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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.

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