Am3+-F- interaction in mixed system of dimethyl sulfoxide and water

The stability constant (β1) of AmF2+ in the mixed solvent system of dimethyl sulfoxide and water with 0.1 mol dm-3 ionic strength was determined using a solvent extraction technique to compare with Nd(III) bearing similar chemical properties to Am(III). The value of ln β1 was nearly constant when the mole fraction of DMSO in the bulk solution (Xs) was less than 0.04 and it increased with an increase in Xs in the range of 0.04 < Xs < 0.25. The variation of ln β1 with Xs is obviously different from the result of Nd(III). The difference between the variations of ln β1 of Am(III) and Nd(III) with Xs could be associated with (1) the cation solvation effect (containing the variation effect of the solvation number in the primary solvation sphere of tripositive cation) and (2) the variation effect of interionic distance between tripositive cation and fluoride ion. The variation of ln β1 of Am(III) in 0.04 < Xs < 0.25 of the DMSO-H2O system is considerably influenced by both effects due to the variation of solvation number in the primary solvation sphere and the variation of interionic distance, but that of Nd(III) in Xs < 0.28 hardly contains those effects. (orig.)