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

666 A. Discussion of selected referencesconstants or discuss possible correlation. In the present review, the reported value forthe LiClO 4 medium has been used with the specific ion interaction theory coefficientsε (Np 4+ , ClO − 4 ) = (0.84 ± 0.06) kg·mol−1 and ε (NpOH 3+ , ClO − 4 ) = (0.50 ± 0.05) kg·mol−1(estimated) to calculate log10 ∗β◦1 = (0.56 ± 0.5). Similarly, using ε (Np(OH) 2+2 , ClO− 4 ) =(0.4 ± 0.2) kg·mol −1 (estimated), log10 ∗β◦2=−0.1 can be calculated; however, thisvalue is rejected because of the lack of details concerning hydrolysis beyond the firstdeprotonation step.[77ERM/PER]In this study, measurements were made of the Np(VI)/Np(V) and Pu(VI)/Pu(V) couples in aqueous LiOH solutions (2.5 M to 4.0 M). The potentialswere measured relative to a mercuric oxide electrode. Using = 0.881[59ROB/STO], (a H2 O = 0.924) and the specific ion interaction theory coefficients,including the estimated values ε (NpO2 (OH) − 3 , Li+ ) = (0.05 ± 0.10) kg·mol −1 ,ε (NpO2 (OH) 2−4 , Li+ ) = −(0.05 ± 0.10) kg·mol−1 ,valuesofE ◦ can be calculated forthe reactions involving the most probable hydrolysis species. Assuming the Np(V)species to be NpO 2 (OH) − 2 , the potentials E◦ = 0.153 V, 0.987 V and 1.821 V forReactions (A.54), (A.55), (A.56), respectively, are calculated using the data for the2.5 M LiOH solutions.NpO 2 (OH) 2 (aq) + e − Å NpO 2 (OH) − 2(A.54)NpO 2 (OH) − 3 + e− + H + Å NpO 2 (OH) − 2 + H 2O(l) (A.55)NpO 2 (OH) 2−4+ e − + 2H + Å NpO 2 (OH) − 2 + 2H 2O(l) (A.56)From these and the selected formation constant for NpO 2 (OH) − 2 ,log 10 ∗β◦2 (8.2) =−(23.6 ± 0.5), and assuming only one non-polymeric Np(VI) hydrolysis speciesin the hydroxide solutions, the calculated possible hydrolysis constants for NpO 2+2are ∗ β2 ◦ = 10−6.6 , ∗ β3 ◦ = 10−(20.7±1.7) or ∗ β4 ◦ = 10−(34.8±1.7) . The value for ∗ β2◦is not consistent with the hydrolysis constants measured in acidic solutions, andit is concluded in this review that NpO 2 (OH) 2 (aq) is not the predominant Np(VI)hydrolysis species in highly alkaline solutions. The latter two constants are similar tothe values selected for the corresponding uranium species [92GRE/FUG].The measured potentials do not vary significantly with changing hydroxide concentration,but the hydroxide concentrations and ionic strengths are correlated. Therefore,it is not possible to distinguish between the hydrogen ion dependence of the potentialsand activity effects.[77SAI/UEN]Np(IV), (V) and (VI) were precipitated in 0.05 to 0.8 M Na 2 CO 3 aqueous solutions usingthe hexamminecobalt(III) cation. The composition of the solid phase (or phases ?)were deduced from chemical analysis. Absorption spectra were shown for aqueoussolutions containing neptunium in each valence state.