Solubility and osmotic coefficient of phthalic acid aqueous solutions from isopiestic measurements

Solubility values reported in the literature for phthalic acid in water, at 25 °C, range from 0.0418 mol kg^-1 to 0.08548 mol dm^-3. A correct value was necessary for the preparation of solutions for the undergoing studies of the phthalate pH reference buffer system, aiming at improved activity coefficients. These require isopiestic measurements of the osmotic coefficients, (phi), of the phthalate solute species, namely H2Phth in its pure aqueous solutions. Equilibration of NaCl reference aqueous solutions and phthalic acid test solutions implies liquid–vapour equilibrium of both solutions where water has the same activity. Since one of the most sensitive methods for the assessment of solubilities is also the isopiestic method, use has been made of the facility. Preliminary experiments led to the identification of the approximate solubility values as well as the establishment of equilibration times. Equilibrium between rigorously known amounts of standard NaCl reference solutions, phthalic acid solutions of initial concentration close to saturation and a saturated solution containing phthalic acid crystals was attained, final weights were measured and molalities were assessed. Extrapolation of equilibrium concentrations to infinite equilibration time led to the solubility value of 0.0427 mol kg^-1, to which corresponds the osmotic coefficient, (phi)=0.9713 and the water activity aw=0.9985.