1854

Separatory Funnel

The separatory, or “sep,” funnel is a common sight in any organic chemistry lab, and the principle behind it is used constantly in every part of the science. As anyone who has seen a mixture of oil and water has realized, not all liquids can mix with one another. When you dissolve a sample into an oil-and-water mixture, some of its components will be more soluble in the water, and some will be more soluble in the oil. This provides a quick and reliable way to separate a mixture into its “hydrophilic” (water-loving) and “hydrophobic” (water-fearing) parts. Underlying this are the same principles that are found in most kinds of chromatography.

A sep funnel allows the two liquid layers to be shaken together and mixed, but the key innovation is that when they’ve separated out again, the lower layer can be drained out of the funnel and into another flask. This is such a convenient method to clean up crude reaction mixtures of all kinds that extraction into two layers is second nature to most working organic chemists, and has been for hundreds of years.

The modern sep funnel has been around since roughly 1854 but versions were found earlier in the nineteenth century as well. In fact, even the alchemists used to use tall, thin variants of regular funnels to allow immiscible liquids to be drained into separate containers, and there are containers from much earlier in history that may well have been used for the same purpose.

The split between water-soluble and oil-soluble has been part of the science for a very long time. Most of the time in organic chemistry, the lower layer is the watery one, since water is denser than most organic solvents. But when you’re using a chlorinated solvent (such as dichloromethane) it wins the density competition, and the water floats on top. Beginning chemistry students using that solvent often mistakenly pour away their desired products before they learn to take heed, but it’s the sort of mistake you tend to only make once!

A row of separatory funnels. This classic design has hardly changed over the years, except for the plastic fittings (which used to be glass and sometimes became permanently stuck).