Vapor pressure measurement

The saturation vapor pressure of a compound characterizes the equilibrium pressure between the vapor of this compound and its condensed phase (liquid or solid) in a closed system. It depends on the temperature and can be modeled by Antoine’s equation. Commonly, the expression “vapor pressure” is also used to designate this equilibrium pressure. At the boiling point of a compound, its vapor pressure is equal to atmospheric pressure.

How to measure vapor pressure?

The saturation vapor pressure of a compound is measured in a closed enclosure containing only the compound to be analyzed. The enclosure is placed under vacuum beforehand in order to remove the air from the vapor phase. This step also makes it possible to degas the compound to be analyzed. The temperature of the enclosure is then regulated to the desired temperature. After equilibrium of the system, the pressure of the gaseous phase is measured and corresponds to the vapor pressure of the compound.

The vapor pressure measurement can be done statically or dynamically (ebulliometer). Another method consists of using an isoteniscope to measure it indirectly with a remote pressure sensor.

The vapor pressure is a quantity often requested by the authorities to determine the risks during the transport as well as for the storage of dangerous and/or flammable materials.

And for the mixtures?

The vapor pressure of a pure compound, liquid at room temperature, is fairly easy to determine. This is greatly complicated in the case of a mixture. In fact, more volatile compounds can pass into the gaseous phase very quickly. It therefore becomes difficult to guarantee the composition of the mixture analyzed. Specific techniques can be implemented.

Measurement limits

When determining vapor pressures as a function of temperature, it is possible that some compounds decompose before reaching the boiling point of the product studied at atmospheric pressure. In this case volatile elements can be emitted and can falsify the measurement. However, this type of behavior makes it possible to determine degradation temperatures. Generally a pressure drift is observed during the measurement, due to the decomposition of certain constituents of the mixture.