Constant-Volume Heat Capacity of H2O + Na2CO3 Solutions Near the Critical Point of Pure Water

The results of new isochoric heat capacity measurements of H2O + Na2CO3 solutions as a function of temperature along several isochores in the near-critical region of pure water are reported. The measurements cover temperatures from 331 to 661 K at Na2CO3 mole fraction 0.008869. The experiments were performed at seven densities between 245 and 875 kg-m−3. The measurements were made in a high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter. Uncertainties of the isochoric heat capacity measurements are estimated to be 1–2% in the liquid phase, 2–3% in the vapor phase, and 4–5% near the critical point. Measurements were made in the two-phase (vapor–liquid, liquid–solid, vapor–solid) and three-phase (vapor–liquid–solid) regions. Two peaks in isochoric heat capacity have been found near the critical point of pure water. One of the peaks at TS1 occurs on the three-phase (L–V–S) curve and another peak at TS2 corresponds to a two-phase (L–S or V–S) curve. The experimental values of phase transition temperatures TS(ρ) on each isochore was determined. Uncertainty in the phase transition temperature measured was no greater than ±0.03 to 0.05 K.