Volatile versus a Semi-Volatile Compounds

Ask the Expert Question:
What makes a compound a volatile versus a semi-volatile?

Expert Response:
There is no hard and fast rule to determine what is called a volatile compound and what is called semi-volatile. There are a variety of chemical characteristics that influence volatility, but for the most part the definitions that are used in environmental analysis are operational definitions.

The operational definition is whether a compound will purge from aqueous solution in a purge and trap device. If a compound will do so at a high enough rate to be detected and quantified, then it is considered to be volatile. If an analyte is only found in a semi-volatile analysis and is not present in the volatiles analysis, then almost by definition it is not volatile.

There are also, however, some physical chemical characteristics of volatile compounds. The key characteristics are (1) vapor pressure or boiling point, and (2) aqueous solubility. As the vapor pressure increases, a compound’s volatility increases. As its aqueous solubility decreases, it also becomes more volatile from an aqueous solution.

For example, 1,2,4-trichlorobenzene is about the highest boiling compound routinely included in a volatiles analysis. Its boiling point is listed as 214.4 degrees Celsius, and it is quite water insoluble. Nitrobenzene’s boiling point is 210.9 degrees Celsius, but it is more soluble in water due to the polar nature of the nitro group. It does not purge with sufficient efficiency, so it is not considered a volatile.

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