Rare Earth Element Complexation by Fluoride Ions in Aqueous Solution

Rare earth fluoride stability constants for Ce, Eu, Gd, Tb and Yb at 25°C have been determined by examining the influence of fluoride ions on the distribution of rare earths between tributyl phosphate (TBP) and 0.68M NaClO ₄ . Our results indicate that rare earth mono and difluoro complexation constants show a steady increase as a function of atomic number from La to Tb but remain relatively constant after Dy. This behavior is similar to that which has been observed for dicarboxylic acids. Stepwise stability constant ratios, K ₂ /K ₁ , obtained in our work (where K ₁ =[MF ²⁺ ][M ³⁺ ] ⁻¹ [F ⁻ ] ⁻¹ and K ₂ =[MF ⁺ ₂ ] ⁻¹ [MF ²⁺ ] ⁻¹ [F ⁻ ] ⁻¹ ) indicated that, for all rare earths, K ₂ /K ₁ =0.09±0.03.