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How to Do a Recrystallization With Constant Melting Point

Before you analyze it or use it in a chemical reaction, you must purify a chemical compound. Recrystallization is one of the most common techniques used by chemists to purify impure compounds. Recrystallization involves the dissolution of impure product in a suitable solvent and appropriate temperature changes to induce crystallization of the pure product.

The melting point is a useful indicator of the purity of a compound. Chemists repeatedly recrystallize their products until the melting point no longer changes, indicating that the compound is pure.

Things You'll Need

Solvent
Pasteur pipette
Flask
Water bath or heating mantle
Filter paper
Filter funnel
Ice bath
Vacuum pump
Melting point apparatus

Instructions

- 1
  Choose an appropriate solvent for your compound. Your product must be insoluble at room temperature and soluble at the boiling point of the solvent.
- 2
  Dissolve your product in the minimum quantity of hot solvent. Add hot solvent in small quantities using a Pasteur pipette. Swirl the flask as you add hot solvent and maintain the temperature of the flask at the boiling point of the solvent using a water bath or heating mantle.
- 3
  Filter the solution through filter paper into a flask. This removes any insoluble impurities from the hot solvent.
- 4
  Leave the hot solution to cool to room temperature. Cover the flask with filter paper to prevent contamination from airborne particles. Do not move or disturb the flask.
- 5
  Place the flask in an ice bath and leave for 15 minutes to ensure that all of your product has crystallized.
- 6
  Collect the crystals from the cold solvent using a vacuum pump to draw the liquid through the filter paper. Rinse the inside of the flask using ice cold solvent to collect all of your product from the flask.
- 7
  Measure the melting point of the crystals using a melting point apparatus.
- 8
  Recrystallize your product and then remeasure the melting point. If the melting point changes, then repeatedly recrystallize your product until the melting point is constant.

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