7.1 - Recrystallisation of Benzoic Acid and Determination of its Melting Point

Materials and Apparatus Required

Chemicals

• Impure benzoic acid (1 g sample)
• Deionised water (boiling)
• Ice-cold deionised water

Apparatus

• 100 cm³ and 250 cm³ beakers
• 250 cm³ conical flask
• Oven set at 100°C
• Filter funnel and filter paper
• Hirsch or Buchner funnel
• Small Buchner flask
• Rubber adaptor
• Retort stand, clamp, and boss head
• Hotplate or Bunsen burner with tripod and gauze
• Spatula
• Clock glass
• Safety glasses
• Heat-resistant gloves
• Melting point tubes
• Mortar and pestle
• Thiele tube or oil bath for melting point determination
• Thermometer (-10°C to 150°C)

Safety Precautions

• Wear safety glasses throughout the experiment.
• Benzoic acid may irritate the skin and eyes; avoid direct contact.
• Use heat-resistant gloves when handling hot glassware or working near the hotplate.
• Avoid burns when handling hot equipment or solvents.
• Be cautious when heating liquid paraffin for melting point determination, as it can ignite at high temperatures.

Method

Recrystallisation of Benzoic Acid

1. Boil approximately 100 cm³ of deionised water and keep it at a constant boil.
2. Weigh about 1 g of impure benzoic acid and place it in a 100 cm³ beaker.
3. Add 25 cm³ of boiling water to the beaker containing benzoic acid and stir the mixture while heating gently until the benzoic acid dissolves.
4. If needed, add small amounts of boiling water to dissolve any remaining crystals, using the minimum amount necessary.
5. Prepare a hot filtration setup with a filter funnel and fluted filter paper in a conical flask containing 20 cm³ of boiling water.
6. Pour the hot benzoic acid solution through the filter to remove insoluble impurities.
7. Transfer the filtrate to a warm 100 cm³ beaker and heat until crystals start to form on the sides of the beaker.
8. Then allow the solution to cool to room temperature.
9. Once cooled, further cool the solution in an ice bath to maximise crystal formation.
10. Filter the recrystallised benzoic acid using vacuum filtration and wash the crystals with 2 cm³ of ice-cold deionised water.
11. Dry the crystals on a clock glass in an oven at 80°C or place them on a radiator for 24 hours.

Determination of Melting Point

1. Grind a small amount of the recrystallised benzoic acid using a mortar and pestle.
2. Fill a melting point tube with about 3–5 mm of the crystals.
3. Attach the tube to a thermometer using a rubber band and place it in a Thiele tube filled with liquid paraffin or an oil bath.
4. Heat the tube slowly and observe the temperature at which the crystals begin to melt and when they are fully liquefied.
5. Record this melting point range.
6. Repeat the process using a sample of pure benzoic acid for comparison.

Results

Substance	Melting Point Range (°C)
Impure benzoic acid	118–121°C
Recrystallised benzoic acid	121–123°C
Pure benzoic acid (Analar grade)	121–123°C

Example Questions with Answers