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 cower 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 T-S1 occurs on the three-phase (L-V-S) curve and another peak at T-S2 corresponds to a two-phase (L-S or V-S) curve. The experimental values of phase transition temperatures T-S(rho) on each isochore was determined. Uncertainty in the phase transition temperature measured was no greater than +/-0.03 to 0.05 K.