Measurement of the boiling point of a liquid at different pressures allows you to determine its vapor pressure curve. Besides this, the enthalpy of vaporization of the liquid can be determined from such measurements using the Clausius–Clapeyron equation. In this article, we show how such measurements are performed in a high-pressure DSC using water as an example.
The boiling point of a liquid depends on the pressure of the surroundings: the lower the pressure the lower the temperature at which the liquid boils.
For example, on Mount Everest, the air pressure is only about 325 mbar and water boils at about 70 °C and not at 100 °C like at sea level. The pressure dependence of the boiling point can be easily measured by high-pressure DSC (HPDSC). The enthalpy of vaporization can also be determined from such measurements using the Clausius–Clapeyron equation [1].
The Clausius–Clapeyron equation describes the relationship between the vapor pressure of a substance, p, and the temperature at which it boils, T:
Here ΔH stands for the enthalpy (heat) of vaporization, L is a substance-specific constant, and R is the universal gas constant, 8.314 J/(mol K). In this article, we use water as an example and show how boiling points can be measured by HPDSC at different pressures and how the enthalpy of vaporization can be calculated from this data.
The measurements were performed using an HP DSC 1 equipped with an FRS 5 sensor and a PC10 pressure controller. The evaporation of a liquid leads to DSC curves with broad endothermic peaks. The evaporation must be suppressed as far as possible to obtain a sharp vaporization peak and thus allow...
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The vapor pressure curves of liquids can be easily determined by HPDSC. To do this, the boiling point of the liquid is measured in crucibles sealed with a pierced lid at different pressures.
The vaporization enthalpy can also be determined from such measurements using the Clausius–Clapeyron equation. Using water as an example, we have shown that the resulting vapor pressure curve and the vaporization enthalpy agree well with the literature values.
Measurement of Vapor Pressure Curves and the Enthalpy of Vaporization of Liquids by HPDSC | Thermal Analysis Application No. UC392 | Application published in METTLER TOLEDO Thermal Analysis UserCom 39