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

92 7. Neptunium aqua ionsE ◦′ (7.1, 1 M HNO 3 , 298.15 K) > 2.00 V, respectively, for reactionNpO + 3 + 2H+ + e − Å NpO 2+2+ H 2 O(l) (7.1)It has recently been proposed [91SHI/TAN] that the structure of the cationic Np(VII)species might be better written as Np(OH) + 6rather than NpO+ 3. Heptavalent neptuniumcan be generated in alkaline solutions, but is reduced by water to Np(VI) overa period of hours to weeks at room temperature. Several measurements exist of theformal Np(VII)/Np(VI) potential in basic solution. The alkaline chemistry of thetransuranium elements was recently reviewed by Peretrukhin et al.[95PER/SHI]. Theyderive E ◦′ (7.2, 1 M NaOH, 298.15 K) = (0.59 ± 0.01) V for the reactionNpO 4 (OH) 3−2 + 2H 2 O(l) + e − ÅNpO 4 (H 2 O) 2−2 (or NpO 2 (OH) 2−4 ) + 2OH− (7.2)From the reversibility of the electrochemical reduction reaction, the authors suggestthat the structure of the hepta- and hexavalent ions must be similar and six coordinate.This would exclude the existence of NpO 3−5which was postulated by several otherauthors that are referred to in the review of Peretrukhin et al. Table 7.1 gives thereported formal redox potentials for neptunium in 1 M NaOH [95PER/SHI].Table 7.1: Formal potentials for neptunium in 1 M NaOH at 298.15 K reported by[95PER/SHI]Formal potential (V. vs. SHE)VII/VI VI/V V/IV IV/III0.59 0.14 −0.95 −1.87.2 NpO 2+2The thermodynamic quantities for NpO 2+2are derived from the value of the entropybased on the solubility and the enthalpy of dissolution of the salt NpO 2 (NO 3 ) 2·6H 2 O(s)in water, the enthalpy of formation of Np 4+ and the enthalpy of oxidation of NpO + 2to NpO 2+2as determined calorimetrically and from values of the standard potentialbased on electrochemical measurements. The temperature coefficients for the VI/V andV/IV potentials are used to calculate, sequentially, Sm ◦ (NpO+ 2 ) and S◦ m (Np4+ ), and fromthe latter f G ◦ m (Np4+ ). The values of the V/IV potential are then used to determine f G ◦ m (NpO+ 2 ) and hence fHm ◦ (NpO+ 2). Finally, the values of the VI/V potential andthe calorimetric enthalpy of oxidation of NpO + 2to NpO2+2are examined together todetermine f G ◦ m (NpO2+ 2 ) and hence fHm ◦ (NpO2+ 2 ).