Hydrolysis of plutonium(VI) at variable temperatures (283-343 K)

The hydrolysis of Pu(VI) was studied at variable temperatures (283-343 K) by potentiometry, microcalorimetry, and spectrophotometry. Three hydrolysis reactions, mPuO(2)(2+) + nH(2)O=(PuO(2))(m)(OH)(n) (2m-n)( +) + nH(+)), in which (n,m)=(1,1), (2,2), and (5,3), were invoked to describe the potentiometric and calorimetric data. The equilibrium constants (*β(n,m)) were determined by potentiometry at 283, 298, 313, 328, and 343 K. As the temperature was increased from 283 to 343 K, *β(1,1), *β(2,2), and *β(5,3), increased by 1, 1.5, and 4 orders of magnitude, respectively. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature increases. Measurements by microcalorimetry indicate that the three hydrolysis reactions are all endothermic at 298.15 K, with enthalpies of (35.0±3.4) kJ mol(-1), (65.4±1.0) kJ mol(-1), and (127.7±1.7) kJ mol(-1) for ΔH(1,1), ΔH(2,2), and ΔH(5,3), respectively. The hydrolysis constants at infinite dilution have been obtained with the Specific Ion Interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the DQUANT equation, and the Ryzhenko-Bryzgalin model, were evaluated with the data from this work.