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

12.1 Neptunium carbon compounds and complexes 223are then deduced for the equilibriaNaNpO 2 CO 3·xH 2 O(s) Å Na + + NpO + 2 + CO2− 3+ xH 2 O (12.11)NpO + 2 + iCO2− 3Å NpO 2 (CO 3 )i 1−2i(12.12)As discussed below and in Appendix A (see the discussions of [84VIT,90RIG, 93LEM/BOY]) well-characterised hydrated solid phases are difficultto obtain. These solids may evolve during the course of the equilibrations,and, at least for conditions under which NpO 2 (CO 3 ) 5−3is formed, the hydratedMNpO 2 CO 3 (s) solids are metastable with respect to others with stoichiometriessuch as M 3 NpO 2 (CO 3 ) 2 (s) [95NEC/RUN]. When enough experimentalinformation is available [84VIT, 93LEM/BOY, 98VIT/CAP] these changescan be recognized, and are consistent with published X-ray diffraction patterns[94MEI, 95NEC/RUN, 96RUN/NEU, 98VIT/CAP]; these transformations couldcause problems even within the same set of measurements by the same author. Toavoid this problem, this review used another set of equilibria to interpret solubilitymeasurements. One equilibrium must correspond to the solubility product (Eq. 12.10,i = 0), and its determination is not independent of these kinetic problems, but theother equilibriaNpO 2 (CO 3 ) 3−2ii−1+ CO2− 3Å NpO 2 (CO 3 ) 1−2ii(12.13)corresponding to the stepwise complexation constants, K i = β i /β i−1 = K s,i (12.10)/K s,i−1 (12.10), are less dependent on the solid phase problem. These stepwise constantswere also directly measured when using absorption spectrophotometry or liquid-liquidextraction techniques.Several papers have recently appeared [94NEC/RUN, 95FAN/NEC, 95NEC/FAN,95NOV/ROB, 95NEC/RUN, 96RUN/NEU] proposing and comparing extrapolationsto zero ionic strength using the SIT (Appendix B) and the equations of Pitzer [73PIT].As discussed below and in Appendix A references to these papers, the complexationconstant and solubility product values as extrapolated to zero ionic strength are in reasonableagreement with those based on other publications. However, some of theseresults lead to inconsistencies in the derived SIT parameters (cf. Section 12.1.2.1.3.b).The differences may indicate experimental problems in some of these studies, or someof the earlier studies, or theoretical problems related to neglect of anion-anion interactions,or might result in part from some of the fitted parameters being highly correlated.As discussed below small systematic deviations could not be ruled out for all theexperimental measurements used in the present review to select thermodynamic datafor the Np(V) carbonate system. To minimize the effect of these systematic deviations,this review selected procedures that enhanced cancellation of errors, reduced the errorsas much as possible, or corrected them when feasible; similar weight was given to dataoriginated from each laboratory because duplicating experiments decreases the randomuncertainties, but not the systematic errors. A better treatment of the random uncertaintiesis generally obtained by equally weighting the values from each laboratory to selecta given constant.