline mass. It may be obtained in solution by exposing to the action of sunlight a solution in hydrochloric acid of uranic hydroxide, or of the green hydroxide, in presence of a little alcohol; or by reduction of uranyl chloride by means of copper and hydrochloric acid in presence of traces of platinic chloride; it is obtained as a green crystalline precipitate on adding ether.

Uranous chloride yields lustrous, dark green, octahedral crystals which sublime when heated to redness. The vapour density is 13.33 (air = 1), corresponding to molecular weight 386.0 (UCl₄ = 380.0). By ebullioscopic determinations of the molecular weight, using molten bismuth chloride as solvent, Riigheimer and Gonder obtained the value 373. The crystals fume in the air and are very deliquescent. They readily dissolve in water, with evolution of heat, yielding a green solution which is fairly stable in the cold, but is hydrolysed to a considerable extent and gives an acid reaction. It is a powerful reducing agent, and on evaporation, hydrochloric acid fumes escape, and on prolonged heating or boiling, a black precipitate is produced which appears to be an oxychloride of composition UCl₄.5UO₂.10H₂O. On complete evaporation an amorphous deliquescent mass is obtained which when heated leaves urano-uranic oxide. Uranous chloride dissolves readily in ethyl alcohol, acetone, and other organic solvents.

It is reduced to the metal by the action of alkali metals. Heated in hydrogen, it yields the trichloride; in ammonia, a nitride; in hydrogen sulphide, a sulphide.

The oxychloride, UCl₄.2UO₂.13H₂O, may be obtained as green crystals by the action of light on a solution of uranyl chloride in a mixture of alcohol and ether. When dried over sulphuric acid the salt loses water and leaves the monohydrate, UCl₄.2UO₂.H₂O. This on heating at 100° C. becomes brownish-black and yields UCl₄.4UO₂. Both hydrates dissolve in water and alcohol, forming green solutions.

Double Chlorides of the type R₂UCl₆ (R = Na, K, Li; R₂ = Ca, Sr, Ba) have been obtained by passing uranous chloride vapour over the chloride of the alkali or alkaline earth metal heated to dull redness. The salts, which are green, are hygroscopic, and darken as they absorb moisture from the air. They are unstable, and their solutions oxidise slowly at ordinary temperature and more rapidly when heated. When evaporated in a vacuum over sulphuric acid they decompose, so that the salts cannot be obtained in the crystalline form.

The double salt, uranous chlorophosphate, UCl₄.UH₂(PO₄)₂, separates in green crystals from a solution of uranous phosphate, UH₂(PO₄)₂.5H₂O, in concentrated hydrochloric acid. A chlorophosphate, of composition UCl₄.3UO₂.2P₂O₅, has also been prepared.