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

714 A. Discussion of selected referencesfor Np(IV) complexation at neutral to alkaline pH. However, one must not be misledto conclude that Np(IV) fluoride complexes are non-existent.[85LIE/TRE]Lierse, Treiber and Kim measured the solubility of NpO 2 OH(s) as a function of pH(6.8 < pH < 13) in 1 M NaClO 4 . There is no indication as to how the glass electrodewas calibrated, although corrections are provided in a later publication from the samegroup [92NEC/KIM], but also see the discussion of [91KIM/KLE] in this appendix.The measurements in neutral to alkaline solutions indicate that, for solutions havingpH values greater than ∼11.5, Np(V) exists predominantly in an anionic form. Thereis no indication that the solid in equilibrium with the solution was characterised afterequilibration with the solution, nor that the same solid was at equilibrium with thesolutions having different values of pH (again additional information was provided byNeck, Kim and Kanellakopulos [92NEC/KIM]). Nevertheless, the good linearity ofthe logarithm (base 10) of the equilibrium neptunium concentration as a function ofpH between pH values of 6.8 and 10.8 (with a slope essentially equal to -1.0) suggeststhese measurements are of good quality.Values of log 10 β 1 = (2.33 ± 0.62), log 10 β 2 =−(4.89 ± 0.05) and log 10 K s,1 =−(8.81 ± 0.05) were reported (for a medium of 1.05 m NaClO 4 ). Revised values(log 10 β 1 = (2.11±0.62),log 10 β 2 =−(4.45±0.18) and log 10 K s,1 =−(8.59±0.07))were later published [92NEC/KIM] and are used here. The ion product for water inthe medium is calculated as 10 −13.80 using the specific ion interaction theory. Thevalues log10 ∗β◦∗1=−(11.81 ± 0.62), log10 β◦ 2(5.33 ± 0.07) can then be calculated using ε (NpO+2 , ClO − 4 ) = (0.25 ± 0.05) kg·mol−1 ,=−(23.54 ± 0.18) and log∗10 K ◦ s,1 =ε (NpO2 (OH) − 2 , Na+ ) =−(0.01 ± 0.07) kg·mol−1 (estimated).The increase in Np(V) solubility at pH > 11 is not consistent with the work ofEwart et al. [86EWA/HOW] (Figure 8.3). This solubility increase was attributed toNpO 2 (OH) − 2formation, but this is only one possible interpretation. The authors usedfilters with different pore sizes (220 down to 1 nm). Only use of the filter with thesmallest pore size resulted in slightly lower solubilities at 11.5 < pH < 12.5, but notat 12.5 < pH < 13. As pointed out by the authors, the interpretation of this smalldifference is not clear.There is no way to verify whether the [CO 2−3 ] was low enough (log 10 [CO2− 3 ]