Required Practical 12 (AQA A Level Chemistry): Revision Note

Separation of species by TLC

Required practical 12: Separation of species by thin-layer chromatography

• This practical covers two key laboratory skills

  • Use thin-layer or paper chromatography

  • safely and carefully handle solids and liquids, including corrosive, irritant, flammable, and toxic substances

• A suitable experiment for this is the analysis of common analgesics by thin-layer chromatography

• Separation depends on a balance between each substance's solubility in the mobile phase (the solvent moving up the plate) and its retention/adsorption on the stationary phase (the coating on the plate)

  • Stationary phase: The phase that remains fixed in place and interacts with the components of the mixture

  • Mobile phase: The phase that moves through the stationary phase, carrying the components of the mixture

Key steps in the procedure

• An aspirin tablet is crushed using a mortar and pestle, and 0.1 g of the powder is weighed out into a suitable container such as a small test tube

• The powder is dissolved in 0.5 cm³ of ethanol

• Tablets of ibuprofen and paracetamol are prepared in the same way

• The procedure is repeated with a caffeine tablet and an Anadin Extra tablet, this time using 7 cm³ of ethanol to dissolve the crushed solid

Running the TLC

• Prepare a beaker with a small quantity of ethyl ethanoate

• On a TLC plate, draw a fine horizontal line at the bottom edge (in pencil)

  • This is called the baseline

• Draw five small crosses in pencil where the five spots will be located

• Use a capillary tube to draw up a small amount of the test solutions and transfer it to the crosses

  • Do not allow the spots to become too large, as the quality of the separation will be affected

• Place the TLC plate inside a chromatography tank or beaker with solvent -making sure that the solvent does not cover the spot - and place a lid to cover the beaker

  • The solvent will begin to travel up the plate, dissolving the compounds as it does

  • A lid is needed to prevent evaporation of the solvent (it may also be harmful)

• As the solvent reaches the top, remove the plate and draw another pencil line where the solvent has reached, indicating the solvent front

• The spots will be visible under a UV lamp, so allow the TLC plate to dry and use a pencil to draw around the spots under UV light

• The R_f values of the spots can then be calculated

Finding the R_f values

• A TLC plate can be used to calculate R_f values for compounds

• The R_f value is the distance travelled by the component divided by the distance travelled by the solvent front

  • It has no units

${\mathrm{R}}_{\mathrm{f}}=\frac{\mathrm{distance}\mathrm{ravelled}\mathrm{by}\mathrm{component}}{\mathrm{distance}\mathrm{travelled}\mathrm{by}\mathrm{solvent}}$

• These values can be used alongside other analytical data to deduce the composition of mixtures

• Each spot is identified by matching its R_f value against reference standards run alongside

Related topics

• Thin Layer Chromatography

• Chromatography