Thermodynamics of Cm(III) in concentrated electrolyte solutions. Fluoride complexation in 0-5 m NaCl at 25 C

The fluoride complexation of Cm(III) is investigated by means of time-resolved laser fluorescence spectroscopy (TRLFS) at 25 C. In the first set of experiments, the speciation of Cm(III) is investigated in 1 m NaCl as function of the fluoride concentration (mF < 8 . 10-3 m). The monofluoride complex CmF2+ is identified and its formation constant is determined to be logβ(CmF2+) = 2.46 ± 0.05. At the highest fluoride concentration there is evidence for the formation of a Cm(III) difluoride complex to a small extent. However, as the fluoride concentration interval is limited by the precipitation of CmF3(s), the stability constant of CmF2+ can not be quantified. In the second set of experiments the formation of CmF2+ is studied in 0-5 m NaCl. The ionic strength dependence of the formation constant is described by modeling the activity coefficients with the ion interaction approach (Pitzer equations). The formation constant at l = 0 is calculated to be logβ (CmF2+) = 3.44 ± 0.05. (orig.)