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

260 12. Neptunium group 14 compounds and complexesIn this review the stoichiometry of the Np(IV) limiting complex was assumed tobe Np(CO 3 ) 6−5, despite the lack of experimental confirmation of this stoichiometry. Aquantitative interpretation of the (relatively poor) experimental data leads to a rough estimateof the stability of Np(CO 3 ) 6−5based on the formal potential of the Np(V)/Np(IV)redox couple. However, recent, solubility data are used to calculate the value of theformation constant selected for Np(CO 3 ) 4−4. Spectrophotometric data for the dissociationof the limiting complex are used to calculate the equilibrium constant for thereaction and the stability constant for Np(CO 3 ) 6−5. The model assumed here seemsto be sufficient to explain all published data. The species Np(OH) 4 (aq), Np(CO 3 ) 6−5and Np(CO 3 ) 4−4are probably formed in concentrated carbonate/bicarbonate media.There are insufficient data to allow selection of formation constant values for othercomplexes, but that does not mean they do not exist.12.1.2.1.4.aThe limiting Np(IV) carbonate complexFedoseev, Peretrukhin and Krot [79FED/PER] proposed the rough value E ◦′ (12.21, 1MNa 2 CO 3 ) = 0.1 V/SHE for the redox potential of the equilibriumNp(V) + e − Å Np(IV) (12.21)A reasonable assessment of the uncertainty in this value is probably ± 0.2 V (AppendixA). Delmau, Vitorge and Capdevila [96DEL/VIT] later obtained a steady measurementin the same medium and determined E ◦′ (12.21, 1MNa 2 CO 3 , pH=10.12) =0.244 V/SHE. The major redox equilibrium can now be interpreted in terms of the limitingcarbonate complexes of Np(IV) and Np(V) whose stoichiometries and stabilitiesin these chemical conditions are now reasonably well established (see below in thissection and Section 12.1.2.1.3.a respectively).NpO 2 (CO 3 ) 5−3+ e − + 2CO 2 (g) Å Np(CO 3 ) 6−5(12.22)The potential of the Np(V)/Np(IV) redox couple E(12.21), is corrected for CO 2 (g)partial pressure to obtain the formal potential E ◦′ (12.22). This is the main source ofuncertainty: when a closed 1 M Na 2 CO 3 solution ( p CO2 = 10 −6.3 atm [95VIT]) iscontacted with air (p CO2 = 10 −3.5 atm), p CO2 can progressively increase, decreasingpH and shifting the redox potential of the solution by as much as +330 mV. Conversely,water reduction produced OH − anion in the course of the electrochemical preparationof Np(IV) from Np(V). E ◦′ (12.22, 1MNa 2 CO 3 , p CO2 = 1 atm) = 0.514 and 0.535V/SHE are calculated by this review from the above values E ◦′ (12.21, 1MNa 2 CO 3 )=0.1V/SHE[79FED/PER] assuming equilibrium with air, and from E ◦′ (12.21, 1MNa 2 CO 3 , pH 10.12) = 0.244 V/SHE [96DEL/VIT] respectively. Further discussion isprovided in Appendix A. From the mean of the two values, E ◦′ (12.22, 1MNa 2 CO 3 ,p CO2 = 1 atm, 298.15 K) = (0.524 ± 0.101) V/SHE (the uncertainty reflects the inaccuracyin the value of p CO2 ), this review calculates log 10 β 5 (12.23,1MNa 2 CO 3 , γ Np 4+= 1, 298.15 K) = (41.96 ± 1.70) in molal units for the equilibriumNp 4+ + 5CO 2−3Å Np(CO 3 ) 6−5(12.23)