Wednesday, October 31, 2012


Recrystallization is a technique used to purify organic solids. That is, you will separate molecules of impurity from molecules of desired materials

A Recrystallization solvent should have the following properties:
1.    Does not dissolve the compound to be purified when cold
2.    Does dissolve the compound then hot (near boiling)
3.    Has a relatively low bp for easy evaporation from the purified compound
4.    Does not react with the compound being purified,
5.    The cold solvent will keep impurities dissolved.

So- you’ll start with impure crystals, dissolve them in a hot solvent and the get the crystals back in the end in a pure state
There are several steps to Recrystallization. They are:

  1. Choose a solvent that does not dissolve the compound when cold but does so readily when near boiling
  2. Add hot solvent until the compound you are trying to purify just dissolves; do not add too much solvent. Use a minimal amount
  3. Add decolorizing carbon (charcoal) to remove colored impurities. The step is skipped most often. Do not add the charcoal to a boiling solution because it will bump and boil over. Add a small amount of charcoal because you will lose good compound as well as colored impurities by adsorption to the charcoal.
  4. FILTER BY GRAVITY WHILE HOT (gravity filtration). This is the step that makes the recrystallization a success. It should never be omitted from a recrystallization. Remember this step for future recrystallizations. This step removes the charcoal and any undisclosed impurities. Use fluted filter paper. This provides a large surface area allowed for fast filtration. Recall: when you want the filtrate, use gravity filtration.
  5. Cool the filtrate from #4. Cool first in air and then later in an ice/water bath, ask your TA what to do if crystals don’t form.
  6. Perform suction filtration to isolate the compound, dissolved impurities stay in this filtrate. Recall: when you want the solid, use suction filtration.
  7. Dry the crystals in a safe place in your drawer. Do not bottle them as they need to dry. Do keep them safe.


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