Extraction of Caffeine from Tea Leaves

Introduction:

Caffeine is a stimulant found in tea, coffee, cocoa, OTC medication, and cola drinks.  It stimulates the brain and the central nervous system, so that a person feels less tired and more alert.  It is currently estimated are that over 80% of the adults in the world regularly ingest caffeine, thus making it the number one psychoactive drug in the world! 

Like other drugs, caffeine produces both physical and psychological dependencies.  An overdose can result in rapid heartbeat, hallucinations, and stomach ulcers.   Withdrawal symptoms include headache, irritability, and palpitations.

The amount of caffeine in coffee or tea depends on a number of factors, including the variety of coffee bean or tea leaf, where it is grown, the particular coffee "grind " or tea leaf cut, and the method of brewing.  The following table shows the approximate caffeine content of various beverages:


 Milligrams of Caffeine
Beverage:
 Average per serving: Range: Per ounce:
Coffee (5 oz. cup) 80  40 - 170 16.00 
Cola (12 oz. can) 45  30 - 60 3.75 
Black Tea (one tea bag) 40  25 - 110 5.00 
Oolong Tea (one tea bag) 30  12 - 55 3.75 
Green Tea (one tea bag) 20  8 - 30 2.50 
White Tea (one tea bag) 15  6 - 25 2.00 
Decaf Tea (one tea bag) 2  1 - 4 0.50 
Herbal Tea (one tea bag) 0  0 0.00 

Purpose:

The purpose of this lab is to determine the amount of caffeine in tea.  To accomplish this, you will need to isolate pure caffeine from the hundreds of other compounds in bulk tea leaves.  The major component of tea is cellulose which is a polymer of glucose.  While cellulose is virtually insoluble in water, caffeine is water soluble.  You will take advantage of this fact to separate it from the tea leaves using hot water.  While this solid/liquid extraction does indeed extract most of the caffeine from the leaves, it also extracts many other compounds such as tannins (very large polyphenols).

To isolate the caffeine from these other compounds, you will use liquid/liquid extraction.  This procedure involves the use of two mutually immissible solvents of different polarities, typically water and an organic solvent.  They are placed in a separatory funnel and shaken to ensure intimate mixing.  The solvents are chosen so that the compound of interest, caffeine in this case, preferentially dissolves into one solvent while the other compounds dissolve into the other solvent.  Since the two solvents are immissible, two layers will form, which allows for easy separation.

After the layer containing the caffeine has been separated, it is washed with 10% sodium hydroxide to help remove any acids or polyphenols that were extracted along with the caffeine.  The solvent is then dried (magnesium sulfate or similar drying agent) to remove trace amounts water.  The dried solvent is then removed by distillation and the crude caffeine is isolated.  The crude caffeine can be further purified using recrystallization or sublimation.

Procedure:

Solid/Liquid Extraction:
  1. Obtain 2 tea bags and weigh their contents into a 250 mL beaker.
  2. Add 75 mL of distilled water and a magnetic stir bar to the 250 mL beaker.  Set for a moderate rate of stirring (avoid splashing and splattering).
  3. Cover the beaker with a watch glass and maintain a gentle boil (add water as needed to replace evaporation loses) for 10 minutes.
  4. Allow the mixture to fully cool to room temperature.
  5. Add approximately 26 g of sodium chloride and 1 g of sodium carbonate to the mixture and thoroughly stir.
  6. Filter the mixture through a Büchner filter.  Be sure to clean and rinse the filter flask before you use it because it is the liquid that we are interested in.

Liquid/Liquid Extraction:
  1. Prepare you 125 mL separatory funnel by rinsing it with water and acetone.  Use a minimal amount of silicon grease to lubricate the ground glass stopper.
  2. Pour the water extract into the separatory funnel (Make sure the stop cock is closed!)
  3. Add 15 mL of 1-propanol to the separatory funnel and insert the glass stopper.
  4. Invert the separatory funnel and open the stop cock to vent any vapors that may have built up.  Repeat this step 10 times.  However, do not shake too vigorously since this may result in the formation of an emulsion.
  5. Set the separatory funnel in a ring stand and allow the two layer to fully separate.
  6. Remove the glass stopper and open the stop cock to remove the aqueous layer.  Be carefully to fully remove the aqueous layer without allowing any of the organic layer to escape.
  7. Transfer the organic layer to a beaker.
  8. Pour the aqueous layer back into the separatory funnel and repeat Step 3-6 with a fresh 15 mL portion of 1-propanol.
  9. Combine the two portions of 1-propanol back into the separatory funnel.
  10. Add 25 mL of 10% sodium hydroxide solution to the funnel.
  11. Invert the separatory funnel and open the stop cock to vent any vapors that may have built up.  Repeat this step 10 times.
  12. Remove the glass stopper and open the stop cock to remove the aqueous layer.  Be carefully to fully remove the aqueous layer without allowing any of the organic layer to escape.
  13. Transfer the organic layer to a clean, dry, 100 mL beaker.
  14. Add a small amount of granular anhydrous sodium sulfate to dry the organic layer (remove any water that may have dissolved in the 1-propanol) and stir.  Continue adding sodium sulfate until it swirls like a 'snow globe'.
  15. Transfer the dried 1-propanol to the sublimation apparatus and add a boiling chip.
  16. Evaporate the solvent on a hot plate to obtain your crude caffeine.  Be careful not to over heat it because the caffeine could sublime away.

Vacuum Sublimation:

While caffeine can be purified by recrystallization, we will take advantage of its unique phase diagram to purify it by reduced pressure sublimation.



As you no doubt remember, a phase diagram shows the three state of matter resulting from various combinations of temperature and pressure.  The vast majority of compounds when heated progress from solid to liquid and finally to gas.  The caffeine phase diagram shows that if we heat solid caffeine at atmospheric pressure, it will follow this pattern and will melt before boiling (blue line).  Unfortunately, this requires enough heat that some of the caffeine decomposes, and some of it reacts with other compounds to create undesired by products.  Both of these processes reduce the yield of caffeine.  However, if we reduce the pressure enough, we can see that caffeine will actual sublime (green line).  This happens at much reduced temperature and so avoids both of the problems associated with atmosphere pressure heating.

You will purify your crude caffeine using a simple vacuum sublimation apparatus similar to the one in the figure below.  It consists of a 50 mL or 100 mL filter flask  which has a cold finger (test tube filled with ice) inserted through a rubber stopper:


Your instructor will show you how to construct it, however, the general process is shown below:


Assemble the vacuum sublimation apparatus and place it on the hot plate.
 Start heating at a setting of "4".  The sublimation should be complete in 10-15 minutes.


After the sublimation is complete, remove the hot plate and allow the filter flask to cool to room temperature.  Leave the vacuum line attached while cooling.
 After cooling, remove the vacuum line from the apparatus (DO NOT turn off the water first).  Then carefully remove the cold finger from the apparatus and scrape its contents into a beaker.

Remember:  because caffeine sublimes so readily, you must seal your melting point tube to determine the proper melting point.

Results:

Calculate the percent of caffeine in the brand of tea leaves you used.

(Updated 9/26/03, C.R. Snelling)