Uranous Phosphates

Uranous orthophosphate, U₃(PO₄)₄, is prepared by adding trisodium phosphate to a solution of uranous chloride in an atmosphere of carbon dioxide. It separates as an amorphous precipitate, which, when heated to redness in a current of hydrogen chloride, changes to a green microcrystalline powder. A basic phosphate, of composition U₂O(PO₄)₂ or 2UO₂.P₂O₅, is obtained as a bright green powder when uranyl ortho- or pyro-phosphate is reduced by heating in hydrogen; it becomes crystalline when ignited in hydrogen chloride. The same compound may be obtained by precipitating a solution of uranous chloride by means of excess of trisodium phosphate or sodium pyrophosphate. A chlorophosphate, UClPO₄ or 3UO₂.UCl₄.2P₂O₅, is obtained by passing the vapour of uranous chloride over heated metaphosphoric acid, or any one of the uranous phosphates, after displacement of the air by carbon dioxide. It yields green orthorhombic crystals,

a:b:c = 0.8376:1:0.7473.

An acid uranium phosphate, UH₂(PO₄)₂ or UO₂.P₂O₅.H₂O, is obtained as a gelatinous precipitate when ordinary sodium phosphate, or excess of sodium metaphosphate, is added to a solution of uranous chloride. It may be obtained in the crystalline form as UH₂(PO₄)₂.5H₂O. Aloy also describes a uranous chlorophosphate of composition UH₂(PO₄)₂.UCl₄.

Uranous metaphosphate, U(PO₃)₄, is prepared by heating to redness a mixture of uranous oxide and metaphosphoric acid; the operation should be carried out in a gold Rose crucible through which carbon dioxide is passed. After washing the product with water and boiling nitric acid, green crystals of the metaphosphate remain. The same compound is obtained when uranium orthophosphate is fused with four times its weight of metaphosphoric acid; or when anhydrous uranyl sulphate is dissolved in metaphosphoric acid at 316° C. Uranous metaphosphate yields emerald-green rhombic crystals, isomorphous with the corresponding thorium salt and of density 3.9 at 0° C. It is only slowly attacked by acids, but fused alkalies or alkali chlorides decompose it. When heated strongly, it melts, loses phosphorus pentoxide, and yields the pyrophosphate.

Uranous pyrophosphate, UP₂O₇, may be obtained by heating uranous hydrogen phosphate, UH₂(PO₄)₂.5H₂O, in a current of carbon dioxide; or more readily, by calcining the uranyl salt, UO₂H₄(PO₄)₂.3H₂O, in an atmosphere of hydrogen. The product is amorphous but becomes crystalline without change in composition on heating in a stream of hydrogen chloride. The crystalline form may also be obtained by passing the vapour of phosphorus oxychloride over urano-uranic oxide. The crystals are not well defined and are almost colourless. The trihydrate, UP₂O₇.3H₂O, is formed according to Rammelsberg as a gelatinous precipitate by the addition of sodium pyrophosphate to a solution of uranous chloride.

Complex phosphates of uranium and the alkali or alkaline earth metals have been prepared by fusing an intimate mixture of the alkali phosphate with an excess of uranous oxide in a platinum Rose crucible through which carbon dioxide was passing. The fusion may be facilitated by the addition of a little alkali chloride. On slowly cooling the crucible, well-defined crystals of the double phosphate are formed. If the alkali metaphosphate is used, salts of the type R₂U(PO₄)₂ where R = K, Na; R₂ = Ca, Sr, Ba, are obtained.

The potassium salt, K₂U(PO₄)₂, forms green, orthorhombic crystals,

a:b:c = 0.3711:1:0.3902,

and is soluble in concentrated acids.

The sodium salt, Na₂U(PO₄)₂, is better prepared from the pyrophosphate, using excess of sodium chloride. It forms emerald-green, orthorhombic crystals,

a:b:c = 0.6766:1:0.4006,

and is readily attacked by acids.

The calcium salt, CaU(PO₄)₂, forms green monoclinic crystals,

a:b:c = 1.508:1:1.124; γ = 93° 29'.

The barium salt, BaU(PO₄)₂, forms thin green hexagonal plates.

The strontium salt, SrU(PO₄)₂, forms orthorhombic crystals,

a:b:c = 1.474:1:1.165.

By using the alkali pyrophosphates, the potassium and sodium salts of the type Ri₂U₃(PO₄)₈, or 6R₂O.3UO₂.4P₂O₅, are obtained.

The salt, KU₂(PO₄)₃ or K₂O.4UO₂.3P₂O₅, is obtained by heating the orthophosphate, K₂HPO₄, with a slight excess of potassium uranium chloride; it forms small dark green crystals. The corresponding sodium salt is prepared by fusion of sodium metaphosphate with excess of uranous oxide.