Experiment #: 2
Title: Crystallization – Purification of a Sample of Impure Naphthalene
Purpose:
The purpose of this experiment is to use the crystallization technique to purify a sample of
impure naphthalene. Crystallization is a common method for purifying solid organic
compounds. This experiment involves dissolving impure naphthalene in hot methanol, removing
insoluble and soluble impurities, and recovering purified crystals. Skills gained include solvent
selection, filtration, evaporation, crystallization, and analysis of purity using melting point
determination
Procedure:
1. Choose a solvent:
a. Methanol is chosen as the recrystallization solvent due to its ability to
dissolve naphthalene when hot and allow it to crystallize out upon cooling.
i. This solvent also minimizes solubility of impurities such as sand
and charcoal.
2. Dissolve the impure solid:
a. Weigh and transfer the impure naphthalene (containing sand, charcoal, and
Congo red) into a small Erlenmeyer flask.
i. Add a small volume of methanol to wet the crystals and insert a
wooden boiling stick to prevent bumping.
b. Heat gently on a hot plate, swirling occasionally.
i. Add methanol in small portions until the solid appears fully dissolved
to the naked eye, avoiding excess solvent to maximize crystal recovery.
3. Remove soluble colored impurities:
a. Once the solution is clear, add a small amount of activated charcoal pellets
to absorb colored impurities like the Congo red dye
b. Reheat and allow the solution to boil briefly, then remove from heat.
4. Gravity filtration:
a. Prepare fluted filter paper and place it into a funnel set over a pre-
warmed Erlenmeyer flask.
i. Wet the filter paper with hot methanol.
b. Pour the hot solution through the filter in portions, trapping insoluble
impurities (charcoal and sand)
c. Rinse the original flask and the filter paper with a small amount of hot
methanol to transfer all soluble material.
5. Evaporate excess solvent:
a. Heat the filtered solution gently to reduce the solvent volume to approximately
15 mL.
i. Use a visual comparison with a reference flask containing 15 mL
of methanol to ensure accuracy
1. Avoid overheating or boiling to dryness.
6. Test for saturation and induce crystallization:
a. Add water dropwise to the hot solution. The brief appearance of crystals
indicates saturation.
Title: Crystallization – Purification of a Sample of Impure Naphthalene
Purpose:
The purpose of this experiment is to use the crystallization technique to purify a sample of
impure naphthalene. Crystallization is a common method for purifying solid organic
compounds. This experiment involves dissolving impure naphthalene in hot methanol, removing
insoluble and soluble impurities, and recovering purified crystals. Skills gained include solvent
selection, filtration, evaporation, crystallization, and analysis of purity using melting point
determination
Procedure:
1. Choose a solvent:
a. Methanol is chosen as the recrystallization solvent due to its ability to
dissolve naphthalene when hot and allow it to crystallize out upon cooling.
i. This solvent also minimizes solubility of impurities such as sand
and charcoal.
2. Dissolve the impure solid:
a. Weigh and transfer the impure naphthalene (containing sand, charcoal, and
Congo red) into a small Erlenmeyer flask.
i. Add a small volume of methanol to wet the crystals and insert a
wooden boiling stick to prevent bumping.
b. Heat gently on a hot plate, swirling occasionally.
i. Add methanol in small portions until the solid appears fully dissolved
to the naked eye, avoiding excess solvent to maximize crystal recovery.
3. Remove soluble colored impurities:
a. Once the solution is clear, add a small amount of activated charcoal pellets
to absorb colored impurities like the Congo red dye
b. Reheat and allow the solution to boil briefly, then remove from heat.
4. Gravity filtration:
a. Prepare fluted filter paper and place it into a funnel set over a pre-
warmed Erlenmeyer flask.
i. Wet the filter paper with hot methanol.
b. Pour the hot solution through the filter in portions, trapping insoluble
impurities (charcoal and sand)
c. Rinse the original flask and the filter paper with a small amount of hot
methanol to transfer all soluble material.
5. Evaporate excess solvent:
a. Heat the filtered solution gently to reduce the solvent volume to approximately
15 mL.
i. Use a visual comparison with a reference flask containing 15 mL
of methanol to ensure accuracy
1. Avoid overheating or boiling to dryness.
6. Test for saturation and induce crystallization:
a. Add water dropwise to the hot solution. The brief appearance of crystals
indicates saturation.
