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

A. Discussion of selected references 689[82MOR/FUG]This paper reports a determination of the enthalpy of formation of Ba 2 MgNpO 6 .The enthalpy of formation of MgCl 2 in 1 M HCl is calculated from the CODATA[89COX/WAG] values for Mg 2+ and Cl − and the difference (5.86 kJ·mol −1 )in the enthalpies of solution of MgCl 2 (cr) in 1 M HCl and in water reported byShin and Criss [79SHI/CRI]. The reported enthalpy of reaction of Ba 2 MgNpO 6with 1 M HCl is used with this and other auxiliary data selected in the presentreview (see Chapter 13 and the entry for [83MOR/WIL] in Appendix A) to obtain f H ◦ m (Ba 2MgNpO 6 , cr, 298.15 K) =−(3096.9 ± 8.2) kJ·mol −1 .[82NAI/CHA]Hydrolysis constants were determined by potentiometric titration (“Bjerrum-Calvin pHtitration technique”) for trivalent Pu, Am, Sm and Eu in 1 M NaClO 4 .Dataanalysisusing the method of weighted least squares was a Bjerrum type analysis (¯n functions).The calculated curves deviate systematically from the experimental points for Pu(III),but less so for Am(III), and are quite good for Sm(III) and Eu(III). Results are reportedin Table A.10.Table A.10: Hydrolysis constants for trivalent Pu, Am, Sm and Eu as given by[82NAI/CHA].Metal ion ∗ β 1 ∗ β 2 − log10 ∗β1 − log10 ∗β2Pu(III) 2.75 ± 0.16 × 10 −6 5.56 ± 0.032.85 ± 0.17 × 10 −6 5.54 ± 0.03Am(III) 9.32 ± 0.94 × 10 −8 7.03Sm(III) 1.46 ± 0.06 × 10 −9 1.85 ± 0.16 × 10 −16 8.84 15.73Eu(III) 7.59 ± 0.30 × 10 −9 3.55 ± 0.33 × 10 −16 8.12 15.45The points in Fig. 1 and 2 of [82NAI/CHA] indicate that the errors should be considerablylarger for Pu than given in the table. In the case of Pu and Am the total metalconcentrations were ≤ 0.04 mM, and polymer formation was unlikely. The approachand technique seem very good, and the results, as shown in the figures appear significant.The discrepancy between the Pu and the Am data seems too large. For Pu, thediscrepancy between the calculated curve and experimental data is substantial. Thisindicates the plutonium solution may not have been pure Pu(III), but perhaps containedsome Pu(IV). This would explain the differences. Therefore the values are rejected inthe present review.[82PER/SPI]The Np(VI)/Np(V) redox potential was found to be approximately constant inaqueous 1 to 15 M NaOH solutions. This agrees with the results of Simakin et al.[73SIM/MAT], but not with previous work of one of the authors [74PER/ALE].