Thermodynamics of Liquid-Liquid Distribution Reactions. I. the Dioxouranium(VI) Nitrate-Water-Tri-n-butyl Phosphate-n-Dodecane System

The heat effect of the extraction of uranyl nitrate from aqueous solution into TBP has been measured. The measurements include both forward and backextractions with neat TBP at 23° ± 1°C, extractions with TBP in dodecane at 23° ± 1°C, and extractions with neat TBP at 5 — 40°C. Heats of mixing of TBP and of uranyl nitrate bis(TBP) solvate with dodecane, which are endothermic, the heat of dilution of aqueous uranyl nitrate, and the heat of hydration of TBP have also been measured. The thermodynamic functions for the extraction reaction UO₂²⁺ (aq) + 2 NO₃^- (aq) + 2 TBP(org) = UO₂(NO₃)'₂(TBP)₂(org) have been calculated. The standard state (aq) for the aqueous phase is infinite dilution of all solutes in water. For the organic phase, two distinct standard states (org) are used: infinite dilution of all solutes in dodecane (marked by ∗) and infinite dilution of all solutes in hydrated TBP (marked by °). The standard state functions are, in kJ mol^-1: ΠH^∗ = -54.5, ΔH° = -33.8, ΔG^∗ = -45.6, ΔG° = -41.0, and in J K^-1 mol^-1: ΔS^∗ = -30, ΔS° = +24, and ΔCp° = + 405.