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Science

Separation techniques

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Chromatography

Diagram of a chromatography experiment

The colours separate and move up the paper at different rates

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 near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg 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.

Rf values

Different chromatograms and the separated components of the mixtures can be identified by calculating the Rf value using the equation:

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

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.

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