Recrystallization
Procedure
Identify an appropriate solvent to do the recrystallization. Do not use water or any of the other acidic or basic aqueous solutions for recrystallization. For approximately 2 grams of a solid, use approximately 30-40 ml of recrystallization solvent. Dispense the solvent into a 100ml beaker and gently warm the solvent in a water bath to just below its boiling point. Add the unknown to the warm solvent and swirl or stir the solution. (If the compound does not completely dissolve, try adding ~5-10 ml more of solvent)
The principle of
recrystalllization is to dissolve the compound to be purified
in a warm solvent while the impurities either do not dissolve,
or the impurities remain dissolved after the pure compound
crystallizes from the solution when the solution is cooled
back to room temperature or put in an ice bath. A proper
solvent must be selected. The properties of a good
recystallization solvent is one that the compound dissolves
easily when the solvent is warm, but that is less soluble at
room temperature or when cooled in an ice bath. The
solvent should relatively low boiling (typically below
100C). For most organic compounds, water is not a good
recrystallization solvent.
Recrystallization
requires
significant patience so be prepared to be patient. Procedure
Identify an appropriate solvent to do the recrystallization. Do not use water or any of the other acidic or basic aqueous solutions for recrystallization. For approximately 2 grams of a solid, use approximately 30-40 ml of recrystallization solvent. Dispense the solvent into a 100ml beaker and gently warm the solvent in a water bath to just below its boiling point. Add the unknown to the warm solvent and swirl or stir the solution. (If the compound does not completely dissolve, try adding ~5-10 ml more of solvent)
 Figure 1: Gravity Filtration
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If you
observe particles in the solution still, use a gravity
filter remove them (hot gravity filtration). Keep
the solution warm and set up a clean
Erlenmeyer flask (125ml) with a glass funnel and piece of
filter paper (Figure 1 at left) and quickly pour the
warm solution through the filter. Set the the
Erlenmeyer flask down and let is sit undisturbed for 15-30
minutes, or until crystals or a precipitate forms. If no
precipitate or crytallization results after 30 minutes,
immerse the beakier in an ice bath and allow it to cool
down, undisturbed for 15-20 minutes. Crystals (i.e., the pure compound) should form in the flask and leave the soluble impurities behind in the solution. The pure compound should be isolated using vaccum filtration (Figure 2 at right). (DO NOT ATTEMPT TO FILTER IF THERE DOES NOT APPEAR TO BE ABOUT 0.5-1.0 g of crystals formed). Collect the pure compound from the filter paper and allow the material to dry before attempting to take a melting point. |
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 Figure 2: Vacuum Filtration |