1879

Soxhlet Extractor

Franz Ritter von Soxhlet (1848–1926)

The Soxhlet extractor is an ingenious solution to a tedious problem: How do you wash a slightly soluble material (e.g., oil of peppermint) out of a mass of solid impurities (e.g., a handful of peppermint leaves)? Or if you’re an organic chemist, how do you extract your reaction product from a mass of inorganic salts and by-products? One rinse is not enough, but rinsing over and over isn’t a good use of a person’s time (and wastes plenty of solvent, too). If only there was a way to recycle that solvent, sending it through fresh every time and leaving the extracted material behind . . .

Enter German chemist Franz Ritter von Soxhlet, who (sadly, or not) is remembered only for this piece of glassware, although he was the first to propose pasteurization of milk and other liquids. Soxhlet spent much of his career in agricultural chemistry, and his original goal for his invention was to extract lipids from milk. Its use has expanded, however, throughout the chemistry lab. Here’s how it works: You put your solid in a porous pressed-paper “thimble” and put the whole extractor on top of a flask of solvent. The solvent is brought to a boil (or “to reflux,” as chemists say), and the vapors are condensed back down into the sample. Once the thimble fills up, the function of the little curved tube on the side becomes clear. When the level of liquid goes higher than the top of the curved tube, the resulting siphoning action drains the chamber around the thimble, which starts filling up again with freshly distilled solvent for another cycle. Meanwhile, the extract concentrates in the flask below. As long as the material you’re extracting is stable in the boiling solvent, the apparatus can be left running for days, while you go do something more useful.

Actually, what many chemists tend to do is watch the Soxhlet extractor itself. It’s fun to try to guess how far it can get before siphoning off, and, in a large extractor, the siphoning action is fairly dramatic. Watching something else do your work for you somehow never gets old.

Modern Soxhlet extractors, like the ones pictured here, are a marvel to watch.