Paper chromatography

Different substances travel through a piece of chromatography paper at different speeds.

Paper chromatography uses this to separate mixtures of soluble substances. It provides clues on the possible identity of the substances in the mixture. These substances are often coloured, such as food colourings, inks, dyes or plant pigments.

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Image caption,
1. Spots of ink or plant dye are placed on a pencil line.
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2. Some of the dye spreads up the paper as it is lowered into the solvent.
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3. The dye spreads up the paper as it absorbs the solvent.

Chromatography phases - Higher tier only

Chromatography relies on two phases, each with different properties. In paper chromatography, one phase is the paper (the stationary phase) and the other phase is the solvent (the mobile phase).

The dissolved substances move at different rates because the strength of their attraction to each phase is different. Substances that have a strong attraction to the paper move slowly and only travel a short distance. Substances that have a strong attraction to the solvent move quickly and travel further.

Interpreting a chromatogram

A paper can by used to distinguish between pure and impure substances.

A pure substance produces a single spot on the chromatogram.
An impure substance produces two or more spots.

It can also help to identify substances by comparing them to known substances. Two substances are likely to be the same if:

they produce the same number of spots with matching colours
the spots travel the same distance up the paper and have the same R_f value (see below)

Results of a chromatogram of three pure substances and brown ink — Figure caption,
In this chromatogram, the brown ink is made of a mixture of red, blue and yellow inks. This is because the spots in the brown ink are at the same heights (and have the same R_f value) as the reference inks.

R_f values - Higher tier only

The R_f value is a measure of the distance the substance travels, relative to the distance travelled by the solvent in the paper. You can identify an unknown substance by comparing its R_f value to the R_f values of a range of known substances.

R_f=\(\frac{distance~travelled~by~the~substance}{distance~travelled~by~the~solvent}\)

R_f values vary from 0 (the substance is not attracted to the solvent) to 1 (the substance is not attracted to the paper).

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 — Figure caption,
The measurements you need to calculate an R_f value.

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Elements, compounds and mixtures - (CCEA) Paper chromatography