Internal pressure measurements of the binary 0.7393H 2O + 0.2607NH 3 mixture near the critical and maxcondetherm points

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The pressure, P, and their temperature derivative ( ∂ P / ∂ T ) V , of binary mixture 0.7393H 2O + 0.2607NH 3 has been measured in the near- and supercritical regions. Measurements were made in the immediate vicinity of the liquid–gas coexistence curve (one- and two-phase regions) using a high-temperature, high-pressure, nearly constant-volume adiabatic piezo-calorimeter. Measurements were made along 40 liquid and vapor isochores in the range from 120.03 kg m −3 to 671.23 kg m −3 and at temperatures from 403 K to 633 K and at pressures up to 28 MPa. Temperatures at the liquid–gas phase transition curve, T S ( ρ ) , for each measured density (isochore) were obtained using the quasi-static thermograms technique. The expanded uncertainty of the pressure, P, and temperature derivative, ( ∂ P / ∂ T ) V , measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.05% and 0.12–1.5% (depending on temperature and pressure), respectively. The direct measured pressures, P, and temperature derivatives, ( ∂ P / ∂ T ) V , has been used to calculate the internal pressure (or energy-volume coefficient) as ( ∂ U / ∂ V ) T = T ( ∂ P / ∂ T ) V − P . We also measured the temperature derivatives of the internal energy ( ∂ U / ∂ T ) V = C V (isochoric heat capacity) using the same apparatus. The measurements were made at isochoric heating of the system at quasi-equilibrium conditions. The effect of pressure, temperature, and concentration on the internal pressure was studied. The measured values of pressure ( P S ), temperature derivative, ( ∂ P / ∂ T ) V S , temperature ( T S ) and density ( ρ S ) at the saturation curve together with isochoric heat capacity measurements were used to calculate other thermodynamic properties of the mixture at the bubble- and dew-pressure points curves. Some unusual “appendix type shape” behavior of the ( ∂ P / ∂ T ) V and internal pressure, P int , near the critical and maxcondetherm points (in the retrograde region) was found.