Filtration under Reduced Pressure: Need Buchner flask, Buchner funnel, pressure tubing, filter
paper and vacuum pump. This separates solid product from liquid.
Process: Connect tube from vacuum pump to Buchner flask. Fit Buchner funnel to flask
using Buchner rubber bung. Pour mixture onto filter paper. Rinse out beaker with excess
solid with solvent to get all solid. Rinse solid in funnel with more solvent.
Recrystallization: Solid product obtained from reduced pressure filtration contains
impurities, which removed during recrystallization. Impurities and product must have
different solubilities in chosen solvent to work.
Recrystallization always after reduced pressure to remove solid impurities.
Melting Point Determination: Used to determine if compound is pure.
– Melting range is when sample starts to melt and completes melting.
– Pure organic substance has sharp melting range of one/ two degrees. If impure
compound, solid has wide melting range.
– Impure sample has lower melting point than pure sample.
Process: Use dry sample. Rotate one end of capillary tube flame to seal. Cool tube.
Push capillary tube into sample and tap to fill. Place tube in apparatus. Heat up the
sample whilst observing sample in window. Record melting range. Can do another
sample but raise temperature slowly around range for accuracy
Another method is using oil bath or Thiele tube. Dip thermometer and capillary tube
in oil through cork. Using micro burner, slowly heat side arm of Thiele tube. Solid will
melt.
When ‘purification of impure aspirin from hot reaction mixture’.
– Impure aspirin from hot reaction mixture: Cool reaction mixture. Filter product under
reduced pressure
– Recrystallization of impure aspirin: Dissolve impure solid in minimum volume of hot
water/solvent. Cool solution and filter solid. Wash with cold water/solvent and dry.
