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Narration : 11. Column Chromatography

• Column chromatography is used to isolate the target organic compound from the mixture if it is impossible by distillation or recrystallization.
• We will separate two organic compounds from each other with the silica gel column chromatography.
• At first stuff cotton wool in the glass tube.
• Next, place sea sand. This will not only prevent the leakage of silica gel but also flatten the base.
• Pour the eluent to fill one forth of the glass tube.
• Add the eluent to the silica gel and make a slurry. Stir well till there are no air bubbles. Depending on the solvent heat is generated.
• Pour the slurry into the glass tube.
• Open the cock and allow the eluent to flow. Allow the silica gel to sediment.
• Pour slurry so that the top of the silica gel is a little above the requirement.
• Allow a little eluent to remain on the silica gel and close the cock.
• Tap the glass tube lightly. As a result, the silica gel sticks well and the level drops.
• Repetition of tapping and allowing the eluent to flow causes the silica gel to stick well.
• Be careful not to drain the eluent by keeping the cock open, which results in drying up of the silica gel.
• You can reuse the collected eluent.
• Gently place sea sand on the top of the silica gel to protect the silica gel layer.
• Now, the preparation for silica gel column chromatography is complete.
• Next, load the sample to be purified.
• Run the eluent to expose the sea sand slightly.
• A sample solution is pippeted gently.
• After the sample is pippeted, open the cock to introduce it into the column.
• Wash the flask with a little amount of eluent and pour the wash gently into the column.
• If one drop of the wash is preserved, it will be helpful later when confirming the separation.
• Repeat the washing more than twice. With this the sample can be transferred completely and introduced into the column.
• Pour the eluent carefully.
• Open the cock and begin developing.
• Compounds weakly absorbed by the silica gel move faster than the ones strongly absorbed. As a result, the mixture separates from each other and bands are formed.
• Collect a series of fractions in test tubes or flasks.
• You can obtain the pure compound by collecting appropriate fractions.
• Fine silica gel usually makes good separation but the flow rate becomes poor.
• The flow rate can be increased with the help of an air pump. This technique is called flash chromatography.