BP305P – Organic Chemistry-II Lab Manual Practical No 2 [Recrystallization of Benzoic acid]
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Table of Contents
Practical No 2
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Aim
To purify impure benzoic acid by the recrystallization method using distilled water as the solvent and determine the percentage yield of the purified product.
Learning Objectives
Upon completing this experiment, I will be able to:
- Explain the basic principle of recrystallization and its role in purifying organic compounds.
- Choose a suitable solvent for recrystallization based on the solubility of the compound.
- Carry out all the steps involved in recrystallization, including dissolving the sample, hot filtration, crystallization, filtration, washing, and drying the crystals.
- Calculate the percentage yield of the purified benzoic acid.
- Determine the purity of the recrystallized benzoic acid by observing its crystal appearance and measuring its melting point.
Learning Outcomes
After successfully completing this practical, I will be able to:
- Use the recrystallization technique to purify an organic compound.
- Select an appropriate solvent for effective recrystallization based on solubility principles.
- Perform the complete recrystallization process using standard laboratory procedures.
- Calculate the percentage yield of the purified product accurately.
- Evaluate the purity of recrystallized benzoic acid by examining its crystal quality and determining its melting point.
Reference
Vogel’s Textbook of Practical Organic Chemistry, Longman Scientific & Technical, 1989.
Requirements
Apparatus
250 mL beaker, 250 mL conical flask, Measuring cylinder, Glass rod, Funnel, Fluted filter paper, Filter paper and Buchner funnel with suction flask (if available), water bath, Ice bath, Watch glass, Spatula
Chemicals
Impure benzoic acid, distilled water, activated charcoal (optional)
Instruments
Hot plate, Analytical weighing balance, Drying oven or desiccator
Principle
Recrystallization is the most widely used laboratory technique for the purification of solid organic compounds. It is based on the principle that the solubility of a substance varies with temperature. An impure solid is dissolved in the minimum amount of a suitable hot solvent to form a saturated solution. As the solution cools, the solubility of the compound decreases and pure crystals separate from the solution, while most soluble impurities remain in the mother liquor. Insoluble impurities are removed by hot filtration before crystallization.
Water is an ideal solvent for recrystallizing benzoic acid because benzoic acid is sparingly soluble in cold water but readily soluble in boiling water. This large difference in solubility allows efficient recovery of pure crystals upon cooling.
Theory
A suitable solvent for recrystallization should possess the following properties:
- Dissolve the compound readily at its boiling point.
- Dissolve only a small amount of the compound at room temperature.
- Not react chemically with the compound.
- Dissolve impurities either completely or not at all.
- Be sufficiently volatile for easy removal after crystallization.
Benzoic acid (C₆H₅COOH) is an aromatic carboxylic acid that forms colourless needle-shaped crystals. It exhibits a significant increase in solubility with increasing temperature, making water a suitable recrystallization solvent.
Approximate Solubility of Benzoic Acid in Water
Temperature | Solubility |
25°C | 0.34 g/100 mL |
50°C | 0.95 g/100 mL |
75°C | 2.2 g/100 mL |
100°C | 6.8 g/100 mL |
This marked change in solubility is responsible for the separation of pure crystals during cooling.
Procedure
Step 1: Weighing
Accurately weigh 1 g of impure benzoic acid and transfer it to a clean 250 mL beaker.
Step 2: Dissolution
- Add approximately 75 mL of distilled water.
- Heat the mixture gently while stirring continuously.
- Add small portions of hot distilled water only if necessary until the benzoic acid dissolves completely.
- Avoid adding excess solvent.
Step 3: Decolorization (If Required)
- If the solution is coloured, add a small pinch of activated charcoal.
- Boil the mixture for about 2 minutes.
- Do not use excess charcoal, as it may adsorb the product.
Step 4: Hot Filtration
- Prepare a fluted filter paper in a warmed funnel.
- Filter the hot solution immediately into a clean, warm conical flask.
- The insoluble impurities and activated charcoal remain on the filter paper.
Step 5: Crystallization
- Allow the filtrate to cool slowly to room temperature without disturbing it.
- After crystallization begins, place the flask in an ice bath for 10–15 minutes to maximize crystal formation.
Step 6: Collection of Crystals
- Collect the crystals by vacuum filtration using a Buchner funnel or by gravity filtration.
- Wash the crystals with 5–10 mL of ice-cold distilled water.
Step 7: Drying
Dry the crystals in a desiccator, or in a hot air oven maintained at 50–60°C.
Step 8: Weighing
After complete drying, weigh the purified crystals accurately.
Step 9: Checking the purity
The purity of recrystallized benzoic acid can be check by thin layer chromatography and via melting point. Benzoic acid has sharp melting point of 121°C.
Precautions
- Use the minimum quantity of hot solvent required for dissolution.
- Avoid prolonged boiling to minimize solvent loss.
- Perform hot filtration immediately to prevent premature crystallization.
- Warm the funnel before hot filtration.
- Cool the filtrate slowly before placing it in an ice bath.
- Wash crystals only with ice-cold water.
- Dry crystals completely before weighing.
- Use only a small quantity of activated charcoal.
- Avoid scratching or disturbing the flask during slow cooling.
Sources of Error
- Excess solvent reduces crystal yield.
- Incomplete dissolution of benzoic acid.
- Premature crystallization during filtration.
- Mechanical loss during transfer.
- Washing crystals with warm water.
- Incomplete drying before weighing.
- Excess activated charcoal causing product adsorption.
Result
Impure benzoic acid was successfully purified by recrystallization using distilled water with % yield of ________ %, and _______ °C
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Aim
To purify impure benzoic acid by the recrystallization method using distilled water as the solvent and determine the percentage yield of the purified product.
Observations
- Weight of watch glass + crude benzoic acid: _________ g
- Weight of watch glass after transfer: _________ g
- Weight of crude benzoic acid taken (1-2): _________ g
- Weight of watch glass + recrystallized benzoic acid: _________ g
- Weight of watch glass after transfer: _________ g
- Weight of recrystallized benzoic acid taken (4-5): _________ g
- Melting point of recrystallized benzoic acid: _________ °C
Calculations
Result
Impure benzoic acid was successfully purified by recrystallization using distilled water with % yield of ________ %, and _______ °C
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