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 10(4) 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%).