For binary aqueous solvent mixtures the solvent composition in the first solvation sphere of Eu3+ has been determined at 25 "C by measuring the luminescence lifetime. In the systems water-acetone, water-acetonitrile and water-l,4-dioxane the first solvation sphere of Eu3+ is exclusively occupied by water molecules even at bulk-phase mole fractions of water as low as 0.1. Eu3+ is preferentially solvated by water in water-methanol and by DMSO in water-DMSO mixtures over the whole range of solvent composition. In water-DMF mixtures Eu3+ is preferentially solvated by DMF when the mole fraction of DMF is below 0.7 and by water when it is above that value. In all the systems the degree of preferential solvation varies considerably with solvent composition. These results for the preferential solvation of Eu3+ are compared with those of A13+ and of alkali-metal ions obtained by the n.m.r. method.
The luminescence lifetime for Eu2(SO4)3 and EuCl3 in H2O and D2O has been measured at 0.01 and 0.001 mol dm−3 of Eu3+ and at various temperatures from 5 to 45 °C. The average number of water molecules coordinated to Eu3+ ion, q, is evaluated from the difference of the luminescence decay constants in the H2O and D2O solutions. By using the q value of 8.3 for aquaeuropium(III) ion in EuCl3 solutions, q values for Eu2(SO4)3 solutions were determined as a function of concentration and temperature. The average number of water molecules released from the primary hydration sphere by the inner-sphere complexation between Eu3+ and SO42− ions is given as Δq=8.3–q, and Δq value increases with concentration and temperature. From the values of Δq and the known equilibrium constants on the overall complexation, it is revealed that for both the unis(sulfato) and bis(sulfato) complexation the ratio of inner-sphere complex to outer-sphere complex increases with temperature and that about one water molecule is released from the inner hydration sphere surrounding Eu3+ ion on the formation of an inner-sphere unis(sulfato) complex. The coordination state of the unis(sulfato) complex and the thermodynamic quantities for the equilibrium between outer- and inner-sphere complexes are discussed.
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