Paper chromatography

Chromatography can be used to separate mixtures of coloured compounds. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food.

Simple chromatography is carried out on paper. A spot of the mixture is placed on a pencil line near the bottom of a piece of chromatography paper – the line must be in pencil because pencil is insoluble in water and so will not move as the chromatography progresses. The paper is then placed upright in a suitable solvent, such as water.

As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.

A pencil line is drawn across a sheet of chromatography paper and spots of ink or plant dye are placed along it. The paper is held above a basin containing solvent.

1. Spots of ink or plant dye are placed on a pencil line

Rf values

Different chromatograms and the separated components of the mixtures can be identified by calculating the retardation factor (Rf). The Rf value is worked out by using this equation:

Rf = distance moved by the compound ÷ distance moved by the solvent

The Rf value is a measure of how soluble a particular substance is in a given solvent. The further the substance moves, the larger the value of Rf for that substance, and so the more soluble it is in the solvent that you are using.

The Rf value of a particular compound is always the same if the chromatography has been carried out in the same way. This allows industry to use chromatography to identify compounds in mixtures.

Chromatography can also be done when the different substances in the mixture are colourless. The chromatogram can be exposed to a locating agent, which reacts with the invisible chemicals so that they can be seen.

Chromatography paper next to a measurement scale, showing the distance travelled by the solvent, and a coloured dot showing the distance travelled by the substance.Chromatography paper next to a measurement scale shows distances moved by the solvent and substance