Chromatography
Paper Chromatography is used to separate different substances dissolved in a liquid. is used to separate mixtures of Able to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution. 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.
Image caption, Paper chromatography
1. Spots of ink or plant dye are placed on a pencil line
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Phases
Chromatography relies on two different 'phases':
- the Phase in chromatography that moves, usually a solvent or mixture of solvents. is the The liquid in which the solute dissolves to form a solution. that moves through the paper, carrying different substances with it
- the Phase in chromatography that does not move, for instance, the paper in chromatography. is contained on the paper and does not move through it
The different A substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. 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 The results of separating mixtures by chromatography.. A paper chromatogram can be used to distinguish between A substance that consists of only one element or only one compound. and A substance that consists of more than one element or compound. 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)
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).