Column Chromatography (AQA A Level Chemistry)

Column Chromatography

Column Chromatography

• In column chromatography, the column is often a long vertical glass tube or in the laboratory a burette is suitable

• An inert solid (usually powdered silica gel or alumina) is the stationary phase which is placed in the column

• A liquid solvent phase, mobile phase, is added into the column until it is saturated with solvent

  • Care should be taken when setting up the column because cracks in the stationary phase can lead to issues during separation

• The sample mixture is dissolved in the solvent and introduced at the top of the column

  • A pipette is usually used to carefully add the dissolved sample to the top of the column

  • The aim here is to add the sample without disturbing the surface of the column so that the sample runs from one level through the column

• Once the sample has been added, more solvent (eluent) is added on top of the sample

• As the solvent runs through, fresh solvent is added to the top of the column so that it does not dry out

• The sample flows through the column via gravity

  • This process can be sped up by pushing the sample and mobile phase through the column

  • In school laboratories, this can be achieved by attaching a gas syringe to the top of the chromatography column

  • In industrial / research laboratories, this is achieved by attaching an air line to the top of the chromatography column

• The component with the greatest attraction / affinity to the stationary phase takes the longest time to flow through the column

• If the components are coloured, then they can be identified using the R_f value

• If the components are colourless, then other techniques such as fluorescence under UV light can be used to show their position in the column

Column chromatography showing the separation of coloured compounds in a glass burette