Plants and other natural materials are sources of many chemicals. Sometimes you want to isolate a single compound from the thousands that may be present. Here is an example of how to use solvent extraction to isolate and purify caffeine from tea. The same principle may be used to extract other chemicals from natural sources.
Caffeine From Tea: Materials List
- 2 tea bags
- 0.2 M NaOH (sodium hydroxide)
- Celite (diatomaceous earth - silicon dioxide)
- Diethyl ether
- 2-propanol (isopropyl alcohol)
Extraction of Caffeine:
- Open the tea bags and weigh the contents. This will help you determine how well your procedure worked.
- Place the tea leaves in a 125-ml Erlenmeyer flask.
- Add 20 ml dichloromethane and 10 ml 0.2 M NaOH.
- Extraction: Seal the flask and gently swirl it for 5-10 minutes to allow the solvent mixture to penetrate the leaves. Caffeine dissolves in the solvent, while most of the other compounds in the leaves do not. Also, caffeine is more soluble in dichloromethane than it is in water.
- Filtration: Use a Buchner funnel, filter paper, and Celite to use vacuum filtration to separate the tea leaves from the solution. To do this, dampen the filter paper with dichloromethane, add a Celite pad (about 3 grams Celite). Turn on the vacuum and slowly pour the solution over the Celite. Rinse the Celite with 15 ml dichloromethane. At this point, you may discard the tea leaves. Retain the liquid you have collected -- it contains the caffeine.
- In a fume hood, gently heat a 100-ml beaker containing the washings to evaporate the solvent.
Purification of Caffeine: The solid that remains after the solvent has evaporated contains caffeine and several other compounds. You need to separate the caffeine from these compounds. One method is to use the different solubility of caffeine versus other compounds to purify it.
- Allow the beaker to cool. Wash the crude caffeine with 1 ml portions of a 1:1 mixture of hexane and diethyl ether.
- Carefully use a pipette to remove the liquid. Retain the solid caffeine.
- Dissolve the impure caffeine in 2 ml dichloromethane. Filter the liquid through a thin layer of cotton into a small test tube. Rinse the beaker twice with 0.5 ml portions of dichloromethane and filter the liquid through the cotton to minimize the loss of caffeine.
- in a fume hood, heat the test tube in a warm water bath (50-60 °C) to evaporate the solvent.
- Leave the test tube in the warm water bath. Add 2-propanol a drop at a time until the solid dissolves. Use the minimum amount required. This should be no more than 2 milliliters.
- Now you can remove the test tube from the water bath and allow it to cool to room temperature.
- Add 1 ml of hexane to the test tube. This will cause the caffeine to crystallize out of solution.
- Carefully remove the liquid using a pipette, leaving the purified caffeine.
- Wash the caffeine with 1 ml of a 1:1 mix of hexane and diethyl ether. Use a pipette to remove the liquid. Allow the solid to dry before weighing it to determine your yield.
- With any purification, it's a good idea to check the melting point of the sample. This will give you an idea of how pure it is. The melting point of caffeine is 234 °C.
Another way to extract caffeine from tea is to brew tea in hot water, allow it to cool to room temperature or below, and add dichloromethane to the tea. The caffeine preferentially dissolves in dichloromethane, so if you swirl the solution and allow the solvent layers to separate. you will get caffeine in the heavier dichloromethane layer. The top layer is decaffeinated tea. If you remove the dichloromethane layer and evaporate the solvent, you will get slightly impure greenish-yellow crystalline caffeine.
There are hazards associated with these and any chemicals used in a lab procedure. Be sure to read the MSDS for each chemical and wear safety goggles, a lab coat, gloves, and other appropriate lab attire. In general, be aware the solvents are flammable and should be kept away from open flames. A fume hood is used because the chemicals may be irritating or toxic. Avoid contact with sodium hydroxide solution, as it is caustic and can cause a chemical burn on contact. Although you encounter caffeine in coffee, tea, and other foods, it is toxic in relatively low doses. Don't taste your product!