Paper Chromatography from Leaves
Introduction
Paper chromatography is a simple technique used to separate mixtures into their individual components. In plants, different pigments absorb light for photosynthesis, and paper chromatography allows us to separate and identify these pigments. Leaves contain pigments like chlorophyll a, chlorophyll b, xanthophylls, and carotenoids, each with distinct roles in capturing light energy. By grinding leaves in a solvent and allowing the extract to travel through filter paper, the pigments separate based on solubility and affinity for the paper. This experiment provides a hands-on method to observe the hidden variety of pigments present in green leaves, beyond just chlorophyll.
Precautions
- Use fresh, healthy leaves for accurate results.
- Handle solvents like acetone, ethanol, or isopropyl alcohol carefully as they are flammable.
- Avoid touching the chromatography paper with bare hands to prevent contamination.
- Keep the chromatography setup stable to avoid uneven pigment separation.
- Work in a well-ventilated area when using organic solvents.
Materials Required
- Fresh green leaves (spinach or similar)
- Mortar and pestle
- Solvent (acetone or ethanol)
- Chromatography paper/coffee filter paper
- Glass jar or beaker with lid
- Pencil and ruler
- Scissors
- Capillary tube or dropper
- Tape
Science Behind It
Paper chromatography works on the principle of differential solubility and capillary action. Plant pigments have varying chemical structures, which make them differ in their solubility in the solvent and their attraction (adhesion) to the paper.
When the crushed leaf extract is applied as a spot on the chromatography paper and placed in a solvent, the solvent moves upward by capillary action. As the solvent passes the pigments, they dissolve and travel with it. However, different pigments move at different rates depending on two factors:
- Solubility in the solvent – Pigments that dissolve easily in the solvent move further up the paper.
- Affinity for the paper – Pigments that strongly adhere to the cellulose fibers of the paper travel more slowly.
As a result, distinct colored bands appear at different positions:
- Chlorophyll a: blue-green
- Chlorophyll b: yellow-green
- Xanthophylls: yellow
- Carotenoids: orange
This separation shows that photosynthesis involves multiple pigments working together to absorb a broader spectrum of light. Paper chromatography thus reveals the diversity of pigments hidden within what appears to be just “green” leaves.