Paper Chromatography

Paper chromatography separates mixtures of soluble substances using a strip of paper (the paper) and a liquid (the solvent). It is used to identify substances by how far they travel up the paper. Colourless substances can be made visible using a locating agent or by viewing under UV light.

Key idea

There are two parts: the paper is the stationary phase (it stays still) and the solvent is the mobile phase (it moves). Imagine runners in a race up the paper. Substances that prefer the solvent run further; substances that stick to the paper stay lower. This difference separates the mixture.

How to set up

• Draw a baseline in pencil about 1–2 cm from the bottom. Pencil is used because it is insoluble in most solvents.
• Spot the samples onto the baseline: small spots of the unknown and known references.
• Place the paper in a beaker with a shallow layer of solvent. The solvent level must be below the baseline so the spots do not wash off.
• Cover the beaker to reduce evaporation. Let the solvent rise by capillary action.
• Remove and mark the solvent front in pencil before it evaporates. Dry the paper. Use a locating agent/UV if spots are colourless.

Reading a chromatogram

• A pure substance gives one spot; a mixture gives several spots.
• Identify substances by comparing how far they travel (or their Rf values) with known references in the same solvent.

Retention factor, Rf: $R_f = \frac{\text{distance travelled by substance (from baseline)}}{\text{distance travelled by solvent front (from baseline)}}$. Rf has no units and is between 0 and 1. Measure to the centre of each spot.

Real-world uses

• Checking food colourings and inks
• Separating plant pigments
• Forensic comparison of pen inks