Interaction of trivalent f-element cations with iodide in methanol + water solvents

Abstract

The stability constants (β ₁) of iodide ion-pairs of trivalent f-block element ions (lanthanoids Ce, Eu, Gd, Tb and Tm, and actinoid Am) were determined in the vicinity of pH 2.5 of mixed methanol/water solvent solutions of an ionic strength of 1.00 mol·dm⁻³ at 298±1 K. The values were less than 2. From the variation in distance between Eu³⁺ and I⁻, which was calculated using a Born-type equation for Gibbs’ free energy derived from β ₁(Eu), the Eu³⁺-I⁻ interaction was shown to be solvent-shared ion-pair formation when the mole fraction of methanol (X _MeOH)≤0.40. In contrast, it was suggested that the interaction of Am³⁺-I⁻ changed from solvent-separated ion-pair to solvent-shared ion-pair with increasing X _MeOH when X _MeOH≤0.10, but remained as solvent-shared ion-pair in the range 0.16≤X _MeOH≤0.40. Furthermore, β ₂(Am) was measured in the range 0.31≤X _MeOH≤0.40. It was also shown that the β ₁ values of lanthanoids at X _MeOH = 0.40, except for that of Gd(III), decreased with increasing atomic number.