Extraction of Caffeine from Tea Leaves: Principles, Procedures, and Calculations

Extraction of Caffeine from Tea Leaves

Background

Caffeine (C8H10N4O2) is a naturally occurring, water-soluble alkaloid found in the leaves, seeds, or fruits of several plants, including coffee and tea. It is well known for its stimulant effects on the central nervous system. In general, fresh green tea leaves contain about 1.5% caffeine by mass.

• Chemical Structure: Caffeine is a purine alkaloid with the following structure:

Figure 1. Molecular structure of caffeine, C8H10N4O2

Principles of Liquid-Liquid Extraction

Liquid-liquid extraction is a separation technique where a solute transfers from one solvent to another. In this experiment, the immiscible solvents are water and dichloromethane (DCM, also called CH2Cl2). The process relies on the distribution coefficient (KD), which is the ratio of the solute's concentration in the two phases at equilibrium.

• Distribution Coefficient (KD): $K_D = \frac{C_{DCM}}{C_{water}}$

• Partitioning: Mixing the solvents allows caffeine to distribute between the layers based on its solubility.

• Example Calculation: If 250 mL of tea solution contains 10 grams of caffeine, and 30 mL of DCM is used, the amount of caffeine extracted into the DCM layer can be calculated as:

$K_D = \frac{x}{10 - x} = 7.8$

Solving for x (amount in DCM):

$x = \frac{7.8 \times 10}{7.8 + 1} = 9.66$ grams

Thus, 9.66 grams of caffeine are extracted into the DCM layer, while 0.34 grams remain in the aqueous layer.

Crystallization of Caffeine

After extraction, caffeine is purified by crystallization. Crystallization occurs when a solution becomes supersaturated, either by cooling or concentration, leading to nucleation and crystal growth. This process is widely used in industry for purification.

• Supersaturation: Required for crystal formation.

• Nucleation: Initial formation of small crystals.

• Crystal Growth: Enlargement of crystals from the supersaturated solution.

Objectives

• To extract crude caffeine from tea leaves.

• To purify crude caffeine by means of crystallization.

• To determine the percentage yield of caffeine.

Materials and Equipment

• Tea leaves in teabags

• Watch glass

• Separatory funnel

• Bunsen burner

• Dropper

• Wooden stick

• Filter papers

• Ice

Procedures

1. Carefully open 3 teabags and weigh the combined mass of the tea leaves. Return the leaves to the bags and secure them to prevent spillage.

2. Boil approximately 100 mL of water in a beaker and steep the teabags for 5 minutes. Cover and mix gently. If bags burst, filter the extract through paper into a flask.

3. Cool the flask in running tap water for 3 minutes, then in an ice bath to room temperature.

4. Add 1 gram of Na2CO3 to the tea extract.

5. Transfer the tea extract to a separatory funnel, ensuring the stopcock is closed.

6. Add 20 mL of CH2Cl2 (DCM) to the funnel and shake gently, releasing pressure regularly.

7. Allow the layers to separate. If needed, break up bubbles using a glass stirrer.

8. Drain the DCM (lower) layer into a clean beaker.

9. Repeat extraction with two more portions of 20 mL DCM.

10. Weigh a dry, clean 250-mL beaker on an analytical balance.

11. Separate the solution from the solid Na2SO4 and transfer to the 250-mL beaker.

12. Heat DCM under the fume hood to evaporate the solvent (DCM is a potent carcinogen; avoid inhalation).

13. Weigh the beaker and determine the weight of crude caffeine by difference.

14. Scrape caffeine from the beaker walls and transfer as needed.

Key Equations

• Distribution Coefficient: $K_D = \frac{C_{DCM}}{C_{water}}$

• Percentage Yield: $\text{\% yield} = \frac{\text{weight of crude caffeine}}{\text{weight of tea leaves}} \times 100\%$

Hazards and Toxicities Table

Substances Involved	Hazards and Toxicities	Precautions and First Aid
DCM (Dichloromethane)	Carcinogenic; inhalation of vapors may cause dizziness, headache, nausea, vomiting	Use under fume hood; avoid inhalation; seek medical attention if symptoms occur
Na2CO3 (Sodium carbonate)	Irritant to skin and eyes	Wear gloves and goggles; rinse with water if contact occurs
Tea extract	Generally safe; may stain surfaces	Clean spills promptly
Hot water/steam	Burn hazard	Handle with care; use tongs or heat-resistant gloves
Ice	Cold burn hazard	Handle with care

Guide Questions and Academic Context

• Layer Identification: The DCM layer is denser and forms the lower layer in the separatory funnel.

• Role of Na2CO3: Sodium carbonate neutralizes acidic tannins, improving caffeine extraction.

• Tannins: Polyphenolic compounds in tea that can interfere with extraction; removed by Na2CO3.

• Multiple Extractions: Washing with several portions of DCM increases yield by maximizing caffeine transfer.

• Distribution Coefficient Application: Theoretical yield can be calculated using $K_D$ and compared to experimental results.

• Improving Extraction: Use fresh solvent portions, ensure thorough mixing, and minimize losses during transfer.

Example Calculation

• If 10 grams of caffeine are present in 250 mL tea solution and $K_D = 7.8$, then using 30 mL DCM:

$K_D = \frac{x}{10 - x} = 7.8$

$x = \frac{7.8 \times 10}{7.8 + 1} = 9.66$ grams

Thus, 9.66 grams are extracted into the DCM layer, and 0.34 grams remain in the aqueous layer.

References

• Berk, Z. (2018). Chapter 11 - Extraction. In Z. Berk (Ed.), Food Process Engineering and Technology (3rd Edition), pp. 289-310: Academic Press.

• Berk, Z. (2018). Chapter 14 - Crystallization and dissolution. In Z. Berk (Ed.), Food Process Engineering and Technology (3rd Edition), pp. 353-371: Academic Press.

• Garcia, C. (2018). Laboratory Experiments in Organic Chemistry. Academic Press.

Additional info: Academic context and explanations have been expanded for clarity and completeness. Safety information and chemical properties have been inferred from standard laboratory practice.