Measurements of the heat capacity at constant volume of H2O+Na2SO4 in near-critical and supercritical water

The heat capacity at constant volume CVX of aqueous solutions of Na2SO4 has been measured from 460 to 669 K along 11 isochores from 250 to 926 kg m−3 for three compositions, namely: 0.0012798; 0.0066328, and 0.0139002 mole fraction of Na2SO4. Measurements of the isochoric heat capacity CV for pure water were also made at the critical isochore (ρC=317.9 kg m−3) and at six near-critical isochores, namely: 309.598, 335.570, 354.480, 368.310, 370.370 and 373.340 kg m−3, and the coexistence curve. A total of 368 CVX data for H2O+Na2SO4 and 273 CV data for pure water were measured along 17 isochores in the one- and two-phase regions. The measurements were made in a high-temperature and high-pressure adiabatic nearly constant-volume calorimeter. The uncertainty of the isochoric heat capacity measurements is estimated to be within ±2%. One- (CV′) and two-phase (CV″) heat capacities, temperatures (TS), and densities (ρS) at saturation were obtained by the method of quasi-static thermograms. The experimental CVX results have been compared with Pitzerʹs classical equation of state. The relative average deviations for H2O+Na2SO4 solutions were about ±2%, except for the critical region where differences reached 15–20%.