Features of the thermodynamics of trivalent lanthanide/actinide distribution reactions by tri-n-octylphosphine oxide and bis(2-ethylhexyl) phosphoric acid

(Graph Presented) A new methodology has been developed to study the thermochemical features of the biphasic transfer reactions of trisnitrato complexes of lanthanides and americium by a monofunctional solvating ligand (tri-n-octylphosphine oxide, TOPO). Stability constants for successive nitrato complexes (M(NO₃)_x^3-x_(aq) where M is Eu³⁺, Am³⁺, or Cm³⁺) were determined to assist in the calculation of the extraction constant, K_ex, for the metal ions under study. Enthalpies of extraction (ΔH_extr) for the lanthanide series (excluding Pm³⁺) and Am³⁺ by TOPO have been measured using isothermal titration calorimetry. The observed ΔH_extr were found to be constant at ∼29 kJ mol^-1 across the series from La³⁺ to Er³⁺, with a slight decrease observed from Tm³⁺ to Lu³⁺. These heats were found to be consistent with enthalpies determined using van't Hoff analysis of temperature dependent extraction studies. A complete set of thermodynamic parameters (ΔG, ΔH, ΔS) was calculated for Eu(NO₃)₃, Am(NO₃)₃, and Cm(NO₃)₃ extraction by TOPO and Am³⁺ and Cm³⁺ extraction by bis(2-ethylhexyl) phosphoric acid (HDEHP). A discussion comparing the energetics of these systems is offered. The measured biphasic extraction heats for the transplutonium elements, ΔH_extr, presented in these studies are the first ever direct measurements offered using two-phase calorimetric techniques.