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

Chapter 8Neptunium oxygen andhydrogen compounds andcomplexes8.1 Aqueous neptunium hydroxide complexes8.1.1 Neptunium(VII) hydroxide complexesOnly limited information on these species appears to be available. There isevidence for a cationic species of Np(VII) in acidic solutions [69SUL/ZIE,72CHA/LEI, 91SHI/TAN], and an anionic species in strongly alkaline solutions[69COH/FRI, 70ZIE/COH, 75CHA/MAT], but no thermodynamic parameters havebeen estimated in this review (see also Section 6.1)8.1.2 Neptunium(VI) hydroxide complexesThe hydrolysis data of NpO 2+2reported in the literature are summarised in Table 8.1.Cassol et al.[72CAS/MAG2] obtained hydrolysis constants for NpO 2+2in 1 M aqueousNaClO 4 at 25 ◦ C. The value for log10 ∗β◦1based on this work (−(5.1 ± 0.4)) isinfairagreement with the value (−5.5) reported by Schmidt et al. [83SCH/GOR]. The selfconsistentset of values of Cassol et al. [72CAS/MAG2] is accepted in the presentreview for reasons discussed in Appendix A. Hence, using the estimated values for theinteraction coefficients,ε (NpO2 OH + ,ClO − 4 ) = −(0.06 ± 0.40) kg·mol −1ε ((NpO2 ) 2 (OH) 2+2 ,ClO− 4 ) = −(0.57 ± 0.10) kg·mol −1ε ((NpO2 ) 3 (OH) + 5 ,ClO− 4 ) = −(0.45 ± 0.20) kg·mol −1the following formation constants are selectedFrom these,log ∗ 10 β1 ◦ = −(5.1 ± 0.4)log ∗ 10 β2,2 ◦ = −(6.27 ± 0.21)log ∗ 10 β5,3 ◦ = −(17.12 ± 0.22)105