Extraction Equilibria of Hydrochloric Acid by Trioctylamine in Low-Polar Organic Solvents

Two-phase emf titrations have been performed for the extraction of hydrochloric acid from aqueous solution containing 1 M NaCl (M=mol dm−3) by trioctylamine (TOA) dissolved in cyclohexane, toluene, benzene, chlorobenzene, nitrobenzene, cyclohexane–benzene mixtures, and benzene–nitrobenzene mixtures. It has been found that the extraction is accompanied with the formation of monomeric trioctylammonium chloride and its successive aggregation from dimer to micell (n-mer) in organic phase: ‾TOA+H++Cl−=‾TOAHCl, Kd; (‾TOAHCl)i−1+‾TOAHCl=(‾TOAHCl)i, (i=2,3,···,n), where the concentration quotients of successive aggregation, , satisfy the relation 2 2=33⁄2=···=n⁄(n−1). The quotients Kd and 2 vary with the total volume fraction of TOA in organic phase, ΨR°, according to equations, In Kd=1nKd°+gΨR° and 1n 2=1n2°+hΨR°. Diluent effect on the equilibrium constants Kd° and 2° are discussed in detail based on the theory of regular solution.