Methods of purification

Sublimation separates a solid that passes directly into vapour on heating from non-sublimable impurities.

Crystallisation relies on the compound being much more soluble in a hot solvent than in the cold one; impurities stay in solution. Repeated crystallisation removes impurities of similar solubility.

Simple distillation separates liquids with large boiling point differences, or a volatile liquid from non-volatile impurities.

Fractional distillation uses a fractionating column to separate liquids whose boiling points are close; it is used to separate crude oil fractions.

Distillation under reduced pressure lets liquids that decompose at or below their normal boiling point boil at a lower temperature; glycerol is recovered from spent-lye this way in soap manufacture.

Steam distillation purifies substances that are volatile in steam and immiscible with water, such as aniline; the mixture boils when the sum of the vapour pressures of water and the compound equals atmospheric pressure, below 373 K.

Differential extraction shakes an aqueous solution with an organic solvent in which the compound is more soluble and which does not mix with water; the layers are separated in a separating funnel.

Chromatography separates components by their different affinities for a stationary phase and a moving (mobile) phase.

Adsorption chromatography uses a solid adsorbent such as silica gel or alumina, in column chromatography or thin layer chromatography (TLC); partition chromatography, as in paper chromatography, uses water held in the paper as the stationary phase.

Retardation factor Rf = distance moved by the substance from the baseline / distance moved by the solvent front; colourless spots are located under UV light, with iodine vapour or with a reagent such as ninhydrin for amino acids.