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

9.2 Aqueous neptunium group 17 (halogen) complexes 157[69KRY/KOM3] are corrected for the formation of NpNO 3+3(see Appendix A) andused in the evaluation procedure of log ∗ 10 β1 ◦ . The values reported for log 10 ∗ β 1 at20 ◦ C[66AHR/BRA] and 23 ◦ C[90SAW/CHA2] are corrected to 25 ◦ Cbyusingtheenthalpy value selected below.The values for log ∗ 10 β 1 are in good agreement with the exception of the mostrecent determination of this constant by Sawant, Chaudhuri and Patil [90SAW/CHA2],who used a fluoride ion selective electrode. The constants reported in that study forU(IV) and Pu(IV) are also inconsistent with those of other determinations. Furtherirregularities, cf. Appendix A, have led us to exclude the constants reported in[90SAW/CHA2] from the selection procedure. Having done this, a simultaneousdetermination of log ∗ 10 β1 ◦ and ε by weighted linear regression appears feasible, cf.Figure 9.1, wherethe25 ◦ C data from [66AHR/BRA, 69KRY/KOM, 69KRY/KOM3,75PAT/RAM, 76BAG/RAM, 76CHO/UNR] are used. The resulting value islog ∗ 10 β1 ◦ (9.4, q = 1, 298.15 K) = (5.78 ± 0.14). The slope of the straight line inFigure 9.1 corresponds to ε(9.4, q = 1) = −(0.12 ± 0.04)kg·mol −1 , whichcompares well with ε =−0.14kg·mol −1 estimated for the corresponding U(IV)-HFreaction [92GRE/FUG]. We convert this constant to conform to Equation (9.5) usingour selected protonation constant of fluoride [92GRE/FUG]. The resulting selectedvalue islog 10 β1 ◦ (9.5, q = 1, 298.15 K) = 8.96 ± 0.14.It should be noted that Krupka et al. [85KRU/RAI] observed no measurable increaseof Np(IV) solubility in the presence of up to 100 ppm fluoride. However, thisobservation is not necessarily in contradiction with our selected value, cf. Appendix A.For the formation of the 1:2 complex, NpF 2+2, Ahrland and Brandt [66AHR/BRA]published two values measured independently with a cation exchange method andwith a redox electrode, respectively. The values are in very good agreement, as isthe value published by Patil and Ramakrishna [75PAT/RAM] who used a solventextraction technique. Again, the value published by Sawant, Chaudhuri and Patil[90SAW/CHA2] differs considerably from the other values and is omitted in the selectionprocedure. The values of log ∗ 10 β2 ◦ listed in Table 9.3 are obtained by extrapolatingthe reported constants to I = 0byusingε(9.4, q = 2) =−(0.18 ± 0.15)kg·mol −1from the corresponding U(IV) fluoride system [92GRE/FUG], where the uncertaintyhas been increased by 0.05. The weighted average of the three valuesis log ∗ 10 β2 ◦ (9.4, q = 2) = (9.34 ± 0.29). We convert this value to conform toEquation (9.5) using our selected protonation constant of fluoride [92GRE/FUG]. Theresulting selected value islog 10 β2 ◦ (9.5, q = 2, 298.15 K) = 15.7 ± 0.3Data for the 1:3 and 1:4 complexes were published by Ahrland and Brandt[66AHR/BRA], and by Sawant, Chaudhuri and Patil [90SAW/CHA2]. The valuesdiffer considerably, even after correction to I = 0 using the ε values fromthe corresponding U(IV) fluoride system [92GRE/FUG], cf. Table 9.3. If weuse ε (NpF+3 ,ClO − 4 ) ≈ ε (M + ,ClO − 4 ) ≈ 0.3kg·mol −1 (cf. Appendix B) instead of