chemical thermodynamics of neptunium and plutonium - U.S. ...

292 14. Neptunium group 1 (alkali) compounds14.2.2 β-Na 2 NpO 4The value f Hm ◦ (Na 2NpO 4 ,β,298.15 K) = −(1748.5 ± 6.1) kJ·mol −1is accepted, based on a recalculation (cf. Appendix A) of the value reported by Goudiakas,Jemine and Fuger [91GOU/JEM] from measurements of the enthalpy of solutionof the solid in 1 M HCl. The difference in the enthalpies of solution of the α- andβforms,(15.3 ± 3.5) kJ·mol −1 , is similar to that for the corresponding uranium compounds(13.1 ± 0.8) kJ·mol −1 [92GRE/FUG].14.2.3 β-Na 4 NpO 5Fuger [85FUG] reported a preliminary value of f Hm ◦ (Na 4NpO 5 ,β,298.15 K) =−(2311.7±7.2) kJ·mol −1 based on two preparations with five determinations on each.He cautioned that the result was suspect because the enthalpy of solution results (dissolutionin 1 M HCl) from solids from the two preparations differed by 5 kJ·mol −1 .Thedetails were not published, and the value is not accepted in the present review. Nevertheless,the results are in good agreement with those from another set of experimentsfrom the same group f Hm ◦ (Na 4NpO 5 ,β,298.15 K) =−(2315.7 ± 5.3) kJ·mol −1[91GOU/JEM]). After adjustment of the auxiliary data to values consistent with thoseused in the present review (cf. Appendix A), the value f Hm ◦ (Na 4NpO 5 ,β,298.15 K) = −(2315.4 ± 5.7) kJ·mol −1is selected.14.2.4 Na 2 Np 2 O 7 (cr)Judge, Brown and Fuger, as cited by Fuger [85FUG], measured the enthalpy of solutionof Na 2 Np 2 O 7 (cr) in aqueous HCl (1 M). From this and auxiliary data discussed inAppendix A, the standard enthalpy of formation of Na 2 Np 2 O 7 (cr) is calculated to be f Hm ◦ (Na 2Np 2 O 7 , cr, 298.15 K) = −(2894 ± 11) kJ·mol −1This value is accepted in this review.No data on S ◦ m and fG ◦ m of Na 2Np 2 O 7 (cr) appear to be available.14.3 Neptunium potassium compounds14.3.1 K 2 NpO 4 (cr)The value f Hm ◦ (K 2NpO 4 , cr, 298.15 K) = −(1784.3 ± 6.4) kJ·mol −1is accepted, based on a recalculation (cf. Appendix A) of the value reported by Goudiakas,Jemine and Fuger [91GOU/JEM] from measurements of the enthalpy of solutionof the solid in 1 M HCl.