Soxhlet Extractor

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Page Author: Lauren Josey

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What is it?

The Soxhlet extractor is used for liquid-solid extractions when the compound to be extracted has limited solubility in the chosen solvent and the impurities are insoluble.

During the extraction, solvent vapour will flow up the distillation path, into the main chamber and up into the condenser where it will condense and drip down. The solvent will fill the main chamber, dissolving some of the desired compound from the solid sample. Once the chamber is almost full, it is emptied by the siphon, returning the solvent to the round bottom flask to begin the process again. Each time the extraction is repeated, more of the desired compound is dissolved, leaving the insoluble impurities in the thimble. This is how a compound is removed from the sample.

Image user left; Daniele Pugliesi, right user; Elrond. 1) Stirrer bar/anti-bumping granules 2) Extraction pot (round bottom flask) 3) Distillation path 4) Soxhlet Thimble 5) Extraction solid 6) Syphon arm inlet 7) Syphon arm outlet 8) Reduction adapter 9) Condenser 10) Cooling water out 11) Cooling water in

Why use it?

Unlike a traditional extraction method, a small amount of solvent is reused to perform an extraction many times. This means that much less solvent is used in a Soxhlet extraction, making it more time and cost effective. Also, the Soxhlet extractor can run continuously without any further operation, making it an excellent choice for extracting compounds over hours or even days.

Franz Ritter von Soxhlet first invented the apparatus to extract lipids (fats) from milk solids. Now, the Soxhlet extractor is used whenever exhaustive extractions are needed, particularly in the oil and food industries. It is also widely used for extracting bioactive compounds from natural resources which is crucial in environmental analysis of soils and wastes.

How to use it?

The Soxhlet extractor will run continuously once set up correctly:

  • Load the sample material containing the desired compound into the thimble
  • Place the thimble into the main chamber of the Soxhlet extractor
  • Add the chosen solvent to a round bottom flask and place onto a heating mantle
  • Attach the Soxhlet extractor above the round bottom flask
  • Attach a reflux condenser above the extractor, with cold water entering at the bottom and exiting above
  • Now the apparatus is set up, heat the solvent to reflux and leave to extract for the required amount of time
Video showing the use of a Soxhlet extractor (using cotton wool instead of a thimble)


Fun fact

A Soxhlet extractor is based upon the principle of a Pythagorean or ‘greedy’ cup – a cup used as a practical joke. Once filled past a certain point, the liquid inside starts to be drained out through a hole in the bottom. This was Pythagoras’ way of making sure each of his students had the same amount of wine and that anyone being greedy would be punished.

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