Lanthanide trifluorides hydration

The direct measurement of enthalpies of solution of lanthanide trifluorides in water is hardly practicable, in view of their very small solubilities in this solvent. Enthalpies of hydration of lanthanide trifluorides have been measured (217). It should be possible to derive enthalpies of solution, at least for the hemihydrates LnF3-y2H20, by measuring enthalpies of precipitation. 

Dehydration of the dehydrated trihalides is a final method for oxide conversion. The hydrated trichlorides, tribromides and triiodides, RXj- H20, are obtained by dissolution of the oxides in aqueous hydrohalic acid and condensation by warming and desiccation (Ashcroft and Mortimer, 1968 Brown et al., 1968). The hydrated trifluorides are prepared by dissolution of the oxide in HNO3 or HCl and precipitation with aqueous HF. The filtered trifluoride may be dehydrated by heating slowly to 600°C in an inert gas stream or in vacuum (Strizhkov et al., 1972). Products obtained by heating in air are contaminated with oxide fluoride (Batsanova, 1971). Thermal decomposition studies of the hydrated trichlorides, cf. section 5.1, have shown that the oxide chlorides are readily formed, but careful dehydration under N2 flow has apparently been successful for the chlorides (Ashcroft and Mortimer, 1968). Tribromides have also been prepared by careful vacuum dehydration, but the lutetium products were contaminated with oxide bromide (Brown et al., 1968). In general simple dehydration becomes increasingly difficult with increasing atomic number of both the lanthanide and the halide. 

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