A Soxhlet extractor works by boiling a solution that has a solute of limited solubility in a percolator, then cooling and collecting the condensate in a reservoir from which the concentrated solute can be extracted. A siphon connecting the percolator to the reservoir flushes excess solvent back into the reservoir.
The Soxhlet extractor was invented in 1879 by Franz von Soxhlet to extract lipids from solid materials. The extractor only works when the solute being extracted has limited solubility in the solvent in which it is dissolved. This means that the Soxhlet extractor cannot be used to extract ionic salts from polar solvents such as water, because the solubility limit of such salts in water is extremely high.
The material containing the compound to be extracted is loaded in a thimble and placed inside the percolator. An extraction solvent is added to the material if the material is not already dissolved. The percolator is placed on top of a heating element, and a chilled condenser is placed at the top of the extractor. As the extraction solvent heats up, it dissolves some of the solute. The pressure in the percolator continues to build up until the heated solvent refluxes into a reservoir. The excess solvent is flushed back into the percolator, leaving the solute behind in the reservoir.