Recrystallization Lab

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• Recrystallization Lab

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New Techniques

• Recrystallization method of purifying an organic
  solid
• Gravity Filtration method of removing insoluble
  impurities from recrystallization solution
• Suction Filtration method of isolating pure
  solid from liquid (filtrate) using vacuum

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In Lab

• Recrystallization
• Determine best solvent for recrystallization
• Weigh crude solid
• Recrystallize
• Weigh purified product

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After Lab

• Let recrystallized solid air dry on shelf.
• Weigh dry crystals and take a melting point (next
  week).
• Calculations
• recovery
• mass crude/mass recrystallized x 100
• Conclusions Discuss effectiveness of
  recrystallization process in terms of purity and
  percent recovery. Discuss role/attributes of
  solvent, sources of error, etc.

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Theory of Recrystallization
Pure solid Tight crystal lattice
Impurities disrupt the crystal lattice
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Steps in Recrystallization

• 1. Dissolve crude solid in hot solvent (saturated
  solution)
• 2. Let solution cool to room temperature, so that
  crystal lattice reforms.
• 3. Cool solution in an ice bath.
• 4. Suction filter the pure solid, leaving
  impurities in solution.
• 5. Let solid air dry to remove traces of
  solvent.

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Recrystallization
1. Add hot solvent until solid dissolves (use
minimum amount of hot solvent)
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Recrystallization
2. Let solvent cool slowly to room temperature,
so crystal lattice can reform. 3. Cool in ice
bath to make sure that the maximum amount
of solid has crystallized. (Suction filtration
gets cold!)
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Recrystallization
4. Suction filter solid away from
impurities. 5. Let crystals air dry.
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An Ideal Recrystallization Solvent Should

• should dissolve all of the compound when the
  solvent is hot (boiling).
• should dissolve none of the compound when the
  solvent is at room temperature.
• should have different solubilities for the
  compound and the impurities.
• should have a lower boiling point than the
  melting point of the compound.
• should have a fairly low boiling point
• should be cheap, non-toxic, non-reactive, and
  non-smelly

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Pour a small amount of the hot solvent into the
flask containing the solid.
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Swirl the flask to dissolve the solid.
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Place the flask on the hot plate to keep the
solution warm.
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If the solid is still not dissolved, add a tiny
amount more solvent and swirl again.
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When the solid is all in solution, set it on the
bench top. Do not disturb it!
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Hot Gravity Filtration
HOT Gravity filtration is the method of choice to
remove solid impurities from an organic liquid.
The "impurity" can be a activated carbon or an
insoluble impurity. Gravity filtration can be
used to collect solid product, although generally
vacuum filtration is used for this purpose
because it is faster.
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After a while, crystals should appear in the
flask.
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You can now place the flask in an ice bath to
finish the crystallization process.
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Suction Filtration Apparatus.
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