Solvent extraction study of Nd3+-F-, Eu3+-F-, and Tm3+-F- interaction in mixed system of dimethyl sulfoxide and water

The stability constants of the monofluoride complex of Nd(III), Eu(III), and Tm(III) have been determined in mixed solvents of dimethyl sulfoxide (DMSO) and H2O at 0.1 mol dm(-3) ionic strength using a solvent extraction technique. The values for Nd(III) and Eu(III) increase as the mole fraction of DMSO in the mixed solvent system increases. However, the stability constant for Tm(III) reaches a maximum in the vicinity of the mole fraction of DMSO of 0.02 and then decreases. The variation of the stability constants was equally dependent on following the two factors: (1) the Gibbs' free energy of transfer of F- between H2O and the mixed solvents; (2) the difference between the solvation energies of the tripositive cation and of the monofluoride complex in H2O and the mixed solvent solutions ([Delta G(Ln,solv)(H2O) - Delta G(LnF,solv)(H2O)] - [Delta G(Ln,solv)(mix) - Delta G(LnF,solv)(mix)]). The difference between solvation energies mentioned in (2) was proportional to the mole fraction of DMSO in the bulk solution and the proportionality constant was dependent on a reciprocal of the effective ionic radius of the Ln(III).