SOLID-LIQUID EQUILIBRIUM REACTIONS OF NEPTUNIUM(V) IN CARBONATE SOLUTION AT DIFFERENT IONIC-STRENGTH

The carbonate complexation of the NpO2+ ion has been studied in 0.1 M, 1.0 M, 3.0 M and 5.0 M NaClO4 under CO2 partial pressure of 10(-3.5) at 25 degrees C, varying pH from 6.8 to 10.2. A radiometric study of solid-liquid equilibria is accompanied by the solution speciation with UV spectroscopy. The solid phase produced in equilibria at pH <8.5 is found to be NaNpO2CO3(s). At pH >9.0 this phase is then very slowly converted to Na3NpO2(CO3)(2)(s). The solubility product of NaNpO2CO3(s) and compiexation constants of NpO2CO3-, NpO2(CO3)(2)(3-) and NpO2(CO3)(3)(5-) are determined in each NaClO4 solution. The intercomparison of each set of constants from this work with the literature data is made by the S.I.T. formalism up to 3.0 M NaClO4 which is found to be the limiting salt concentration for its application. After normalization to zero ionic strength by the S.I.T., an excellent agreement is observed among all available constants of each reaction from the literature and present work.