Chromatography Case

Partition - Remember the solvent extraction activity with the separating funnel. The iodine moved from the water (aqueous) layer into the organic layer. However there was some iodine left in the aqueous layer.

Chemists say that the iodine is partitioned between the two layers: water and organic solvent. If more organic solvent were added and the mixture shaken again then more iodine would move from the aqueous layer to the organic layer.

Chromatography is used to analyze small quantities of a mixture of substances which are chemically similar to each other.

It involves the partition of the components of the mixture between a stationary phase and a mobile phase. The mixture to be separated is introduced on to the stationary phase which stays still. The mobile phase is then allowed to move over the stationary phase for separation. Partition depends on the different solubilities of the components in the mobile phase and the different adsorption forces of the components with the stationary phase.

Adsorption is the temporary attraction of molecules of a gas or liquid to a solid surface. Components with greater solubilities will dissolve in the mobile phase and move along with it readily. Components with stronger adsorption forces will be held on the stationary phase and not move along readily with the mobile phase. The differences in solubilities and adsorption bring about separation

Paper Chromatography

In Paper Chromatography, a piece of filter paper or chromatography paper is used. The paper consists of stationary water molecules embedded in a cellulose matrix. The water molecules act as the stationary phase. The mobile phase consists of a suitable solvent that travels up the stationary phase. The mixture to be separated is spotted a short distance from one end of the paper (BASE LINE). The end below the spot is placed in the solvent (THE SOLVENT MUST NOT TOUCH THE SPOT ON THE BASELINE). As the solvent moves along the paper it carries the mixture with it. The distance the solvent moves from the baseline is called the SOLVENT FRONT. Components of the mixture will separate readily according to how strongly they adsorb on the stationary phase and how readily they dissolve in the mobile phase.

If the separated components are colourless, then a visualizing agent can be used to convert them into coloured spots. The positions of certain substances can also be determined by fluorescing (to cause to glow) under a UV lamp.

The ratio of the distance moved by a component of the mixture to the distance moved by the solvent is called the RETENTION FACTOR,

Rf = distance moved by a component (dye)

distance moved by the solvent

Each component has a characteristic