Phthalic acid and sodium hydroxide were chosen as the reactants and a product of hydrated sodium phthalate was synthesized by the method of liquid phase synthesis. An X-ray powder diffraction technique, chemical and elemental analyses were applied to characterize the composition and structure of the compound. Low-temperature heat capacities of the substance were measured by a precision automated adiabatic calorimeter over a temperature range from 78 to 366 K. A polynomial equation of the heat capacities as a function of the temperature was fitted by the least-squares method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated at an interval of 5 K. A reasonable thermochemical cycle was designed on the basis of the Hess law, and the standard molar enthalpies of dissolution for the reactants and products of the designed solid phase reaction in the selected solvents were measured by an isoperibol solution-reaction calorimeter, respectively. In addition, the enthalpy change of the reaction was determined to be 29.073±1.05 kJ•mol^－1 from the data of the above standard molar enthalpies of dissolution. Eventually, the standard molar enthalpy of formation of hydrated sodium phthalate was derived to be －1493.637±1.11 kJ•mol^－1 by the combination of the enthalpy change of the reaction with other auxiliary thermodynamic quantities.

Key words: hydrated sodium phthalate, adiabatic calorimetry, low-temperature heat capacity, isoperibol solution-reaction calorimetry, standard molar enthalpy of formation