Column Chromatography (Oxford AQA International A Level Chemistry)

Column Chromatography

• Column chromatography (CC) is a technique typically used to separate larger samples

• The column is often a long vertical glass tube

  • In the laboratory, a burette can be used

• The column contains an inert solid as the stationary phase

  • The most common stationary phases are powdered silica or alumina

• The mobile phase is a liquid solvent

  • This 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 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

Column chromatography example

• 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

• Components can collected in a beaker as they exit the column