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

632 A. Discussion of selected referencesb) Np(V)In a Np(V) solubility study also reported in this publication, Moskvin proposedthe stoichiometry of Np(V) soluble carbonate complexes (NpO 2 OHCO 2−3andNpO 2 OH(CO 3 ) 4−2), and their stability constants. The author did not try any othermodel (i.e., set of soluble complexes and solid phases). There was no proof offeredfor his model, which is not consistent with later work. The experimental solubilitymeasurements in concentrated (NH 4 ) 2 CO 3 solutions, using either NpO 2 OH(s)or hydrated NH 4 NpO 2 CO 3 (s) solids as starting materials, are compatible withNp(V) behaviour in concentrated carbonate media found in previous [66GOR/ZEN]and later studies [77SIM, 84VIT, 86GRE/ROB, 91KIM/KLE, 94NEC/KIM]. Nocharacterisation of the solid phase (assumed not to change during the equilibrations)was reported. The initial solid phase had little influence on most of the solubilityresults, indicating that at least one of the initial solid phases was transformed duringthe solubility equilibration [98VIT/CAP]. No indication was given as to the methodof pH calibration (junction potential, ionic strength effects). It was assumed the freecarbonate concentration, [CO 2−3], was equal to the total carbonate concentration; thisis incorrect as the measured pH values were always less than the pK a value for thefirst protonation step for CO 2−3 .In carrying out the present review, attempts were made to reinterpret the datafrom this study. It has been assumed that, on addition of (NH 4 ) 2 CO 3 to the solution,NpO 2 OH(s) solid was transformed into a hydrated NH 4 NpO 2 CO 3 (s), which wasslowly transformed into a thermodynamically more stable (hence less soluble) hydrated(NH 4 ) 3 NpO 2 (CO 3 ) 2 (s) solid in the most concentrated (NH 4 ) 2 CO 3 aqueous solutions(2 and 2.2 M). The data for 2 and 2.2 M (NH 4 ) 2 CO 3 solutions were therefore rejected,and the NpO 2 OH(s) solubility data were also excluded because the resultswere scattered, suggesting equilibrium was not always achieved. The Np(V) complexationconstants and the specific interaction coefficients selected or estimated inthe present review were used, but the uncertainty has been increased for the valuesestimated by analogy: ε (NH+4 ,CO2− 3 ) ≈ ε (K + ,CO 2−3 ) = (0.02 ± 0.10) kg·mol −1 andε (NH+4 ,HCO − 3 ) ≈ ε (Na + ,HCO − 3 ) = (0.0 ± 0.2) kg·mol−1 . Values for ε (NpO2 (CO 3 ) 5−3 ,NH+),4and K s,3 (A.25, M + = NH + 4 ) = K s,3β 3 =[NH + 4 ][NpO 2(CO 3 ) 5−3]/ [CO2−3 ]2 were determinedby fitting two different models to the data. One model assumed the 1:2 complexwas important (and required a fixed value of log 10 K3 ◦ (A.26) = (−0.87) to achieveconvergence and yield a value for ε (NpO2 (CO 3 ) 3−2 ,NH+ 4)), the other that only the limitingcomplex was important. Both models resulted in a value for ε (NpO2 (CO 3 ) 5−3 ,NH+ 4 )near 0.6 a surprisingly large positive value that again leads to questions about whetherMoskvin’s solutions were at equilibrium.The values −(2.6±1.0), −(2.8±0.4), −(3.0±0.3), −(2.9±0.2) and −(2.9±0.3)were calculated in the present review for log 10 K s,3 for solutions 0.2, 0.6, 1.0, 1.25and 1.5 M in (NH 4 ) 2 CO 3 , respectively. The solid phase was not characterised by theauthor so this review has tabulated (Table 12.4) only the value calculated for 0.6 M(NH 4 ) 2 CO 3 (this is similar to the value for the corresponding sodium system as calculatedfrom the data selected in the present review). At higher ammonium carbonate