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

718 A. Discussion of selected referencesaccording to our calculations). It is certainly correct to interpret the potential difference,measured with Method A in the presence and absence of Np(V), in terms ofthe formation of NpO 2 F(aq). However, the reliability of the resulting constants cannotbe high. These values can only be given a small weight in the selection procedureof the present review. We use log 10 β 1 (A.62, I = 0.1 M) = (1.51 ± 0.50) andlog 10 β 1 (A.62, I = 1M) = (1.15 ± 0.80).[85SCH/FRI]This work reports the results of calorimetric titrations of uranium(VI) and neptunium(VI)in aqueous Na 2 SO 4 solutions with Na 2 CO 3 and (for uranium(VI)) NaHCO 3aqueous solutions at 25 ◦ C. The uranium experiments were done at I = 1.6 Musingsamples containing 50 µmoles of U(VI), and similar conditions were used for theneptunium experiments. The uranium part of this work has already been discussed inthe NEA uranium review [92GRE/FUG], and the same reinterpretation is used in thepresent review.The authors [85SCH/FRI] reported enthalpy changes forUO 2+2+ 2CO 2−3Å UO 2 (CO 3 ) 2−2and for M = U and NpMO 2+2+ 3CO 2−3Å MO 2 (CO 3 ) 4−3Grenthe et al. [92GRE/FUG] concluded that the technical quality of the enthalpy dataseem to be satisfactory, but they found that the two main reactions taking place underthe conditions of experiment areUO 2 SO 4 (aq) + 2CO 2−3Å 1 3 (UO 2) 3 (CO 3 ) 6−6+ SO 2−4UO 2 SO 4 (aq) + 3CO 2−3Å UO 2 (CO 3 ) 4−3+ SO 2−4.The authors gave a value of the enthalpy of the reaction r H m (I = 0.3to1.6MNa 2 SO 4 ) = −(50 ± 2) kJ·mol −1A re-evaluation of the three experimental determinations at high excess CO 2−3concentrationsyields a more realistic estimate of r H m =−(50.1 ± 12.2) kJ·mol −1 .Assuming that NpO 2 SO 4 (aq) is the only species initially present in the calorimeter, r H m (10.1) =−(16.7 ± 0.5) kJ·mol −1 can be used to estimate r H m for the reactionNpO 2+2+ 3CO 2−3Å NpO 2 (CO 3 ) 4−3(A.63) r H m (A.63) = −(33.4 ± 12.2) kJ·mol −1consistent with the subsequent determinations made by Ullman and Schreiner[88ULL/SCH] using the same calorimeter.