Recrystallisation is a method that is often used to purify solid samples. Recrystallisation depends on differences in the solubility of the sample and its impurities.
Choosing an appropriate recrystallisation solvent is critical to the success of this technique. The solid to be purified must have low solubility in the solvent at room temperature or below.
Procedure:
Solids are generally recrystallised in a conical flask.
The sample is dissolved in the minimum amount of recrystallisation solvent heated to its boiling temperature
Insoluble impurities are removed from the hot solution by gravity filtration (apparatus at left).
The filtrate is allowed to stand, cooling to room temperature when crystallisation is complete.
Why is a fluted filter paper used in gravity filtration?
The surface area to which the solution is exposed is greater than for a simple folded filter paper. This promotes more rapid transfer of the solution through the paper.
Why is gravity filtration used for hot solutions?
If a suction filtration were used, the solution containing the sample to be purified would drain into the suction flask under vacuum. This causes the solution to cool rapidly and results in a less pure product,
Choosing an appropriate recrystallisation solvent is critical to the success of this technique. The solid to be purified must have low solubility in the solvent at room temperature or below.
Procedure:
gravity
filtration
suction
filtration
filtration
suction
filtration
Solids are generally recrystallised in a conical flask.
The sample is dissolved in the minimum amount of recrystallisation solvent heated to its boiling temperature
Boiling stones (chips) or a boiling stick may be put into the solvent to ensure boiling without "bumping".
Insoluble impurities are removed from the hot solution by gravity filtration (apparatus at left).
This step may be omitted if it is not necessary.
The filtrate is allowed to stand, cooling to room temperature when crystallisation is complete.
The solution should not be disturbed during this period so that crystallisation occurs slowly so that impurities are not co-crystallised with the desired product.
The re-crystallised solid is separated from soluble impurities using suction filtration.The side-arm on the flask is connected to a vacuum source by a piece of thick-walled rubber tubing. The solution is poured into the funnel and the solid is sucked to neara dryness. Washing of the precipitate may be done by passing a few mL of cooled pure recrystallisation solvent through the solid.
Why is a fluted filter paper used in gravity filtration?
The surface area to which the solution is exposed is greater than for a simple folded filter paper. This promotes more rapid transfer of the solution through the paper.
This is important because if the solution cools too much, crystallisation occurs in the funnel.
Why is gravity filtration used for hot solutions?
If a suction filtration were used, the solution containing the sample to be purified would drain into the suction flask under vacuum. This causes the solution to cool rapidly and results in a less pure product,