Complex Formation between Tributyl Phosphate and Nitric Acid and the Hydration of the Complexes in Dodecane

Complex-formation of nitric acid with tributyl phosphate (TBP) and the hydration of the complexes in dodecane were examined from the acid and water distribution data between the aqueous and organic phases at 298 K. The equilibrium constants were determined by analyzing the acid and water distribution data as functions of the TBP concentration in the organic phase, the aqueous acid activity, and the water activity. Three complexes (TBP·HNO3, (TBP)2·HNO3, and (TBP)3·HNO3) were found in the organic phase when the aqueous nitric acid concentration was not higher than 3 mol dm−3. The hydration numbers for these complexes were also obtained. The TBP·HNO3 was chiefly anhydrous, but partially a monohydrate. In contrast, the (TBP)2·HNO3 was only a monohydrate and (TBP)3·HNO3 was only a tetrahydrate. The TBP–HNO3–H2O stoichiometry, by taking account of these complexes in addition to the TBP hydrates and free nitric acid in the organic phase, applied very well, up to 2 mol dm−3 (54.6 vol%) TBP in dodecane without any activity correction.