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

16.5 Pu 3+ 31316.5 Pu 3+The values of the standard molar enthalpy of formation, the Gibbs energy of formation,and the molar entropy of the plutonium(III) cation were very thoroughly reviewedby Fuger and Oetting [76FUG/OET]. Their selected value f H ◦ m (Pu3+ ,aq,298.15 K) = −(592.0 ± 2.1) kJ·mol −1 , was derived from calorimetric measurementsof the enthalpies of solution of Pu and PuCl 3 (cr) in HCl(aq) [76FUG/OET]. Theiranalysis used the value of Robinson and Westrum [49ROB/WES] for the enthalpy ofsolution of PuCl 3 (cr) in 6 M HCl(aq)(−(92.68 ± 0.42) kJ·mol −1 ) and the result ofthe extrapolation of the enthalpy of solution of PuCl 3 (cr) in HCl(aq) to infinite dilution(−(133.47 ± 0.84) kJ·mol −1 ) by the same authors. For the enthalpy of solution ofPu(cr,α) in 6 M HCl(aq) Akhachinskij’s [66AKH] value of −(592.04±1.67) kJ·mol −1was accepted. Using revised auxiliary data (cf. Section 18.1), f H ◦ m (Pu3+ , aq, 298.15 K) = −(591.8 ± 2.0) kJ·mol −1Based on the Gibbs energy and enthalpy values for the dissolution reaction,PuCl 3·6H 2 O(cr) → Pu 3+ + 3Cl − + 6H 2 O(l) (16.13)(−(30.125 ± 0.840) kJ·mol −1 and −(34.60 ± 0.64) kJ·mol −1 , as discussed inSections 18.1.3.3.a and 18.1.3.3.b), r Sm ◦ (16.13 ) =−(15.01 ± 3.53) J·K−1·mol−1 .From this value and using new CODATA auxiliary data and values fromthis review for Sm ◦ (Cl− , aq, 298.15 K) = (56.60 ± 0.20) J·K −1·mol−1 andSm ◦ (PuCl 3·6H 2 O, cr, 298.15 K) = (420 ± 5) J·K −1·mol−1 , cf. Section 18.1.3.3,S ◦ m (Pu3+ , aq, 298.15 K) =−(184.5 ± 6.2) J·K −1·mol −1 .Then, using S ◦ m (Pu, cr, 298.15 K)= (54.46 ± 0.80) J·K−1·mol −1 as selected bythis review (cf. Section 15.1.2), r S ◦ m (16.14 )= −(42.95 ± 6.21) J·K−1·mol −1 wascalculated.α-Pu + 3H + Å Pu 3+ + 3 2 H 2(g) (16.14)From this and the selected value of f Hm ◦ , the selected value f G ◦ m (Pu3+ , aq, 298.15 K) = −(579.0 ± 2.7) kJ·mol −1is calculated. The standard potential for the α-Pu/Pu 3+ couple, E ◦ (16.15),is then calculated asPu 3+ (aq) + 3e − = α-Pu (16.15)E ◦ (16.15) = −(2.000 ± 0.009) V.For the value of the Pu(VI)/Pu(III) redox couple, Capdevila [92CAP, 92CAP/VIT,95CAP/VIT] obtained E ◦′ (16.6, 1MHClO 4 ) = (1.013 ± 0.017) V from K (16.5), and