Two-phase calorimetry. I. Studies on the thermodynamics of lanthanide extraction by bis(2-ethylHexyl) phosphoric acid

Enthalpies of extraction of lanthanide ions by bis(2-ethylhexyl) phosphoric acid (HDEHP) from aqueous nitrate solutions have been determined using isothermal titration microcalorimetry. This solvent extraction system was chosen as a model for developing a direct method of calorimetric investigation of thermochemistry of liquid-liquid distribution reactions because it has been well-studied and is predictable. To provide a quantitative validation for a two-phase calorimetric technique, the enthalpy of extraction of europium(III) by HDEHP also was determined applying the more conventional method of van't Hoff analysis of temperature dependent extraction equilibrium constants. Within the accuracy of the measurements, the determination of thermodynamic parameters by the complementary techniques agreed. The extraction enthalpies determined in this study have been correlated with reported free energies of extraction of the lanthanides to reveal the patterns of extraction entropies observed across the lanthanide series. Challenges and opportunities for application of two-phase calorimetry as a new analytical method for routine analyses of the thermochemistry of solvent extraction reactions are discussed.