 Steam distillation

Steam distillation is co-distillation with water, in which water vapor carries volatile material into a laboratory condenser and receiver. It can recover volatile organic compounds from less volatile material without requiring the isolated compound to reach its own pure-component boiling point. The University of York's teaching apparatus combines material and water in a boiling flask connected to a distillation head, condenser and collection flask.^[ref]

A useful ideal case is liquid immiscibility: two separate liquid phases remain present, and each supplies approximately its pure-component vapor pressure. Their sum reaches the external pressure at boiling:Because both terms contribute, the required temperature can be below either pure liquid's boiling point at that same external pressure. This limit assumes both liquid phases persist and neglects mutual solubility and vapor nonideality.

For invented equilibrium data, let the water and organic partial pressures be $95$ and $5\mathrm{kPa}$ at the boiling temperature. The vapor mole ratio is $5/95$. If the organic compound has molar mass $150\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and water is approximated by $18\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, the condensed mass ratio isThis is an equilibrium model, not an extraction yield.

A 1964 methanol dehydrogenase study used steam distillation before a formaldehyde assay. Its authors explicitly left unresolved whether the product had been free or bound before separation.^[ref] Recovering a volatile product and establishing its earlier molecular state are different experimental claims.