Thermodynamics of the actinoid elements. Part 6. The preparation and heats of formation of some sodium uranates(VI)

Abstract

The preparation of sodium uranates(VI) has been investigated by heating together appropriate amounts of Na₂O₂ and UO₂ and also by the thermal decomposition of complexes such as Na₄U(C₂O₄)₄·3H₂O, Na₄UO₂(CO₃)₃·nH₂O, Na₂UO₂(C₂O₄)₂·4H₂O, and NaUO₂(CH₃COO)₃. The enthalpies of solution in 1.00 mol dm^–3 HCl of high-purity samples of β-Na₄UO₅, α- and β-Na₂UO₄, and Na₂U₂O₇, prepared by the latter method, are –386.6 ± 0.6, –171.9 ± 0.4, –184.2 ± 0.4 and –171.8 ± 1.0 kJ mol^–1, respectively. The resulting enthalpies of formation, –2 457.3 ± 2.8, –1 901.3 ± 2.3, –1 889.0 ± 2.3, and –3 203.8 ± 2.8 kJ mol^–1, are discussed in relationship to published values. X-Ray powder diffraction studies appear to confirm the existence of α-Na₄UO₅.