Required Practical 12 (AQA A Level Chemistry)

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

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 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

Steps in the TLC analysis of some common analgesics

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 need to prevent evaporation of the solvent (it also may be harmful)

• As 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

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

R_f values can be calculated by taking 2 measurements from the TLC plate