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

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776 A. Discussion <strong>of</strong> selected references(for ε (Na + ,ClO − )), equilibrium constant (for ε 4 (Na + ,CO 2− )) <strong>and</strong> redox (for ε (NpO + 3 2 ,ClO−))4measurements. For this last value an analogy between NpO 2+2<strong>and</strong> UO 2+2was also assumedsince the experimental determinations <strong>of</strong> the values <strong>of</strong> the MO 2+2 /MO+ 2 redoxcouples were found to be the same for M = U, Np, Pu [87RIG/VIT, 89RIG/ROB,90RIG, 94CAP/VIT]. The ε value <strong>of</strong> the two solubility product reactions as determinedby Kim et al. <strong>and</strong> Neck et al. [91KIM/KLE, 95NEC/RUN] are not consistent withthe value calculated in the present review (Table A.29). The two ε (NpO+2 ,ClO− 4 ) valuescalculated from those ε values are also not consistent with later determinations bythe same group [95NEC/FAN]. A simplified Pitzer equation was used from which thisreview determined ε (NpO+2 ,ClO− 4 ) = 0.20 ± 0.03 kg·mol−1 , a value which is in accordwith the value 0.25 ± 0.05 accepted in the present review (Table A.29).Table A.29: Values <strong>of</strong> ε (NpO+2 ,ClO− 4 )recalculated in this review from the measurements<strong>of</strong> [91KIM/KLE, 94NEC/KIM, 95NEC/FAN, 95NEC/RUN] (a) <strong>and</strong> auxiliary valuesfrom Appendix B. Similar values using auxiliary values calculated in this review fromexperimental data published in these references are reported in Table 12.5.ε (h) ε (e) ε (f) log 10 Ks,1◦this review (b) (0.25±0.05)[91KIM/KLE] (0.36±0.04)[95NEC/RUN] (0.34±0.04) (0.37±0.09) (0.42±0.09)0.1to3M (c) (0.38±0.07) −(10.95 ± 0.11)0.1to5M (c) (0.38±0.04) −(10.95 ± 0.07)[95NEC/FAN] (d) (0.20±0.05)[94NEC/KIM] (g) (0.18±0.03)(a) Unless noted otherwise, ε is ε(NpO +,ClO − 2 4 ) from ε s,i = iε (Na + ,CO2−) + ε (NpO + 3 2 ,ClO− 4 ) + (2i −1)ε (Na + ,ClO−4) + x ir as determined by SIT linear regression from the values determined at different ionicstrength for K s,i ,theNa (2i−1) NpO 2 (CO 3 ) i·x i H 2 O(s) solubility product, where r is a H2 O/m NaClO4 meanvalue, x 1 = 3.5 <strong>and</strong> x 2 is unknown; i=1 in column 2; i=2 in columns 3 <strong>and</strong> 4.(b) ε values calculated with the auxiliary values used by this review.(c) Calculated in the present review from the data reported in [91KIM/KLE].(d) Calculated in the present review from the Pitzer set <strong>of</strong> parameters proposed in [95NEC/FAN]. The uncertaintyhas been estimated in this review as the difference (ε(3 M) - ε(0.5 M)) where the ε values were calculatedfrom the mean activity coefficients.(e) Calculated from K s,2 using the arbitrary x 2 value x 2 = 0 in the solid phase.(f) Calculated from K s,2 using the arbitrary x 2 value x 2 = 3.5.(g) From liquid-liquid extraction data also published in [95NEC/FAN](h) Calculated using K s,1 (unless otherwise noted)Thus, the ionic strength corrections from this <strong>and</strong> related work [95NEC/RUN,96RUN/NEU] are not self consistent, <strong>and</strong> the ε values from the solubility productsare not consistent with currently accepted ε values. These two inconsistencies could

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