Acid-base equilibria of phthalic acid in saline media: ion association from Pitzer equations

The protonation constants of 1,2-benzenedicarboxylic acid (phthalic acid) at 25 °C in NaCl and CaCl₂ (ionic strengths ranging from 0.1 to 2 mol kg⁻¹) were determined potentiometrically, and the results were analyzed by means of Pitzer equations. The values of the logarithm of the first and second dissociation constants ranged from 2.730 and 4.948 (in 0.101 mol kg⁻¹ NaCl) to 2.449 and 3.984 (in 0.624 mol kg⁻¹ CaCl₂), respectively. Tabulated interaction parameters for the inorganic species involved along with salting coefficients estimated from literature allowed the calculation of new interaction parameters for the phthalate-Na⁺/Ca²⁺ systems. The specific ion interaction model considered seems to account for the influence of the formation of weak complexes of phthalate with calcium ions on the equilibrium constants.