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

596 A. Discussion of selected referenceswith those of Newton and Baker [57NEW/BAK]. However, while Newton and Baker[57NEW/BAK] needed to assume the formation of PuO 2 Cl 2 (aq) to explain their data,Rabideau and Masters [61RAB/MAS] obtained equally good agreement by using eitheronly PuO 2 Cl + or both PuO 2 Cl + and PuO 2 Cl 2 (aq) in the evaluation of their measurements.This observation is compatible with that of Mazumdar and Sivaramakrishna[65MAZ/SIV] who observed complexes higher than PuO 2 Cl + only above 1.75 M Cl − .The same conclusion can be drawn from the series of visible absorption spectra measuredby Giffaut [94GIF]. With our selected formation constant, we can calculate thatthe proportion of 1:2 complex in Rabideau and Master’s experiments was between 2%and 12%. This is a rather small amount from which to determine a stability constant,and thus the constants reported here [57NEW/BAK, 61RAB/MAS] for the formationof PuO 2 Cl 2 (aq) probably have rather larger uncertainties.From the variation of the constants over the temperature range between 2.4 and29.6 ◦ C, Rabideau and Masters [61RAB/MAS] derived a reaction enthalpy value underthe assumption that only PuO 2 Cl + was formed. They obtained r H m = 14 kJ·mol −1for the formation reaction of PuO 2 Cl + . The IAEA review [92FUG/KHO]usedtheRabideauand Masters’ values from the 1:2 model to derive r H m = (8 ± 4) kJ·mol −1for the formation of PuO 2 Cl + and r H m = (17 ± 4) kJ·mol −1 for the formation ofPuO 2 Cl 2 (aq). In light of the above discussion (only a small proportion of 1:2 complexformed), the value of the enthalpy of reaction for the 1:1 complex should not bestrongly affected, regardless of whether the formation of the 1:2 complex is taken intoaccount. Therefore, the value r H m = 14 kJ·mol −1 seems more likely to be correct,but on the other hand the value r H m = 8kJ·mol −1 selected in the IAEA review isconsistent with the value for the corresponding U(VI) system [92GRE/FUG]. In viewof these uncertainties, we do not select enthalpy data from this study.[61SUL/HIN]The authors used a cell arrangement which eliminated liquid junction potentials todetermine the formal potential of the reactionNpO 2+2+ 1 2 H 2(g) Å NpO + 2 + H+ .They determined E ◦′ = (1.13638 ± 0.00016) Vin1MHClO 4 at 25 ◦ C. The authorsalso recalculated the value of Cohen and Hindman [52COH/HIN] which was derivedin 1 molal HClO 4 .Using1m ∧ = 0.9581 M HClO 4 , the formal potential was correctedfrom (1.1373 ± 0.0008) V to (1.1361 ± 0.0010) V. Cohen and Hindman, however,used 1.027 molal HClO 4 . Therefore, the corrected value determined by this review isE ◦′ = (1.1366 ± 0.0010) V.[62GEL/MOS]This paper reports four experiments on the solubility of ammonium diplutonate((NH 4 ) 2 PuO 2 O 7 ) in distilled water and in carbonate solutions (0.175 to 0.438 M).There are four difficulties in interpreting this data: 1) From three solubility