Question

A yellow compound is placed on a column and the column is eluted with hexanes. After a large volume of solvent has been collected but the yellow band is still at the top of the column. What should be done to recover the yellow compound? How could this experiment have been done better?

Answer 1

Also try this:

• Open the tap of column to allow the solvent already in the column to drain so that it is level with the top of the packing material, and then add the solution carefully to the top of the column.
• Then you open the tap again so that the coloured mixture is all absorbed into the top of the packing material.

  Hexane is non-polar than the yellow compound because hexane travels through the column very quickly. That means that it must adsorb less strongly to the silica gel or alumina than the yellow one. The more polar yellow one spends less of its time in the solvent and therefore does not wash through the column much faster. You should try to use more polar solvent instead of hexane i.e. acetone or ethyl acetate.

  The process of washing a compound through a column using a solvent is known as elution. The solvent is sometimes known as the eluent.

• Collect what comes out of the bottom of the column in a whole series of labelled tubes.

• Collect 1 cm³ samples or 5 cm³ samples.

• Take a drop from each solution and make a thin layer chromatogram from it.

• Place the drop on the base line alongside a drop from a pure sample of the compound that you are making.

• By doing this repeatedly, you can identify which of your samples collected at the bottom of the column contain the desired product, and only the desired product.