Chromatography

Paper chromatography is used to separate mixtures of soluble substances and to provide information on the possible identity of the substances present in the mixture. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

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 abovea basin containing solvent.

Paper chromatography

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

Phases

Chromatography relies on two different 'phases':

The different dissolved substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.

Interpreting a chromatogram

Separation by chromatography produces a chromatogram. A paper chromatogram can be used to distinguish between pure and impure substances:

  • a pure substance produces one spot on the chromatogram
  • an impure substance produces two or more spots

A paper chromatogram can also be used to identify substances by comparing them with known substances. Two substances are likely to be the same if:

  • they produce the same number of spots, and these match in colour
  • the spots travel the same distance up the paper (have the same Rf value)
Results of a chromatogram of three pure substances and brown inkInterpreting the chromatogram for a brown ink

In this chromatogram, the brown ink is made of a mixture of the red, blue and yellow inks. This is because the spots in the brown ink are at the same heights (and have the same Rf value) as the reference inks.

Rf values

Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value is always the same for a particular substance if run in the same solvent system.

The Rf value of a spot is calculated using:

R_{f} = \frac{distance \: travelled \: by \: substance}{distance \: travelled \: by \: solvent}

Rf values vary from 0 (the substance is not attracted to the mobile phase) to 1 (the substance is not attracted to the stationary phase).

Chromatography paper next to a measurement scale, showing the distance travelled by the solvent, and a coloured dot showing the distance travelled by the substanceMeasurements needed to calculate an Rf value