Complex Uranyl Phosphates

The following naturally occurring phosphates have been described:

Autunite, Ca(UO₂)₂(PO₄)₂.8H₂O;
Uranocircite, Ba(UO₂)₂(PO₄)₂.8H₂O; and
Chalcolite, Cu(UO₂)₂(PO₄)₂.8H₂O.

The last named may be prepared synthetically 1 by boiling a solution of copper acetate with uranyl phosphate and treating the precipitate with acetic acid. Corresponding salts of potassium and ammonium have been prepared. The potassium salt, KUO₂PO₄, may be formed by fusing together uranyl hydrogen phosphate, UO₂HPO₄, with potassium sulphate; or by dissolving uranic anhydride in molten potassium pyrophosphate. It is obtained as the trihydrate, KUO₂PO₄.3H₂O, in fluorescent crystals, by the addition of potassium dihydrogen phosphate to an acid solution of uranyl nitrate; the reactants should be in the proportion KH₂PO₄: HNO₃: UO₃. The crystals are insoluble in water or acetic acid, but dissolve readily in solutions of alkali carbonates. The corresponding ammonium salt, NH₄UO₂PO₄.3H₂O, is obtained in a similar manner and has similar properties. By fusing together tripotassium phosphate and uranic anhydride the compound K₄UO₂(PO₄)₂ is obtained.

When uranyl pyrophosphate is dissolved in excess of sodium pyrophosphate solution, the freezing-point rises to a maximum corresponding with 3Na₄P₂O₇.(UO₂)₂P₂O₇, and then descends to a minimum at 2Na₄P₂O₇.(UO₂)₂P₂O₇. The solution up to this point exhibits none of the characteristic reactions of uranyl salts, so that complex formation has evidently taken place. If the solution is evaporated to dryness, and the resulting gummy residue treated with alcohol, a very soluble hygroscopic powder is obtained, to which Pascal gives the formula Na₈[(UO₂)₂(P₂O₇)₃].6H₂O. If alcohol is added to a solution of sodium pyrophosphate saturated with uranyl pyrophosphate a yellow insoluble powder, of composition Na₂(UO₂)P₂O₇.H₂O, is formed. The stability of these complexes decreases with rise in temperature, and they cannot therefore be obtained from boiling solutions. The anhydrous salt, Na₂(UO₂)P₂O₇, may be obtained by fusing together sodium meta- phosphate and uranic anhydride. It yields glistening yellow monoclinic prisms.