Low-Temperature Heat Capacities and Thermodynamic Properties of Aqueous Na2B4O7 Solution

The molar heat capacities of aqueous Na2B4O7 solution with concentration of 0.033 55 mol . kg(-1) were measured by a precision automated adiabatic calorimeter in the temperature range from 78 K to 351 K. The fusion transition at T=272.905 K was observed from the heat capacity measurements. According to the polynomial equations of heat capacity as a function of temperature and thermodynamic relationship, the thermodynamic functions (H-r-H-298.15K) and (S-r-S-298.15 (K)) of the aqueous borax decahydrate solution were derived in the temperature range from about 80 to 350 K with an interval of 5 K. The molar enthalpy and entropy of the phase transition were determined to be 4.536 kJ . mol(-1) and 16.62 J . K-1 . mol(-1). The activity of the solution was determined to be 0.997 63 by using the freezing point depression of the solution based on the liquid-solid phase equilibrium rule.