Chromatography (Paper Chromatography)

Principles of Paper Chromatography

Paper chromatography is a method used to separate mixtures of substances. It works because different substances dissolve and move at different rates when carried by a solvent through a stationary phase.

Mobile phase: This is the solvent that moves through the paper, carrying the substances with it.

Stationary phase: This is the chromatography paper itself, which does not move.

Separation happens because substances have different solubilities in the solvent and different tendencies to adsorb (stick) to the paper. Substances that dissolve well in the solvent travel further, while those that adsorb strongly to the paper move less.

For example, when separating coloured inks, some dyes dissolve better in the solvent and travel further up the paper, forming distinct spots.

Method of Paper Chromatography

The basic steps to carry out paper chromatography are:

• Spotting the sample: Use a capillary tube or a fine pipette to place a small spot of the mixture near the bottom of the chromatography paper (about 1 cm from the edge).
• Placing in solvent: Place the paper upright in a container with a shallow layer of solvent (the mobile phase). The solvent level must be below the spot to prevent it dissolving directly into the solvent.
• Developing the chromatogram: The solvent moves up the paper by capillary action, carrying the substances with it. Different substances move different distances.
• Drying and observing spots: When the solvent has nearly reached the top, remove the paper and let it dry. The separated substances appear as spots on the paper.

The pattern of spots is called a chromatogram. The number of spots and their positions help identify the components in the mixture.

Interpreting Chromatograms

To analyse a chromatogram, you can calculate the Rf value for each spot. The Rf value is a ratio that compares how far a substance travels to how far the solvent travels.

The formula is:

$$R_f = \frac{\text{Distance travelled by substance}}{\text{Distance travelled by solvent}}$$

Rf values are always between 0 and 1. Substances with higher Rf values travel further up the paper.

By comparing Rf values with known standards, you can identify substances in a mixture. If a sample produces only one spot with the same Rf value as a pure substance, it is likely pure. Multiple spots indicate an impure or mixed sample.

For instance, if a blue ink produces three spots with different Rf values, it shows the ink is a mixture of three dyes.

Example: If the solvent travels 12 cm and a spot travels 9 cm, the Rf value is 9 divided by 12, which equals 0.75.

Applications of Paper Chromatography

Paper chromatography is widely used to:

• Test purity: Pure substances produce a single spot; mixtures produce multiple spots.
• Identify components: By comparing Rf values with known substances, components in mixtures can be identified.
• Analyse food and drugs: It helps check for additives, contaminants, or to identify ingredients in food and pharmaceutical products.

For example, it can be used to check if a food colouring is pure or a mixture of dyes, or to identify the active ingredients in medicines.