Chemical equilibrium constants of rare earth nitrates and tri-n-butyl phosphate complex formation

Mixed rare earth nitrates (REi(NO3)3) in the aqueous solution was mixed with tri-n-butyl phosphate (TBP, (n-C4H9O)3PO) dissolved in kerosene for the formation of their corresponding complexes (REi(NO3)3·ni(n-C4H9O)3PO) at 303 K. The effects of initial concentrations of both TBP and mixed rare earth nitrates on the equilibrium constants of their complex formations were investigated. The complexes were formed almost immediately after mixing. The simultaneous formations reached their chemical equilibria within a few minutes by shaking the mixture at 200 r/min. The chemical equilibrium constants of the complex formations were independent of the initial TBP concentrations. However, they were decreased by reducing the concentration of REi(NO3)3. All equilibrium constants of the simultaneous complex formations were less than 0.7, while the average molar ratio of TBP to REi(NO3)3 of the complexes varied between 1.0 and 1.6. The chemical equilibrium constant for the formation of La(NO3)3·(n-C4H9O)3PO was 0.09, while that of Dy(NO3)3·(n-C4H9O)3PO was 0.68. The ascending sequence of chemical equilibrium constants for the simultaneous formations was La, Ce, Pr, Nd, Eu, Y, Sm, Gd, and Dy.