Question

What is the purpose of adding sodium carbonate to the water-tea extract?Draw the general reaction equation that results from the addition of the sodium carbonate.

List one advantage and one disadvantage of shaking the separatory funnel during extraction

List two different types of analyses that could be carried out to confirm the recrystallized solid is indeed caffeine

List two different methods besides the one used in this experiment to isolate caffeine from tea.

Answer 1

Answer-

Sodium carbonate is added to the extraction medium to ensure that the acidic components in the tea leaves remain water soluble and that caffeine is the free base.

Sodium carbonate is basic. Tannins are acidic compounds with a high molecular weight that have an –OH directly bound to an aromatic ring. For this add about 10g, of sodium carbonate to your aqueous tea extract. You should notice a distinct fizzing when you add the first of the sodium carbonate, as it reacts with the acidic tannins to release carbon dioxide gas. Simmer the solution for 5 minutes and then boil most of the water off.

(A) Advantage of using shaking the separatory funnel,

1)Ability to separate at least two immiscible liquids from each others.

2) It can be operated easily and rapidly.

3) It is transparent to observe different layers easily.

4) The upper layer or the lower layer can be selectively removed. The upper layer is removed by the stopper and the lower by the stopcock.

(B) The major disadvantage of using shaking the separatory funnel is the separation of phases may be not extremely accurate.

1-Recrystalization Method-Add 5-8 mL of hot acetone to dissolve the crude caffeine and transfer the solution to a 50 mL Erlenmeyer flask for recrystallization. Add a few drops of petroleum ether until you reach the cloud point (caffeine is less soluble in this mixed solvent and is just beginning to precipitate) and then cool the solution.

2-Recrystalization Method-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.

Procedure :- Isolation Of Caffeine From Tea

Reagents Required:

1. Dichloromethane.
2. Anhydrous sodium sulfite.
3. Distilled water.

Materials Required:

1. Tea bags.
2. Beaker (500ml).
3. Hot plate.
4. Separating funnel.
5. Melting point apparatus.
6. Cool the aqueous solution to near room temperature.
7. Continue cooling in an ice box, the tea must be cool (20° C) before coming in contact with dichloromethane (boiling point = 40° C).
8. Extract the solution three times with 30-mL portions of dichloromethane (CH2Cl2). Do not get dichloromethane on your hands.

Tea bags are used as the source of caffeine for this experiment.

Extraction step:

a. The tea solution is poured into a separating funnel and 20ml of dichloromethane is added to it. The mixture will separate into two layers - the top layer is the tea layer and bottom layer is the dichloromethane since it is denser than tea.
b. Remove the funnel from the stand and keep your fingers on the stopper and carefully shake the separating funnel.
c. Vent the separating funnel periodically (every 30 sec) to relieve vapour pressure created inside the funnel.
d. When the contents have been sufficiently shaken place the separating funnel back on the ring stand and let the two layers separate.
e. Drain the bottom layer into a conical flask because now the caffeine is extracted into the dichloromethane layer. Cover the mouth of the conical flask to avoid evaporation of solution.
f. Repeat steps a) through e) twice.  

9.Dry the combined dichloromethane solutions with anhydrous Sodium sulfite. Add about 1 teaspoon of the drying agent until it no longer clumps together at the bottom of the flask. Mix well and leave it for 10 minutes.

10.Decant the dichloromethane into a conical flask (100ml). Evaporate the dichloromethane solvent in a hot water bath.

11.When all the solvent is removed you observe a residue of yellowish green - white crystalline caffeine.

Sublimation step:

a.Take the conical flask containing crystalline caffeine.
b.Sublime the crude caffeine at atmospheric pressure by placing the flask directly on a pre-heated hot plate. Caffeine melts at 238°C and sublimes at 178°C.
c.Collect your sublimed caffeine by keeping a test tube on the mouth of the conical flask.
d.White vapour of caffeine sticking onto the test tube and the walls of the conical flask is observed.
e.Now cool the conical flask.

12.Take a clean watch glass and record its weight in a weigh balance.

13.Now strip off the caffeine from the conical flask and the walls of the test tube into the watch glass using a spatula.

14.Record the weight of the watch glass + caffeine in a weigh balance and then find out the weight of extracted pure caffeine.

15.The melting point of the extracted caffeine is determined using the melting point apparatus.