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

Chapter 16Plutonium aqua ionsPlutonium in aqueous solution can exist in the oxidation states, III, IV, V, VI, and VII. Inacidic solution, the reduction potentials of the Pu 4+ /Pu 3+ ,PuO + 2 /Pu4+ ,PuO 2+2 /Pu4+ ,and PuO 2+2 /PuO+ 2couples lie relatively close to each other, and multiple oxidationstates can coexist in solution. Thus, the determination of molar enthalpies, Gibbs energiesand entropies of reaction may be hampered by the presence of plutonium in oxidationstates that are unrelated to the redox couple under investigation, and the presenceof the plutonium in these other oxidation states may remain undetected by the potentialmeasurements. In solutions less than 1 M in acid, tetravalent plutonium slowly undergoesdisproportionation to Pu(III) and Pu(VI). Also, it is partially hydrolyzed evenin 1 M acid solution. Pentavalent plutonium disproportionates to Pu(III), Pu(IV) andPu(VI) in acidic solutions, and if the acid concentration is of the order of 0.1 M or less,Pu(IV) may also be precipitated. The selected thermodynamic quantities for Pu 3+ ,Pu 4+ ,PuO + 2and PuO2+2are strongly connected, and there is a minimum amount ofredundant information to provide confirmation for these values. The selection processused in the present review relies primarily on values for Pu 3+ derived from solubilityand enthalpy of solution measurements. Values for Pu 4+ are based on potentialmeasurements of the Pu 4+ /Pu 3+ couple. Next, for PuO 2+2, the equilibrium constantfor the disproportionation reaction of Pu(IV) to Pu(VI) and Pu(III) and values for thePu 4+ /Pu 3+ couple and the enthalpy of oxidation of Pu(III) to Pu(VI) are used. Finally,values for PuO + 2are based on potential measurements of the PuO2+2 /PuO+ 2couple. Itis assumed, that the value of the enthalpy of a reaction at I = 0 is equal to that in1MHClO 4 within the uncertainties of the measurements. This approximation leadsto the differences between values of the formal potentials in 1 M HClO 4 (aq) and thestandard potentials being assigned primarily to entropy differences. The assumption isundoubtedly incorrect (and would actually be consistent with temperature-independentactivity coefficients) but, used consistently, in the absence of experimental information,is probably subject to smaller errors than would be introduced by some of theother possible assumptions.16.1 Pu(VII) and redox potentials in alkaline solutionsHeptavalent plutonium is only stable in strongly basic solution. The formal potential in1 M NaOH for the Pu(VII)/Pu(VI) couple was measured by Krot et al., [70KRO/GEL]and by Peretrukhin et al. [72PER/KRO] asE ◦′ = 0.95 V and 0.85 V, respectively, for301