Measurements of heat capacities at constant volume for aqueous salt solutions (H 2O+NaCl, H 2O+KCl and H 2O+NaOH) near the critical point of pure water

Abstract

Measurements of isochoric heat capacities C VX for pure water and aqueous salt solutions (H 2O+NaCl, H 2O+KCl and H 2O+NaOH) as a function of temperature along the critical isochores and on the coexistance curves for different concentrations are reported. For binary system H 2O+KCl, measurements of C VX along four isochores (333.11, 368.32, 415.97, and 598.09 kg m −3) near the critical point of pure water are reported. Measurements of isochoric heat capacity C V for pure water were also made at critical isochore ( ρ c=317.9 kg m −3) and at five near-critical isochores, namely: 309.598, 335.570, 354.480, 370.370, and 373.340 kg m −3, and the coexistence curve. The measurements cover the range in temperature from 371 to 827 K at concentrations from 0 to 0.0932 mole fraction of salt (KCl, NaCl and NaOH). The measurements were made in a high-temperature and high pressure adiabatic nearly constant-volume calorimeter. Uncertainties of the isochoric heat capacity measurements are estimated to be less than ±2%. One- and two-phase heat capacities ( C VX′, C VX″), temperatures ( T s), and densities ( ρ s) at saturation were obtained by the method of quasi-static thermograms. The experimental C VX results has been compared with a Pitzer's classical equation of state and with the results of our previous measurements. The relative average deviations for H 2O+NaCl, H 2O+KCl and H 2O+NaOH solutions within about ±2%, except critical region where deviations reached 15–20%.