Variation of the solvation number of Eu(III) in mixed system of methanol and water

The stability constants (beta(1)) of the monofluoride complex of Eu(III) ha ve been determined in mixed solvents of methanol and water at a 0.10 M ioni c strength using a solvent extraction technique. The values of ln beta(1) i ncrease as the mole fraction of methanol in the mixed solvent system increa ses. The variation in the stability constants can be correlated with both t he large effect due to the solvation of F- and the small effect due to both (1) the solvation of cations in connection with complexation and (2) the e lectrostatic attraction between Eu3+ and F-. Based on the variation in the sum of (1) and (2) in water and the mixed solvent solutions, it was determi ned that the coordination number (CN) of Eu(III) varied from a mixture of C N = 9 and 8 to CN = 8 at about a 0.03 mole fraction of methanol in the mixe d solvent.