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

8.1 Aqueous neptunium hydroxide complexes 115As discussed in Appendix A and Section 8.2.5.2, after recalculationlog10 ∗Ks,0(8.13) = (1.5 3 ± 1.0) is selected.NpO 2 (hyd, am) + 2H 2 O(l) Å Np(OH) 4 (aq) (8.7)Np 4+ + 4H 2 O(l) Å Np(OH) 4 (aq) + 4H + (8.8)Using f G ◦ m (NpO 2, hyd, am, 298.15 K) = −(957.3±8.0) kJ·mol −1 and log 10 K(8.7) =−(8.3 ± 0.3), the value log 10 K(8.8) =−(9.8 ± 1.1) is calculated, and the value f G ◦ m (Np(OH) 4, aq, 298.15 K) = −(1384.2 ± 8.2) kJ·mol −1is selected. The reported uncertainties do not include those arising from the fact that thehydrated amorphous oxide is not thermodynamically stable, and may not be a uniquecompound reproducibly precipitated under different conditions. Colloids may formduring the solubility experiments, and the work of Silber et al. [94SIL/NIT] suggestsradiolytic effects may also be important in long term experiments involving the use ofNp(IV) solids.It is clear from the work of Rai and Ryan [85RAI/RYA], Ewart et al.[85EWA/GOR] and Pratopo, Moriyama and Higasi [89PRA/MOR] that Np(OH) − 5is not an important hydrolysis species for neptunium(IV). The thermodynamicparameters previously suggested for Np(OH) − 5in [82ALL] and[84LEM] are rejectedin the present review, and no new values are proposed for this species.8.1.5 Neptunium(III) hydroxide complexesNp(III) is stable in 1 M HClO 4 , but it is rapidly oxidised by air to the Np(IV) state. Hydrolysiswill shift the [Np(IV)]/[Np(III)] ratio towards the (IV) state, and so will solventextraction with an extractant like HTTA; under these conditions, Np(III) will be stableonly in the presence of strong reductants like hydroxylamine. However, studies of theformation of hydrophilic Np(III) complexes have been carried out in an oxygen-freeatmosphere without interference from hydrolysis, e.g. [49HIN/MAG, 49MAG/LAC].The literature appears to contain only one experimental study of the equilibriumNp 3+ + H 2 O Å NpOH 2+ + H + (8.9)Mefodeva et al. [74MEF/KRO] determined log10 ∗β1(8.9) =−(7.43 ± 0.11) for25 ◦ Cand0.3 MNaClO 4 from potentiometric measurements in the pH-range 6-8 in0.1 M NaClO 4 at 20 ◦ C and comparison with the hydrolysis behaviour of Pr 3+ andNd 3+ under the same conditions (as discussed in Appendix A). Table 8.4 compares theresult with various estimates for the first hydrolysis constant of Np(III).The results seem reliable, except that the uncertainty in log ∗ 10 β 1 (8.9, 298.15 K,0.3 MNaClO 4 ) is probably overly optimistic, as can be inferred from the uncertaintyin the reference Nd/Pr system [66FRO/KUM].This value was extrapolated to I = 0, using ε = (0.04 ± 0.09) kg·mol −1 fromthe corresponding Am system [95SIL/BID] (with uncertainties expanded by 0.05), resultingin log10 ∗β◦1=−(6.8 ± 0.3). The uncertainty is an estimate.