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

742 A. Discussion of selected referencesof NpO + 2 and NpO 2(CO 3 ) 3−2, and has an intermediate molar absorptivity, so qualitativeinterpretation (prior to curve fitting or to help in interpretation of the results) is notpossible. Riglet noted that these problems did not cause difficulties in the study of theinfluence of ionic strength on the shift of the curves of absorbance vs.log 10 [CO 2−3 ] (todetermine values of ε 2 and ε 3 ), and her reported values are indeed in fair agreementwith the values determined in the present review from other published data (a graphicalrecalculation led to ε 3 =−(0.08 ± 0.05) kg·mol −1 ). However, these data are notused in the selection of thermodynamic parameters in the present review. An attemptby Riglet to fit all her spectrophotometric and solubility results within the uncertaintiesof the measurements was unsuccessful, and she concluded this inconsistency was dueto trying to simultaneously determine the unknown molar absorptivity coefficients ofNpO 2 (CO 3 ) 3−2from the fit of the data.b) Np(V) solubility in sodium carbonate mediaThis thesis also reported and discussed a study of Np(V) solubility in carbonicacid/bicarbonate/carbonate aqueous solutions. The experimental data were thosereported in several earlier publications [84VIT, 86GRE/ROB], and reports ofEuropean joint research contracts [85CÔM, 85KIM]. Sensitivity analysis was done(page 153 [90RIG]). The thermodynamic values deduced by the author and in thosefrom recalculation in the present review are consistent within the stated uncertainties(Table A.20). Riglet noted that the main uncertainty was due to poor crystallisationof the solid phase when NpO + 2was the major soluble neptunium species. This seemsto have been substantiated in later studies (e.g., [93LEM/BOY]). As mentionedelsewhere in this appendix (see the discussion of [84VIT]), this relatively pooraccuracy probably cannot be attributed to ion exchange equilibria in the solid phase asproposed by Volkov et al.[79VOL/VIS2]2(x − 1)Na + + NaNpO 2 CO 3 (s) Å Na (2x−1) NpO 2 (CO 3 ) x (s) + (1 − x)CO 2−3The solids, typically hydrated Na 0.6 NpO 2 (CO 3 ) 0.8 (s), did not usually result fromlengthy equilibration with aqueous solutions. Vitorge carried out measurements[84VIT], for the chemical conditions under which NpO + 2was the major solublecomplex by assuming the equilibriumNa (2x−1) NpO 2 (CO 3 ) x (s) Å (2x − 1)Na + + NpO + 2 + xCO2− 3controlled the solubility and deduced values of the corresponding solubilityproduct, K s,x , and x values (Table A.21) from linear regression (the plot oflog 10 [Np(V)] vs. log 10 [CO 2−3] was a straight line with slope of approximately −1).He found the “best” stoichiometry to be Na 0.72 NpO 2 (CO 3 ) 0.86 (s), but after severalweeks a hydrated NaNpO 2 CO 3 (s) was formed. Reanalysis of the data for eachsuccessive dissolution and precipitation of Np(V) indicates the linear regressionresults were more accurate for dissolution than for precipitation; over time the x valuebecame closer to 1 and the K s,1 decreased (Table A.21). This is evidence that hydratedNa (2x−1) NpO 2 (CO 3 ) x (s) with x < 1 was formed initially, but that this solid phase wasmetastable, and was slowly transformed to a thermodynamic stable phase, hydrated