The gravimetric determination of uranium in uranyl nitrate

A study of the factors which affect the gravimetric determination of uranium in uranyl nitrate is described. In the gravimetry of uranium, the U₃O₈ (weighing form) produced by ignition is usually assumed to deviate <0.02% from theoretical composition; and elemental impurities are assumed to form common oxides within the U₃0₈ matrix. It is shown that these assumptions are incorrect. Ignition temperature and time affect U₃O₈ stoichiometry. Ignitions of uranyl nitrate for 1–3 h at 850° produce U₃O₈ that deviates as much as 0.15% from stoichiometric U₃O₈; deviations are negligible when uranyl nitrate is ignited at 1000° for 2 h. Elemental impurities, particularly calcium and phosphorus, affect the composition of U₃O₈ formed in the ignition of uranyl nitrate. A variety of impurity complexes such as uranates and phosphates are found within the U₃O₈ matrix. Formation of these impurity complexes depends on the elements present, their concentration, and ignition temperature. Therefore, in the gravimetric determination of uranium in uranyl nitrate, the effects of ignition parameters and nonvolatile impurities must be considered in order to obtain accurate uranium determinations.