Chromatography
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. These are often coloured substances such as food colourings, inks, dyes or plant pigments.
Phases
Chromatography relies on two different ‘phases’:
- the Phase in chromatography that does not move, for instance, the paper in chromatography. is paper, or a thin layer of powder coated onto a glass or plastic plate
- the Phase in chromatography that moves, usually a solvent or mixture of solvents. is a The liquid in which the solute dissolves to form a solution. that moves past the stationary phase, carrying different substances with 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.
Image caption, 1. Spots of ink or plant dye are placed on a pencil line
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Identifying substances
Separation by paper chromatography or thin layer chromatography (TLC) produces a The results of separating mixtures by chromatography.. A chromatogram can 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 as they have the same Rf value
Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value for a particular substance is always the same if the same solvent and stationary phase are used.
The Rf value of a spot is calculated using:
Rf =\( \frac{distance \ travelled \ by \ substance}{distance\ travelled\ by\ solvent }\)
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).