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Vapor Pressure

By Little Pro on 2016-01-13 Views:  Update:2017-01-18

Vapor pressure or equilibrium vapour pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature (usually 25 Celcius degress) in a closed system. The unit of vapor pressure is generally in atm (standard atmosphere) or kPa or bar.

1 atm = 101.325 kPa = 1.01325 bar

Examples: Water (2.4kPa), Acetone (30kPa), Benzene (14kPa).

You can often find vapor pressure in the section 9 of a safety data sheet (SDS).

Evaporation Rate

The evaporation rate is a measure of how quickly a substance becomes a vapour at normal room temperature. Usually, the evaporation rate is given in comparison to certain chemicals, such as butyl acetate, which evaporate fairly quickly.

For example, the rate might be given as "0.5 (butyl acetate=1)." This means that, under specific conditions, 0.5 grams of the material evaporates during the same time that 1 gram of butyl acetate evaporates. Often, the evaporation rate is given only as greater or less than 1, which means the material evaporates faster or slower than the comparison chemical.

Regulatory Implications of Vapor Pressure & Evaporation Rate

Vapor pressure measures a chemical’s volatility. It is often used by authorities and experts to predict the concentration of a chemical substance in air, occupational exposure and environmental releases to air. Vapour pressure data is also required as a pre-requisite for animal and environmental studies. It informs whether a substance may be available for inhalation as a vapour and whether occlusive conditions are necessary for dermal studies.

Chemicals with bigger vapor pressure are more likely to enter air and they shall be handled in a way that their emission to air can be minimized. Workers shall wear respiratory protection equipment when handling highly volatile substances (ether, acetone, etc). Volatile substances which are also toxic and persistent can have huge adverse effects on human health or the environment.

Higher vapor pressures and high evaporation rates also lead to lower flash points and higher flammability. Extra attention shall be paid to fire prevention for substances with higher vapor pressure or high evaporation rates.

Under REACH, this study does not need to be conducted if the melting point of a substance is above 300 °C. If the melting point is between 200 °C and 300 °C, a limit value based on measurement or a recognized calculation method is sufficient.

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