Several factors influencing the stability of extracted species in synergistic extraction of lanthanide(III) with pivaloyltrifluoroacetone and 2,2'-bipyridyl.

The stability constants and the hydration number of a complex between tris(pivaloyltrifluoroacetonato)lanthanide(III) (LnA(3)) and 2,2'-bipyridyl (B), LnA(3)B, and the complexation heat were determined across the Ln series by a solvent-extraction technique, Karl Fischer coulometry, and calorimetry, successively. The number of water molecules released from LnA(3) upon the complexation with B as well as the values of the stability constant increased along with increasing Ln atomic number to around Dy(III); via a maximum, it decreased. It is concluded that the largest stability constant of the complex of Ho(III) or this vicinity is derived from the strongest bond energy, due to the fitness between LnA(3) and B. This conclusion was supported by the trend of the variation of the entropy change as well as that of the hydration number across the Ln series.

Synergistic extraction of lanthanoids(III) with 2-thenoyltrifluoroacetone and benzoic acid: thermodynamic parameters in the complexation in organic phases and the hydration.

In order to examine the reason why the magnitude of the synergistic effect observed in the extraction of lanthanoids(III) with a beta-diketone and a monodentate Lewis base generally decreases along with increasing atomic number, the hydration number of the extracted species when lanthanoids(III) are extracted with TTA (2-thenoyltrifluoroacetone, HA) and benzoic acid (HB) into chloroform by Karl Fischer titration and the enthalpy change in complexation between LnA(3) and HB by calorimetric titration were determined across the lanthanoid series at 25 degrees C. It has been concluded that since the decrement of entropy change caused by the change in the number of released water molecules and in the coordination number of lanthanoids(III) upon complexation is larger than the increment of the enthalpy change, the values of the second formation constants of the complexes decrease with increasing the atomic number across lanthanoid series so that the magnitude of the synergistic extraction decreases with increasing the atomic number.