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Analytical chemistry

Caffeine extraction from tea and UV analysis

Brew, extract and quantify caffeine: liquid–liquid extraction, phase separation, dilution to the working range, absorbance at 272 nm and a recovery calculation.

Learning objective

Practise liquid–liquid extraction and phase separation, then quantify the extracted caffeine by UV absorbance and report percent recovery.

Safety and hazards

  • Dichloromethane is a suspected carcinogen and CNS depressant (H315, H319, H336, H351). Use only in a fume hood.
  • Never seal a warm separating funnel; vent frequently to release pressure.
  • Anhydrous sodium sulfate dust is an irritant.
  • Collect all halogenated solvent waste separately.

Reagents and materials

  • Tea leaves — 5.00 g
  • Water (deionised) — 100 mL
  • Sodium carbonate — 2.0 g
  • Dichloromethane — 3 × 20 mL
  • Anhydrous sodium sulfate — ≈2 g
  • Caffeine standard (20.0 mg/L)for the reference absorbance
  • Separating funnel (250 mL), quartz cuvette, UV spectrophotometer

Procedure

1
Extract

Boil 5.00 g of tea leaves in 100 mL of deionised water for 15 minutes, then filter the hot brew and let it cool.

  • 5.00 g Tea leavesReagent
  • 100 mL Water (deionised)Solvent
Temperature:
100 °C
Duration:
15 min
2
Add reagent

Dissolve 2.0 g of sodium carbonate in the cooled brew to keep tannins deprotonated and water-soluble.

  • 2.0 g Sodium carbonateAdditive
pH:
≈10
3
Extract

Transfer to a 250 mL separating funnel and extract with 20 mL of dichloromethane. Swirl gently for 1 minute, venting often. Let the phases separate and drain the lower organic layer. Repeat twice more.

  • 3 × 20 mL DichloromethaneSolvent
Duration:
1 min per extraction

Equipment: 250 mL separating funnel

Dichloromethane (ρ = 1.33 g/mL) is the lower layer. Swirl rather than shake to avoid an emulsion.

4
Filter

Dry the combined organic extracts over anhydrous sodium sulfate, filter, and evaporate the solvent on a rotary evaporator at 40 °C.

  • ≈2 g Anhydrous sodium sulfateDrying agent
Temperature:
40 °C

Equipment: Rotary evaporator

5
Mass

Weigh the crude caffeine residue and dissolve it in water to exactly 100.0 mL in a volumetric flask.

Solution preparation calculator

48.0 mg in 100.0 mL = 480 mg/L stock

c = m / V

Crude residue mass
48.0 mg
Final volume
100.0 mL
6
Dispense

Dilute the stock 1:20 — 5.00 mL of stock into a 100.0 mL volumetric flask — to reach the linear UV range.

Dilution calculator

5.00 mL of stock to 100.0 mL gives 24.0 mg/L

C₁V₁ = C₂V₂

Stock concentration
480 mg/L
Final volume
100.0 mL
Target concentration
24.0 mg/L
7
Absorbance

Measure the absorbance of the diluted sample at 272 nm against a water blank, in a 1.00 cm quartz cuvette.

Wavelength:
272 nm

Beer–Lambert calculator

A = 0.486 → 23.8 mg/L in the diluted sample

× 20 dilution factor = 476 mg/L in the stock = 47.6 mg caffeine recovered

A = ε · c · l

Absorbance at 272 nm
0.486
Path length
1.00 cm
Standard
20.0 mg/L, A = 0.408

Equipment: UV spectrophotometer, 1.00 cm quartz cuvette

8
Custom measurement

Compare the caffeine found with the amount expected from the tea mass and report percent recovery.

Recovery calculator

79.3 % recovery

47.6 mg found against 60.0 mg expected (≈1.2 % caffeine in 5.00 g tea)

recovery % = (found / expected) × 100

Caffeine found
47.6 mg
Expected
60.0 mg

Expected result

  • 40–55 mg of crude caffeine from 5.00 g of black tea.
  • Absorbance maximum near 272 nm.
  • Typical recovery 70–85 %; losses are mostly emulsion and residual aqueous phase.

Calculators used in this protocol

Dilution

C1 × V1 = C2 × V2

Solve C1 × V1 = C2 × V2 for any missing variable.

Beer–Lambert

A = ε × l × c

Solve A = ε × l × c for absorbance, concentration, path length or ε.

Spike recovery

Recovery % = (spiked − unspiked) / added × 100

Recovery percentage from unspiked, spiked and added amounts.

Percentage solutions

% = (solute / total) × 100

% w/v, % v/v and % w/w in both directions.

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