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

A. Discussion of selected references 775This value is used in the present review and, as in the original paper, the differencein the enthalpy effect corresponding to the transfer of the NpO + 2ion from 1.0 M HClto infinite dilution is assumed to be negligible.[94NEC/KIM]This review used the Np(V) solubility measurements tabulated in this report, withother data published by the same group of authors (as discussed in this appendix[91KIM/KLE, 94NEC/RUN]). Experimental data of Np(V) liquid-liquid extraction byNaDNNS from 0.2 to 5 M Na(ClO 4 ,Cl) aqueous solutions were also tabulated in thisreport. These were used by Neck, Kim and Kanellakopulos to estimate Np(V) activitycoefficients, however there are difficulties with the interpretation of the authors as explainedin the discussion of [94NEC/RUN] (work based on the same experimental datafrom the same group). In the present review we have interpreted interactions betweenNpO + 2 and Cl− solely in terms of the selected interaction coefficients rather than invokea model involving the formation of weak chloro-complexes (also see [96RUN/NEU]).The interpretation of the liquid-liquid extraction study neglected activity coefficientcorrections for the organic phase. These corrections might be important compared toactivity coefficient corrections. The corresponding uncertainty could not be deducedfrom statistical analysis since some of the possible errors were systematic (not random).The necessary activity coefficient corrections (especially for chloride media,cf. Section 12.1.2.1.3.c) increased the uncertainty in the interpretation of the data.Therefore these extraction measurements were not used in the present review to estimatethe stability constants of Np(V) carbonate complexes (see also the discussions of[91KIM/KLE, 94NEC/RUN] in this appendix).[94NEC/RUN]This paper incorporates and discusses the solubility and spectroscopic results describedin an earlier report [91KIM/KLE]. New activity coefficient values are also proposed.In this paper the SIT, as used in the present review, was used by Neck et al., while ina later paper [95FAN/NEC] the authors used a simplified Pitzer equation. The junctionpotential was measured (and not set to zero by using the same ionic medium forthe reference electrode and working solutions). The interpretation of the liquid-liquidextraction study [94NEC/KIM, 95NEC/FAN] neglected activity coefficient correctionsfor the organic phase. These corrections might be important compared to activity coefficientcorrections. The corresponding uncertainty could not be deduced from statisticalanalysis since some of the possible errors were systematic (not random). In addition,possible chemical systematic error (discussed above) could be even more important.For these reasons, the activity coefficients proposed in this work [94NEC/RUN] arenot accepted in the present review. However, as mentioned elsewhere in this appendix(see discussions of [91KIM/KLE, 94NEC/KIM]), some of the systematic errors, ifthey exist, will cancel, especially for the values of equilibrium constants extrapolatedto zero ionic strength. The ε value of the solubility products can be calculated fromε values used by this review (see Appendix B) that were determined from isopiestic