Separation Techniques: Paper Chromatography

There are several methods to separate substances from each other, such as: filtration, chromatography, distillation and fractional distillation, evaporation, crystallization, magnetization and the separation of immisicible liquids.

But today, we shall only focus on chromatography.

Paper chromatography is used to separate mixtures of solutes with different solubility and degree of absorption such as ink dyes or sugar mixtures.

How to carry out paper chromatography:

Step 1 - Draw a pencil line 2cm from the edge of the chromatography paper

Step 2 - Place a drop of the mixture on the line. Wait for awhile before dripping about 2 more times, this is to ensure there is a concentration of the mixture

Step 3 - Dip the chromatography paper in a suitable solvent, and make sure the pencil line does NOT touch the solvent

Step 4 - Observe the solvent "run" up the chromatography paper

Food coloring "running" up chromatography paper

Step 5 - Remove the chromatography paper from the solvent once it has reached the solvent front

Step 6 - Calculate the Rf value

What is Rf value?

Rf value stands for retention factor. It is the distance moved by substance over the distance moved by solvent. This means substances with a higher Rf value are less easily absorbed, and smaller Rf values means the substance is more stongly absorbed.

How does it work?

As the solvent travels up the paper, the dyes are dissolved. Some dyes are more soluble, hence they travel faster up the paper, whilst others are less soluble and are absorbed strongly on the paper. This is all based on the principle of solubility.

The difference of solubility of the substances allows separation. The more strongly absorbed substances  travel slower.

What is it used for?

Paper chromatography is used to separate and identify compounds in a mixture. It can also be used for testing the purity of substances.