Uranyl Chloride, UO₂Cl₂

Uranyl Chloride, UO₂Cl₂, is obtained in the anhydrous condition by the action of chlorine on heated uranous oxide. It yields a yellow crystalline mass, stable in dry air but very hygroscopic and readily soluble in water. It is also soluble in alcohol or ether. When heated in air, uranyl chloride loses chlorine and leaves uranous oxide which is oxidised to higher oxides. If heated to redness in hydrogen, it is reduced to uranous oxide. Fusion with excess of caustic potash in an open tube produces a red peruranate, K₂UO₅, which gradually changes to the yellow uranate, K₂UO₄, with loss of oxygen.

It is formed in solution by oxidising uranous chloride with nitric acid; by dissolving uranic oxide in concentrated hydrochloric acid; or by adding barium chloride to a concentrated solution of uranyl sulphate until precipitation is complete. Its aqueous solution on evaporation yields the monohydrate, UO₂Cl₂.H₂O. The solution is unstable at ordinary temperatures and slowly deposits uranic hydroxide, which after a time partly redissolves. The uranyl chloride may be reduced in solution to black uranous oxide by the action of magnesium or aluminium powder. The densities of aqueous solutions of uranyl chloride have been determined as follows:

Temperature, ° C.	Concentration, UO2Cl2 per cent.	Density.
14.6	1	1.0056
16.3	2	1.0112
13.7	3	1.0161
13.1	4	1.0215
14.2	5	1.0260
15.2	6	1.0313
14.3	7	1.0366
14.5	8	1.0418
15.0	9	1.0469
14.8	10	1.0517

The equivalent conductivities of aqueous solutions containing ½UO₂Cl₂ in v litres at 25° C. are as follows:

v	4	8	16	32	64	128	256	512	1024
Λ	79.8	89.3	96.6	104.3	111.3	118.5	127.4	136.4	147.0

From the solution in hydrochloric acid, the trihydrate, UO₂Cl₂.3H₂O, possibly H₄UO₅.2HCl, separates in yellowish-green hygroscopic prisms, which when heated above 100° C. decompose, losing water, hydrogen chloride, and chlorine. The trihydrate dissolves in water, 1 part in 0.134 part of water at 18° C., yielding a viscous solution of density 2.74.

A saturated solution of uranyl chloride in hydrochloric acid when cooled to -10° C. yields yellow crystals of an unstable hydrochloride, UO₂Cl₂.HCl.2H₂O. The compound may also be obtained by the action of hydrochloric acid on uranyl nitrate.

Compounds of uranyl chloride with ammonia, of composition UO₂Cl₂.2NH₃ and UO₂Cl₂.3NH₃, and with organic compounds, for example UO₂Cl₂.2NH₃.(C₂H₅)₂O and UO₂Cl₂.2(C₂H₅)₂O, have been described. Well crystallised compounds with organic bases have also been prepared.

Uranyl chloride reacts with the chlorides of the alkali metals to form a series of complex salts of the type R₂(UO₂)Cl₄. Uranyl potassium chloride, K₂(UO₂)Cl₄, is prepared in the anhydrous condition by passing dry chlorine first over heated uranous oxide and then over heated potassium chloride. It is obtained as a golden-yellow powder which when heated to redness melts without decomposition. It is very soluble in water. The dihydrate, K₂(UO₂)Cl₄.2H₂O, separates on evaporation of a solution containing potassium chloride and uranyl chloride; or of a hydrochloric acid solution of potassium uranate. It yields greenish-yellow, triclinic pinacoidal plates. The double salt can only be obtained pure from the aqueous solution above 60° C., as below this temperature it undergoes partial decomposition with separation of potassium chloride. The ammonium salt, (NH₄)₂(UO₂)Cl₄.2H₂O, prepared by crystallisation from a solution of ammonium and uranyl chlorides in concentrated hydrochloric acid, is isomorphous with the potassium salt, and behaves similarly with water. The sodium salt, Na₂(UO₂)Cl₄, obtained in the dry way by passing chlorine successively over heated uranous oxide and sodium chloride, is a golden-yellow substance with properties similar to the anhydrous potassium salt. The rubidium salt, Rb₂(UO₂)Cl₄.2H₂O, is isomorphous with the potassium and ammonium salts, but differs from them in being undecomposed by water both at high and low temperatures. The caesium compound, Cs₂(UO₂)Cl₄, is obtained in yellow rhombic leaflets from an aqueous solution of the mixed chlorides containing hydrochloric acid. Like the rubidium salt, it is stable towards water.