Chromatography
How paper chromatography works
separates the substances in a mixture, such as the dyes in an ink, and helps identify them.
There are two phases:
- the — the paper, which doesn't move
- the — the solvent, which moves up the paper
Each substance is attracted to the two phases by different amounts. A substance that is more attracted to the solvent (more ) travels further up the paper. So the substances separate into different spots.
A pure substance gives one spot in every solvent. A mixture gives two or more spots.
Key terms
Rf values
Measure both distances from the pencil start line: to the centre of the spot, and to the (how far the solvent reached).
Example: a spot moves 3.2 cm and the solvent front moves 8.0 cm.
Rf = 3.2 ÷ 8.0 = 0.40
An Rf value has no units and is always less than 1. Each compound has its own Rf value in a particular solvent, so you can identify an unknown substance by comparing its Rf with those of known substances in the same solvent.
Key terms
Required practical: paper chromatography
- Draw a pencil line about 2 cm from the bottom of the chromatography paper. Pencil won't dissolve in the solvent; ink would.
- Put small spots of each ink or dye on the line, spaced apart.
- Stand the paper in a beaker of solvent, with the solvent below the pencil line so the spots don't dissolve into it. Cover with a lid.
- Remove the paper when the solvent is near the top, and in pencil.
- Let it dry, measure the distances and calculate the values.
- Compare them with known substances to identify the dyes.
Hazard: some solvents are flammable — keep them away from flames.
Better results: use small, concentrated spots and keep the lid on so the solvent doesn't evaporate.