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

218 12. Neptunium group 14 compounds and complexespolynuclear complexes could not be readily determined. In the present review some ofMaya’s reported formation constants (including some calculated for species that wereeventually rejected), and some values recalculated in this review from Maya’s data, areproposed as maximum possible values (Table 12.2).Table 12.2: Maximum possible values of formation constants. The formation of theNp(VI) species on the right-hand side of these equilibria has not been demonstratedunequivocally, cf. discussion of [71MOS2] and[93PRA/MOR, 95MOR/PRA] inAppendixA.Method Ionic Medium t ( ◦ C) log 10 K Reference3NpO 2 (CO 3 ) 4−3 + 7H 2 O(l) Å 7H + + 9CO 2−3 + (NpO 2 ) 3 (OH) − 7pot 1 M (NaClO 4 ,NaHCO 3 ) 25 (62.35±1.50) [84MAY] (a)3NpO 2 (CO 3 ) 4−3≤ 63.33+ 8H 2 O(l) Å 8H + + 9CO 2−3+ (NpO 2 ) 3 (OH) 2−8pot 1 M (NaClO 4 ,NaHCO 3 ) 25 ≤ 55.45 [84MAY] (a)(b)3NpO 2 (CO 3 ) 4−3 + 3H 2 O(l) Å 3H + + 8CO 2−3 + (NpO 2 ) 3 CO 3 (OH) + 3pot 1 M (NaClO 4 ,NaHCO 3 ) 25 ≤ 75.42 [84MAY] (a)(a) Maximum possible value estimated in the present review from the data of [84MAY].(b) The corresponding species was recently proposed for uranium [95PAL/NGU].The only value accepted here (Table 12.1) islog 10 K (12.7, 1MNaClO 4 ) =(41.75 ± 1.46) for2NpO 2 (CO 3 ) 4−3+ 7H + Å(NpO 2 ) 2 CO 3 (OH) − 3+ 5CO 2 (g) + 2H 2 O(l) (12.7)Using data selected in this review, this corresponds to log 10 K (12.8, 1MNaClO 4 ) =−(21.38 ± 1.62) and log 10 K (12.9, 1MNaClO 4 ) =−(4.16 ± 1.60)2NpO 2+2+ CO 2 (g) + 4H 2 O(l) Å (NpO 2 ) 2 CO 3 (OH) − 3 + 5H+ (12.8)2NpO 2+2+ CO 2−3+ 3H 2 O(l) Å (NpO 2 ) 2 CO 3 (OH) − 3 + 3H+ (12.9)As for the corresponding uranium species, the value ε ((NpO2 ) 2 CO 3 (OH) − 3 ,Na+ ) =(0.00 ± 0.05) kg·mol −1 is assumed and hence,log 10 K ◦ (12.7, 298.15 K) = (49.17 ± 1.59) r G ◦ m (12.7, 298.15 K) = −(280.64 ± 9.05) kJ·mol−1 f G ◦ m ((NpO 2) 2 CO 3 (OH) − 3, 298.15 K) = −(2814.92 ± 14.67) kJ·mol−1