PAPER CHROMATOGRAPHY
Paper chromatography is an example of partition chromatography. In partition chromatography, the stationary phase is a liquid absorbed in water. Absorbed water behaves as an immiscible liquid towards the mobile phase which passes over the paper. The mobile phase is an organic liquid.
There are three common ways of carrying out paper chromatography
- Ascending
- Descending
- Radical/Circular
Here we will discuss only ascending paper chromatography.
In this technique, a solvent mixture that is in accordance with the sample to be separated is fed into the chromatographic tank. The solvent is a pool in the chromatographic tank in which paper is supported and solvent travels upward by the capillary action. The tank is covered to homogenize the inner atmosphere. Take 20cm of Wattmann's chromatographic paper No. 1 and draw a pencil line about 2.5 cm from its one end. Point a spot on the pencil line. To facilitate identification spot of the known compound may also be placed alongside.
Suspend the paper with a clip when the spots dried so that impregnated end dips into the solvent mixture to a depth of 5-6mm. As the solvent front passes the spots, The solute begins to move upward at the rate depending upon their distribution coefficient. As the solvent front rises to the three-fourth of the paper then remove the paper. Marks the solvent front point and let the paper dry.
Chromatogram:
When the paper is dried the pattern on the paper is called a chromatogram. Different components can be easily identified if coloured but if colourless then the chromatogram has to be treated with chemical or physical methods to identify the spots.
Specific Retardation factor:
Every component has a specific value of retardation factor called the Rf value.
\[R_{f}\; =\; \frac{Distance\: travelled\: by\: a\: component\: from\: original\: spot}{Distance\: travelled\: by\: solvent\: from\: the\: original\: spot}\]