Elements, Compounds and Mixtures

What is Paper Chromatography?

Paper Chromatography is a separation technique that is used to separate
and identify the components of a mixture (see also gas chromatography).

Paper chromatography is used to identify colouring agents
(chemicals) for example in food or ink.
The mixture (in this case two green ink spots)
is put on a filter paper that is placed in a suitable solvent.

As the solvent rises up the filter paper the individual components (dyes)
within the green ink spots are separated. Different dyes travel different
distances up the paper. The solvent travels furthest
up the filter paper leaving a line called the solvent front.

What is the R_f value?

The distance travelled up the paper by a component
divided by the distance travelled up the paper by the solvent
is called the R_f value or retention factor.

For example, if a component travelled 4 cm and the solvent travelled 10 cm
then R_f = 4 ÷ 10
        = 0·4

In the above example the green ink spots each have the same blue dye
because they have travelled the same distance (same R_f value)
but different yellow dyes
because they have travelled a different distance (different R_f value).

Paper chromatography is used by the food industry and forensic science.

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