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

788 A. Discussion of selected referencesculated assuming equilibrium with the air (again no pH measurement was provided),from which log 10 β 5(IV) (1.0MNa 2 CO 3 , A.78) = (38.98 ± 1.97) is calculated.c) Temperature influence on the formal potential of the Np(V)/Np(IV) redoxcouple in carbonate mediaThe main source of uncertainty (lack of pH measurement) cancels in the studyof the temperature dependence of the potential measured for a single solution.The potentials measured in each solution varied linearly as a function of temperature,and these changes were reproducible within less than 10 mV. Fromlinear regression, the authors calculated temperature coefficient values of -2.058,-2.168 and -2.241 mV·K −1 for the 0.6, 1.0 and 1.5 M Na 2 CO 3 aqueous solutions,respectively. If the heat capacity of reaction is assumed to be negligible, thesetemperature coefficients can be used to calculate r H(A.77) = -82.40, -85.86 and-89.91 kJ·mol −1 in 0.6, 1.0 and 1.5 M Na 2 CO 3 aqueous solutions, respectively.From linear regression of ( r H(A.77) − 11z 2 T 2 (∂ε/∂T)Rln10)vs.m Na +, thereviewer calculated r H(A.77) =−(84.39 ± 4.38) kJ·mol −1 and T ◦ (∂ε/∂T)(A.77)=−(0.50 ± 0.34)kg·mol −1 (values of (∂D/∂T) were calculated from the Debye HückelA and B values given in Appendix B assuming no variation of a j with T, and that Ba j=1.5at25 ◦ C). This value of r H(A.77) is virtually the same as the mean of the threevalues for the different media, -(85.06 ± 4.63) kJ·mol −1 .d) Dissociation of the Np(IV) limiting complex in bicarbonate mediaTo study the dissociation of the Np(IV) carbonate limiting complex, Np(IV) was preparedas above by electrolysing 0.2, 0.3, 0.4, 0.5, 0.6 and 1.5 M Na 2 CO 3 aqueous solutionsof Np(V). Carbon dioxide gas, preequilibrated with NaCl solutions of the sameionic strength, was then bubbled through these carbonate solutions. During these titrations,pH was measured and spectra were recorded from 600 to 1100 nm. No spectralchange was observed in the 1.5 M Na 2 CO 3 solution from pH 12.64 to 10.34, althoughprecipitation (attributed to NaHCO 3 (s) formation) was observed. The authors interpretedtheir experimental observations, including molar absorbances at 823, 990 and1013 nm, in terms of dissociation of the Np(IV) limiting carbonate complex accordingto the following equilibriumNp(CO 3 ) 6−5Å Np(CO 3 ) 4−4+ CO 2−3(A.79)involving the loss of only one CO 2−3. Ionic strength varied during each titration as aresult of the reactionH 2 O + CO 2−3+ CO 2 (g) Å 2HCO − 3In their data analysis, Delmau, Vitorge and Capdevila used the data atthe half dissociation point for the mean at the three wavelengths. Usingthe data given page 57 of the report [96DEL/VIT], log 10 K ◦ 5 (A.79) =−(1.071 ± 0.136) and ε = (0.377 ± 0.184)kg·mol −1 are calculated. From