Synergistic extraction equilibrium of p-aminobenzoic acid by the mixture extractant of di-2-ethylhexyl phosphoric acid (D2EHPA) and tri-n-butylphosphate (TBP) in heptane was investigated at various initial pH values and D2EHPA

TBP concentrations

and a mathematical model was proposed to describe the extraction equilibrium.The results showed that the extraction of p-aminobenzoic acid with the mixed solvent presented synergistic effect by TBP bonded to —NH2 on p-aminobenzoic acid and D2EHPA associated with —COOH on p-aminobenzoic acid

and the synergistic effect was more obvious when D2EHPA concentration was close to TBP concentration.The apparent extraction equilibrium constants were obtained by fitting the experimental data to the models proposed in this paper

and the apparent extraction equilibrium constants for the mixture solvent of D2EHPA and TBP were much higher than those for D2EHPA and TBP solvent alone.