Written by Jennifer
The experiment was carried out to extract caffeine from tea leaves (Isolation of Caffeine). The experimental procedure was carried out in four weeks. In the first week, two tea bags were used together with anhydrous sodium carbonate to make a solution with water. The tea solution was allowed to cool. In the second week, dichloromethane was added to the solution in three portions in a separatory funnel which formed two layers and the aqueous caffeine layer was separated. The extracted liquid was dried using anhydrous sodium sulfate and then decanted into a beaker. This solution was then concentrated by boiling to dryness to obtain crude caffeine. In the third week, the crude caffeine was purified by recrystallization and the caffeine crystals were collected. In the fourth week the mass of the purified caffeine was measured after air drying it for several days.
- Physical Properties of Components:
|Component||Molecular Weight (g/mol)||Melting Point (ºC)||Boiling Point (ºC)||Density (g/mL)||Refractive Index (η)|
- Experimental Procedure
Two tea bags were taken. They were stapled a few times to ensure that they were secure and do not break open during the process. 5.499g of sodium carbonate were weighed and added to a 125mL Erlenmeyer flask. 50mL water was measured using a graduated cylinder and added to the flask to dissolve the sodium carbonate. The tea bags were then added in the flask making sure that they lie flat in the bottom of the flask. The mouth of the Erlenmeyer flask was covered with a small watch glass and it was placed on the electric hot plate and heated gently for about 30 minutes. During the heating process, the contents of the flask started boiling vigorously at one point so the flask was removed with a pair of tongs from the hot plate to let it cool down a little. After some time the flask was placed back on the hot plate and continued heating. After 30 minutes the flask was removed from the hot plate, covered with Parafilm and allowed to cool down.
An iron ring was clamped to a stand. A clean separatory funnel was assembled in the iron ring. The aqueous tea solution was poured directly into a 125mL separatory funnel with the help of a glass funnel making sure that the stop clock was closed and the solution did not leak. 10mL of water was measured using a graduated cylinder and poured into the flask containing the tea bags. The water was used to further wash the tea bags and a glass rod was used to press them against the side of the flask making sure not to break them open. The washings were also transferred into the separatory funnel. The caffeine was extracted from the aqueous tea solution with three separate 10mL portions of dichloromethane. 10mL of dichloromethane, measured using a graduated cylinder, was poured very slowly into the separatory funnel containing the tea solution. The glass funnel was removed and the separatory funnel was stoppered using a cork. The top of the funnel was held between two fingers and gradually inverted. The stopcock was carefully opened ensuring that the tip of the funnel was aimed away in a safe direction. This released the funnel of any pressure that may have developed from vaporization of dichloromethane which is a volatile solvent. The stopcock was closed and inverted again. It was swirled gently but not vigorously to avoid forming an emulsion. It is important to make sure the two layers mix thoroughly. Swirling increases the surface area of interaction between the two liquids so that the caffeine can be extracted more accurately. After swirling, the funnel was placed in the iron ring and allowed to stand for a few minutes so that the organic and aqueous layer separated completely. After the two layers separated, the stopper was removed and the stopcock was opened to allow the denser lower layer drain through into a beaker. The lighter, clearer layer was the organic layer containing caffeine. The brown layer was the aqueous layer. A second portion of dichloromethane was added to the brown layer in the separatory funnel and the process was repeated two more times. The solutions which were extracted in the second and third extractions were combined with the first extraction.
10.001g of anhydrous sodium sulfate, a drying agent, was measured and added to the beaker containing the organic liquid. The liquid was stirred with a glass rod and dehydrated. This organic liquid was then decanted into a clean 100mL beaker containing a boiling stone. This was placed on a hotplate and heated gently. The solution was concentrated to dryness. During the heating process, it was ensured that the hotplate was set to low heat because the boiling point of dichloromethane is only 40ºC and the hood of the fume chamber was closed because dichloromethane is very volatile. Crude caffeine was collected in the bottom of the beaker after heating was complete.
The crude caffeine was purified by recrystallization. A solvent, 50:50 hexane: toluene was chosen in which the caffeine was not readily soluble at room temperature, but soluble at an elevated temperature. The solvent was taken in a beaker and heated. A small amount of the solvent was added to the crude caffeine. The beaker was placed on a hotplate and stirred to help dissolution. The crude caffeine was dissolved in a minimum amount of the solvent. Once it dissolved, the beaker was cooled to room temperature. It was then cooled in an ice bath for a further 3 minutes to promote crystallization. A vacuum filtration apparatus was set up by clamping a 50mL filter flask. A Hirsch funnel was placed in the mouth of the filter flask. A filter paper was taken and weighed. This was folded and placed on the Hirsch funnel to collect the caffeine crystals. The filter flask was connected to a vacuum pump which was then turned on. The solution in the beaker was very carefully poured into the flask. The caffeine crystals were collected on the filter paper. The filter paper was then carefully removed and placed on a watch glass to air dry for a few days.
After air drying the caffeine, the filter paper containing the caffeine crystals were weighed. The mass of the filter paper was then subtracted from this mass to determine the mass of the pure caffeine crystals.
- Mass of filter paper = 0.287g
- Mass of filter paper + caffeine = 0.292g
- Mass of caffeine = (0.292 – 0.287)g = 0.005g
- Mass of tea in the two tea bags = 4.311g
Percentage by mass of caffeine in tea = formula x 100
According to the calculation, the percentage by mass of caffeine in tea was very small. This might be due to some errors during the experimental procedure. One source of error might be when the aqueous tea solution was transferred from the flask to the separatory funnel, not all caffeine was transferred when it was washed with water. Another possible source of error might be during the general extraction procedure in week 2, the separatory funnel was not swirled twice. When dichloromethane was added to the solution, the separatory funnel was not swirled initially as mentioned in the laboratory manual, and was directly inverted. This might result in lesser caffeine to dissolve in the organic layer and thus resulting in the low percentage by mass.