ChromatographyCambridge IGCSE Chemistry: Revision notes

Chromatography is a technique used to separate mixtures of soluble substances and to help identify what is present in a mixture.

• Paper chromatography separates mixtures of soluble coloured substances (such as dyes in ink or food colourings) using a suitable solvent.
• Different substances in the mixture are carried up the paper at different rates, because they have different solubilities in the solvent and different attractions to the paper.
• This gives each substance its own final position on the paper, producing a pattern called a chromatogram.

Method for coloured substances:

1. Draw a pencil baseline near the bottom of the paper (pencil is used because it does not dissolve in the solvent and move).
2. Place small spots of the mixture(s) on the baseline.
3. Stand the paper in a shallow layer of solvent, keeping the baseline above the solvent surface.
4. As the solvent soaks up the paper, it carries the substances with it at different rates.
5. Remove the paper once the solvent has nearly reached the top, and mark the solvent front before it dries.

Many soluble substances (such as amino acids) are colourless, so their spots cannot be seen directly on the chromatogram.

• Paper chromatography can still separate mixtures of soluble colourless substances, using a suitable solvent.
• A locating agent is then sprayed onto the dried chromatogram to react with the colourless spots and make them visible (often as coloured spots), so their positions can be identified.

A finished chromatogram can be used to identify unknown substances and to check purity.

• Identifying unknowns: compare the spots produced by an unknown sample with spots produced by known reference substances run on the same paper. If an unknown spot lines up at the same position as a known substance, it suggests they are the same substance.
• Pure vs impure substances: a pure substance produces only one spot on a chromatogram, run in any solvent. An impure substance (a mixture) produces two or more spots.
• If a spot does not move at all, or moves with the solvent front, this also gives information about its solubility in that solvent.

The Rf value (retardation factor) is a number that describes how far a substance travelled relative to the solvent, and can be used to identify substances.

Rf = distance travelled by substance ÷ distance travelled by solvent

• Both distances are measured from the baseline: the substance distance is measured to the centre of its spot, and the solvent distance is measured to the solvent front.
• Rf values are always between 0 and 1 (no units).
• For a given solvent and temperature, a particular substance always has the same Rf value, so Rf values can be compared to known/published values to help identify substances.