THERMODYNAMICS OF URANIUM(VI) AND PLUTONIUM(VI) HYDROLYSIS

The thermodynamic parameters for the hydrolysis of hexavalent uranyl a nd plutonyl ions have been determined by calorimetric titration in the presence of 1.0 M tetramethylammonium chloride and 1.0 M sodium perch lorate as background electrolytes for both ions, respectively. The mea surements were obtained at 25 degrees C over a wide range of ($) over bar n values and at different initial metal concentrations to ensure r eliability of data. The values for the constants and the corresponding reactions were; 2UO(2)(2+)+2H(2)O double left right arrow [(UO2)(2)(O H)(2)](2+)+2H(+) Delta H-22 = 44.4+/-1.9kJ mol(-1); Delta S-22 = 36+/- 6 J K-1 mol(-1) PuO22+ + H2O double left right arrow PuO2(OH)(+) + HDelta H-11 = 28.0+/-5.0 kJ mol(-1); Delta S-11 = -4+/-17 J K(-1)mol(-1 ) and PuO22+ + 2H(2)O double left right arrow PuO2(OH)(2) + 2H(+) Delt a H-12 = 42.6+/-7.0 kJ mol(-1); Delta S-12 = -48+/-23 J K(-1)mol(-1). The values obtained are compared to literature data on uranyl(VI) and neptunyl(V) systems.