In the Laboratory
Soxhlet Extraction of Caffeine from Beverage Plants D. J. Adam and J. Mainwaring Department of Science and Mathematics, Halton College of Further Education, Kingsway, Widnes, Cheshire WA8 7QQ, U.K. Michael N. Quigley* Department of Chemistry, Richard King Mellon Hall of Science, Duquesne University, Pittsburgh, PA 15282 We have implemented an extraction procedure as a supplementary exercise to more labor-intensive experiments in organic chemistry undergraduate classes. Provided that safety precautions are observed, the two-hour extraction can be performed safely and efficiently. Doubling up of laboratory procedures has been addressed elsewhere (1). It is most appropriate in environments in which groups of students perform different experiments in the same class and are rotated to new experiments in subsequent classes (2, 3). Hence, only one Soxhlet extraction apparatus is required and the amount of hazardous waste material is small. If necessary, the experiment can be stopped at a convenient break point. In addition to various analytical methods (4–7), most of which are based on either thin-layer chromatography (8–13) or high-performance liquid chromatography (9, 14–20), several procedures for the extraction of caffeine from tea, coffee, and soda drinks have already been reported in this Journal (21–26). Together with a method for extraction of theobromine from cocoa (27), all have been based on fairly simple procedures. However, none of them takes into account the very varied nature of beans, nuts, and leaves. A simple extraction may work perfectly well for one type of sample, but not for another. Soxhlet extraction overcomes doubts about extractive efficiency by using a powerful combination of heat, solvent, and reflux, and from a quantitative point of view is much preferred over simpler procedures (28). Soxhlet extraction’s origins are old enough to accord it “classical” status, but it is often time consuming, and this probably accounts for the scant attention accorded it in the Journal. To compound its apparent fall from grace, Soxhlet extraction has, in recent years, been superseded in many applications by less tedious and more advanced technologies (29–33). Despite its protracted nature, Soxhlet extraction is well suited to the undergraduate laboratory, illustrating, as it does, many of the principles of extractive processes in general. Experimental Procedure CAUTION: The entire procedure should be performed in a fume hood, and gloves and safety glasses should be worn at all times. Exercise care in the use of ethanol (flammable), sulfuric acid (corrosive), potassium hydroxide (corrosive), and dichloromethane (narcotic). Note that others have used less hazardous extractants to good effect (26). Crush a quantity of coffee beans or instant coffee (substitute tea leaves, maté leaves, etc., as appropriate) to a fine powder using a mortar and pestle. Accurately weigh out approximately 10 g of powder into a Soxhlet extraction thimble, and place this in the extraction apparatus. Perform the following in a fume hood. Use a 250mL round-bottom flask as a reservoir, and add to this 100 mL of ethanol and a few anti-bumping granules. Use *Corresponding author. Current address: Bradfield Hall, Cornell University, Ithaca, NY 14853.
a heating mantle to reflux the mixture for 1.5–2 h. If required, the remainder of the procedure can be left until the next class period. Store the contents of the flask in a sealed container. Allow the extract solution to cool before adding it to a 10% (w/v) aqueous solution of magnesium oxide (this causes tannins to form water-insoluble salts). Evaporate the ethanol using a steam bath until a brown pasty mass is obtained. Add approximately 125 mL of distilled water, and then boil this mixture on a steam bath for 30 min. Filter through a cone of coarse filter paper (for example, Whatman No. 1). Add 10 mL of 0.1 M sulfuric acid solution to the filtrate and boil this until the volume is reduced to half. Allow to cool. Transfer to a separating flask and add 12 mL of dichloromethane. After shaking, remove the yellow layer and repeat the procedure two more times with fresh volumes of dichloromethane. Add 8 mL of 0.1 M potassium hydroxide solution to the combined extract contained in a new separating flask. This should be enough to remove the yellow color, but add more if required. Remove the organic layer and wash the base solution layer twice with 5-mL volumes of dichloromethane. Add these volumes to the earlier extracts. In a preweighed beaker, evaporate the combined extracts over a steam bath to approximately 10 mL. Eventually, a white crystalline precipitate should be obtained. Recrystallize from ethanol. Check the melting point of the dried crystals. Caffeine has a melting point within the range 234–239 °C, and the expected yield—depending upon the coffee—is approximately 2%. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
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Vol. 73 No. 12 December 1996 • Journal of Chemical Education
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