Thermodynamics of Extraction of Nitric Acid by Tri-n-Butyl Phosphate-Hydrocarbon Diluent Solutions: I. Distribution Studies with TBP in Amsco 125-82 at Intermediate and Low Acidities

The distribution of 0–8 M HNO₃ between aqueous and 5–100 vol % solutions of TBP in Amsco 125-82 has been measured and used to obtain a quantitative description of this extraction process. Up to ∼5 M aqueous HNO₃, a limit arbitrarily chosen because of uncertainties in HNO₃ activity coefficients, the data of this report at any particular concentration of TBP in Amsco are accurately described by the equation log [(HNO₃)_org/{[HNO₃]_aq(3.75493-(HNO₃)_org)}] = A + B (HNO₃)_org, where parentheses refer to concentrations, square brackets to activities, and subscripts org and aq to organic and aqueous phases. The antilogarithms of the constants A for the six TBP-diluent solutions studied can be described as a linear function of , the sum of the mole fractions, and , of water and TBP in the acid-free water-saturated organic phase. These antilogarithms were interpreted as the product K₁^mγ_T, where K₁^m is the thermodynamic equilibrium constant for the extraction reaction and γ_T is the mean activity coefficient of TBP and TBP·H₂O in the acid-free, water-saturated organic phase. As the concentration of TBP in Amsco 125-82 increases from 0 to 100%, K₁^mγ_T, in molal units, varies from 0.2 to 1.5. The quantities B of the above equation are proportional to while the product B(HNO₃)_org is interpreted as log γ_TN, where γ_TN is the mean activity coefficient of the species TBP·HNO₃ and TBP·HNO₃·H₂O. The organic phase water and acidity values are consistent with formation of the complexes TBP·H₂O, TBP·HNO₃, TBP·HNO₃·H₂O, and TBP·2HNO₃.