Solubility, protonation and activity coefficients of some aminobenzoic acids in NaClaq and (CH3)4NClaq, at different salt concentrations, at T = 298.15 K

Protonation constants of 2-aminobenzoic acid, (anthranilic acid, vitamin L1) (1), 4-aminobenzoic acid (PABA) (2), and 2-aminoterephthalic acid (3) have been obtained from potentiometric measurements at 25 °C (298.15 K) and ionic strengths between 0 and 5 mol⋅dm−3 NaCl and between 0 and 3 mol⋅dm−3 (CH3)4NCl. Values for the step-wise protonation constants log K1 and log K2, extrapolated to zero ionic strength and infinite dilution, were determined as follows. For 2-aminobenzoic acid: 4.958 ± 0.003 (error estimate at the 95% confidence interval) and 2.060 ± 0.007; for 4-aminobenzoic acid: 4.868 ± 0.008 and 2.420 ± 0.004; for 2-aminoterephthalic acid: 5.089 ± 0.004 and 3.594 ± 0.006. Differences between protonation constants obtained using various concentrations of NaCl(aq) and (CH3)4NCl(aq) for ionic strength adjustment are interpreted in terms of formation of weak complexes between the ligands and the ions of the background electrolyte. Values of log K for the formation of 1:1 complexes of the aminobenzoate anion with the tetramethylammonium cation have been estimated. The dependence of the solubility was studied as a function of NaCl(aq) and (CH3)4NCl(aq) concentration. Similar behaviour was found for the three ligands. The total solubility increases with increasing NaCl(aq) concentration, whereas the opposite trend is observed in (CH3)4NCl(aq). Total and specific solubility was also determined, from which Setschenow coefficients and activity coefficients were deduced for the various species. Activity coefficient variation was examined in terms of both a Debye–Hückel type equation (DHt) and a SIT (Specific ion Interaction Theory) model.