Equilibrium studies on enantioselective liquid - liquid amino acid extraction using a cinchona alkaloid extractant

The enantioselective extraction of aqueous 3,5-dinitrobenzoyl-R,S-leucine (A_R,S) by a cinchona alkaloid extractant (C) in 1,2-dichloroethane was studied at room temperature (294 K) in a batch system for a range of intake concentrations (10^-4 - 10^-3 mol/L) and pH values (3.8-6.6). The experimental data were described by a reactive extraction model with a homogeneous organic phase reaction of A_R,S with C. Important parameters of this model were determined experimentally. The acid dissociation constant, K_a, of A_R,S was (1.92 ± 0.07) x 10 ^-4 mol/L. The physical distribution coefficient of A_R,S between the organic and aqueous phase was 8.04 ± 0.39. The equilibrium constants of the organic phase complexation reaction were (9.31 ± 0.76) x 10⁴ L/mol and (2.71 ± 0.76) x 10⁴ L/mol for the S- and R-enantiomers, respectively. With these parameters an optimum performance factor, PF, of 0.19 was predicted. The PF was independent of the pH provided that pH ≫ pK_a. The model was verified experimentally with excellent results (±7.9%).