Required Practical 12 (AQA A-Level Chemistry): Revision Notes

8.3.4 Required Practical 12

This practical focuses on two essential laboratory skills:

• Using thin-layer or paper chromatography to separate substances.
• Safely and carefully handling solids and liquids, including substances that are corrosive, irritant, flammable, or toxic.

To analyse medicine samples (e.g., aspirin):

1. Use a pestle and mortar to crush an aspirin tablet into a fine powder.
2. Transfer the crushed tablet to a weighing boat or bottle.
3. Dissolve approximately 0.1 g of the powdered tablet in 0.5 cm³ of ethanol.
4. Repeat the process with other painkiller tablets. (For caffeine or anadin tablets, dissolve them in 7.0 cm³ of ethanol).

Thin Layer Chromatography (TLC) Method:

1. Prepare the TLC plate:

• Wear gloves and use a pencil to lightly draw a line 1 cm above the bottom of the TLC plate.

• Mark five evenly spaced spots along this line. Explanation:

• Gloves prevent contamination from your hands.

• The pencil line is essential as it does not dissolve in the solvent.

2. Apply the samples:

• Using a capillary tube, carefully apply a small drop of each solution onto a separate spot on the TLC plate. Let the spots air dry.

• If necessary, reapply the sample to make small, concentrated spots. Explanation:

• Applying a drop that is too large could cause the spots to merge.

3. Prepare the development chamber:

• Pour about 10 cm³ of solvent into a development chamber (or any other container with a lid).Explanation:
• Ensure the solvent depth is not too deep, as it could dissolve the spots from the TLC plate.

4. Develop the plate:

• Place the TLC plate into the chamber, ensuring the solvent level is below the line of spots.

• Close the chamber with a tight-fitting lid. Explanation:

• A lid prevents solvent evaporation and ensures that the chamber is saturated with solvent vapour.

5. Allow the solvent to rise:

• Once the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent front with a pencil.

• Allow the plate to dry in a fume cupboard. Explanation:

• Letting the solvent rise near the top of the plate will provide more accurate Rf values, but the Rf can still be calculated if the solvent doesn't reach the top.

• The plate should be dried in a fume cupboard since the solvent can be toxic.

6. Visualise the spots:

• Place the TLC plate under a UV lamp to see the spots if they are not visible.

• Lightly trace around the spots with a pencil. Explanation:

• UV light is used to detect colourless spots that might otherwise be invisible.

7. Calculate Rf values:

• Measure the distances travelled by the solvent and each spot using a millimetre ruler.

• Calculate the Rf value for each spot. Explanation:

• Using a precise ruler ensures more accurate Rf values.

Finding the Rf values:

• A TLC plate can be used to calculate Rf values for various compounds.

$$R_f = \frac{\text{Distance travelled by component}}{\text{Distance travelled by solvent}}$$

• These values can be combined with other analytical data to determine the composition of mixtures.