Chromatography: A technique that can be used to identify and separate substances in a mixed solution

Paper chromatography is a simple technique based on the principle that some substances dissolve better in a solvent than others.

Thin Layer Chromatography is based on the principle that some substances adhere to the molecules of the stationary phase more strongly than others.

Paper chromatography can be used to:

Distinguish between pure and impure substances
Identify substances by comparing the pattern on the chromatogram with the patterns formed by known substances
Identify substances by calculating their \(R_f \text{ value. }\)

\(\text{ Stationary/ Immobile phase: }\) The solvent and dissolved substances move along this. In the case of paper chromatography, this is the paper. In the case of Thin Layer chromatography, the stationary/ immobile phase is aluminium oxide paste or silica gel

\(\text{ Mobile phase: }\) The solvent. This could be an aqueous solvent such as water, or a non-aqueous solvent such as ethanol

\(\text{ Chromatogram: }\) The paper with the separated components on it

\(R_f \text{ value: }\) The distance the compound has risen divided by the distance the solvent has risen. Both measurements are taken from the starting positions of the samples on the paper. This value depends on the mobile phase i.e. what solvent is used and the stationary phase i.e. what type of paper is use

\( R_f \text{ value= }\frac{\text{Distance moved by the spot}}{\text{Distance moved by the solvent}}\)

This value does not change if the chromatography conditions are the same.