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

A. Discussion of selected references 729The authors reported r H m =−(41.9 ± 1.3) kJ·mol −1 for the reaction. Assumingthe difference results from the original authors using unrounded primary results thatwere unavailable to the present reviewers, we accept r H m (0.098 to 0.135 M Na 2 SO 4 + 0.123 to 0.363 M Na 2 CO 3 )= −(41.9 ± 3.2) kJ·mol −1The values for the measured molar enthalpies of reaction remain constant (within±2.3 kJ·mol −1 ), after the formation of the limiting complex. Since the contributionsof Na 2 SO 4 dilution and of added Na 2 CO 3 to the measured heat were subtracted usinga blank, and since the number of moles of Np(VI) is constant in each series of titrations,then in each series of titrations when an excess of Na 2 CO 3 was used the onlycontributions to this (undetected) variation of r H m are due to changes in the activitycoefficients [93GIF/VIT, 94GIF/VIT, 98VIT/CAP]. The experiments confirm thisresult for the limiting complexes.For the heats measured before the end of titration (before the formation of thelimiting complex), a linear variation with the amount of carbonate added was notfound. This is evidence that intermediary species were formed. Under these conditionsone expects some formation of the trinuclear carbonate complex. However,the authors [88ULL/SCH] did not interpret their data using a model incorporating(NpO 2 ) 3 (CO 3 ) 6−6. Using the limited data available, it is not possible to choose betweenthe different possible models. However, the value for H 2 reported for the neptuniumsystem is quite different from the values reported for the corresponding uranium andplutonium systems as measured in the same work [88ULL/SCH]. This could be dueto experimental kinetic problems as noted by Riglet [90RIG] for similar conditions.These problems apparently occur only for Np(VI), but not for U(VI) or Pu(VI). Therefore,this review does not use the H 2 value reported by Ullman and Schreiner forthe Np(VI) system. For the Pu(VI) system, the results were reported as the extrapolatedinfinite dilution values: log 10 β3◦ = (18.5 ± 1.2) (where the uncertainty isassigned in the present review), r G ◦ m,3 =−(105.6 ± 6.8) kJ·mol−1 ,and r Hm,3 ◦ =−(38.65 ± 1.00) kJ·mol −1 . For the PuO 2 (CO 3 ) 2−2complex: log 10 β2 ◦ = 15.1 ± 2.2, r G ◦ m,2 =−(86.2 ± 12.6) kJ·mol−1 ,and r Hm,2 ◦ =−(27 ± 4) kJ·mol−1 . The uncertaintiesin log 10 β2 ◦ and rHm,2 ◦ were expanded above those reported to account forthe non-standard calculation procedure used in the original paper.[89BUD/FED]The structures of neptunium(V) sulphate hydrate (NpO 2 ) 2 SO 4·xH 2 O(s), where x =6,4.5, and 2, were characterised by IR and X-ray diffraction. Instructions are given forthe preparation of these salts. No thermodynamic data are presented for evaluation.[89FEL/RAI]This paper reports the results of a study of the solubility of Pu(OH) 3 (s) ( 239 Pu) in waterand two high ionic strength brines. The measurements were done at (23 ± 2) ◦ C,