Thermodynamic properties of o-phthalic acid and its products of dissociation at 0–200°C and 1–5000 bar

Abstract

Values of the pH for four solutions in a KHPh-HCl-KOH system, where Ph denoting C8O4H4, are measured at 25–70°C depending on pressure (1–1000 bar). The experimental results and the available literature data are processed on the base of Helgeson-Kirkham-Flowers (HKF) equation of state [1] and the GIBBS, OptimA, and OptimB programs from the HCh software package [2]. The obtained standard thermodynamic properties and HKF parameters of H2Phaq, HPh−, and Ph2− aqueous species, provides calculation of the pH of phthalate buffers in a wide range of temperatures (up to 200°C) and pressures (up to 5 kbar). The calculated values for the biphthalate buffer (0.05m KHPh) correspond to the IUPAC recommendations (at 0–50°C and 1 bar) with an accuracy of 0.005 pH units, and to the values of pH measured in this study at elevated Tp parameters (25–70°C and pressures of up to 1 kbar) within the limits of ±0.02 pH units.