Uranyl Phosphates

Well-defined ortho- and pyro-phosphates exist, but little is known of uranyl metaphosphate. Several complex uranyl phosphates have been described.

The normal salt, (UO₂)₃(PO₄)₂, is formed, according to Werther, as a yellow precipitate when a solution of the normal sodium phosphate is poured into a neutral solution of uranyl nitrate. Uranyl monohydrogen phosphate, UO₂HPO₄, is obtained as a yellow amorphous precipitate by adding phosphoric acid, or a soluble phosphate, to a solution of uranyl acetate or nitrate; or by treating uranyl hydroxide with phosphoric acid. If the precipitate is boiled for some considerable time with dilute hydrochloric acid, it becomes converted into tetragonal crystals of composition UO₂HPO₄.4H₂O. A trihydrate, UO₂HPO₄.3H₂O, is also obtained 3 by the addition of sodium dihydrogen phosphate on an acid solution of uranyl nitrate; it separates as a sulphur-yellow crystalline powder. The ammonium salt, UO₂(NH₄)PO₄.xH₂O, is formed when a soluble phosphate is added to a solution containing an ammonium and a uranyl salt. The greenish-yellow precipitate is quite insoluble in presence of acetic acid or ammonium acetate, so that the precipitation is quantitative, and is therefore employed in the volumetric estimation of phosphates, with potassium ferrocyanide or cochineal as indicator. Upon ignition the precipitate is converted to uranyl pyrophosphate, (UO₂)₂P₂O₇.

By cooling a hot concentrated solution of uranyl monohydrogen phosphate in nitric acid, a phosphonitrate of composition UO₂HPO₄.HNO₃.7H₂O, or 2UO₃.P₂O₅.N₂O₅.16H₂O, separates. This is decomposed by water with separation of nitric acid.

Uranyl dihydrogen phosphate, UO₂(H₂PO₄)₂.3H₂O, is obtained by concentrating a solution of uranic oxide in hot dilute phosphoric acid; it separates in small yellow crystals.

Uranyl metaphosphate, UO₂(PO₃)₂, is obtained, according to Rammelsberg, by heating the yellowish-green precipitate produced by the action of nitric acid on uranyl hypophosphite.

Uranyl pyrophosphate, (UO₂)₂P₂O₇, is formed as a precipitate when a solution of sodium metaphosphate is poured into a hot solution of uranyl nitrate. The tetrahydrate, (UO₂)₂P₂O₇.4H₂O, is precipitated in yellow microcrystals by double decomposition of sodium pyrophosphate and uranyl nitrate. The hydrate loses water and yields the anhydrous salt at 120° C. Uranyl pyrophosphate is insoluble in water, alcohol, or ether; but it dissolves in excess of sodium pyrophosphate solution, and also in dilute nitric acid.