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

A. Discussion of selected references 749Table A.23: Formation constants of fluoride complexes of tetravalent actinides[90SAW/CHA2] after correction for the protonation constant of fluoride. The uncertaintiesare estimated in this review. I = 1M(Na,H)ClO 4 ,23 ◦ C.An 4+ + qHF(aq) Å AnF 4−qq + qH +Metal ion log ∗ 10 β 1 log ∗ 10 β 2 log ∗ 10 β 3 log ∗ 10 β 4(An 4+ )Th 4+ (4.66 ± 0.05) (7.52 ± 0.25) (8.80±1.00) (11.87 ± 0.55)U 4+ (5.53 ± 0.05) (8.76 ± 0.05) (10.66±0.15) (12.12 ± 0.30)Np 4+ (5.22 ± 0.20) (8.62 ± 0.65) (11.20±0.65) (14.15 ± 0.90)Pu 4+ (4.66 ± 0.60) (8.87 ± 0.50) (11.26±1.85) (14.27 ± 0.80)we correct the constants reported here [90SAW/CHA2] toI = 0, after conversion tomolal units and by using the same ion interaction coefficients as used by Grenthe et al.[92GRE/FUG]. A temperature correction is not considered necessary. The resultinglog 10 βq ◦ values are compared with those of the uranium book in Table A.24 below.Surprisingly, the first constant is inconsistently high, while the other constants are inagreement with those of other studies and the ones selected in the uranium review[92GRE/FUG]. Partial hydrolysis of U 4+ is negligible under the conditions of thepresent experiment.Table A.24: Comparison of the formation constants of the U(IV) fluoride complexesof [90SAW/CHA2] with those selected in the TDB uranium review [92GRE/FUG].Reference log 10 β1 ◦ log 10 β2 ◦ log 10 β3 ◦ log 10 β4◦[90SAW/CHA2] (10.21 ± 0.11) (16.58 ± 0.14) (21.6 ± 0.2) (26.3 ± 0.3)(see text)[92GRE/FUG] (9.28 ± 0.09) (16.23 ± 0.15) (21.6 ± 1.0) (25.6 ± 1.0)(critical review)In addition to values for U(IV), the log 10 β 1 values for the Np(IV) and Pu(IV)systems also differ significantly from the other values reported in earlier studies forsimilar conditions, see Sections 9.2.1.2 and 18.2.1.2. Sawant, Chaudhuri and Patil[90SAW/CHA, 90SAW/CHA2] did not discuss any conceivable reasons for these discrepancies.An open question is certainly the irregularity in the stepwise constants(K 4 > K 3 ). It is not clear to what extent an error in K 4 and K 3 mayhaveanimpactonthe values of K 2 and K 1 in the statistical treatment used by the authors. In the Np(IV)system, K 1 can be considered as an outlier, the same can be said about K 2 in the Pu(IV)system. Although there is no obvious reason to discard the results of this study, we suspectthat the computer evaluation of the experimental data sets was fragile. It was not