Chemistry – Experimental techniques and chemical analysis - Chromatography | e-Consult

Paper chromatography is a separation technique used to separate mixtures based on differences in the partition coefficient of the components between a mobile phase (the solvent) and a stationary phase (the paper). The principle behind this is that different substances have different affinities for the stationary and mobile phases. Substances with a higher affinity for the mobile phase will travel further up the paper than those with a higher affinity for the stationary phase.

Procedure:

• Preparation: A small spot of the mixture is applied near the bottom of a strip of chromatography paper. The paper is then mounted vertically in a container, ensuring the solvent level is below the spot.
• Solvent Choice: The choice of solvent is crucial. It should be a suitable organic solvent that dissolves all the components of the mixture. The solvent's polarity should be chosen so that it effectively dissolves the components but also has a reasonable affinity for the stationary phase (the paper). A solvent that is too polar will cause all components to travel up the paper very quickly and result in poor separation. A solvent that is too non-polar will not dissolve the components effectively. A test run is often performed to determine the optimal solvent.
• Locating Agent: After the solvent front has reached a suitable height, the paper is removed and allowed to dry. The separated components are then visualized using a locating agent. This is a reagent that reacts with the separated substances to make them visible. For example, if the components are organic, a developing agent like potassium permanganate (KMnO4) or iodine vapor can be used. The components will react with the developing agent, producing coloured spots at their respective locations.

The different components of the mixture will travel at different rates up the paper, resulting in their separation. The distance each component travels is related to its partition coefficient. The Rf value (retention factor) is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. This provides a characteristic value for each compound, allowing for identification.