Question

a) Why do you use a minimum of hot water solvent in carrying out a recrystallization of a particular compound?

b) Why is it desirable to use suction filtration rather than gravimetric filtration in recovering the purified crystals?

c) Why should the suction be released before the solid is washed with fresh solvent?

Answer 1

A) The most common method of purifying solid compounds is by recrystallization.

In this technique, an impure solid compound is dissolved in a solvent and then allowed to slowly crystallize out as the solution cools.

As the compound crystallizes from the solution, the molecules of the other compounds dissolved in solution are excluded from the growing crystal lattice, giving a pure solid.

The process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases.

For example, much more table sugar can be dissolved in very hot water (just below the boiling point) than in water at room temperature.

What will happen if a concentrated solution of hot water and sugar is allowed to cool to room temperature? As the temperature of the solution decreases, the solubility of the sugar in the water also decreases, and the sugar molecules will begin to crystallize out of the solution.

This is the basic process that goes on in the recrystallization of a solid.

B)In Suction filtration the rate of filtration is more compared to gravimetric filtration. Suction filtration is also called as vacuum filtration.

In suction filtration the Büchner flask, is fixed to a filtration flask. Then we should place a filter paper in the Buchner funnel. The size of the filter paper should not be more or should not be smaller than the size of the funnel.It should nicely fit into the funnel.

Then we need to connect the filtration flask to the vacuum liner through a rubber tube.

First ,wet the filter paper with the appropriate solvent, turn on the vacuum. The partial vacuum or the suction action will make the filter paper stick firmly onto the filter funnel.

Then the solid which is to be filtered from its solvent is used and it is dried also by using this equipment.

So here fitration is carried out with the help of pressure factor ,so it is faster compared to the normal gravimetric filtration.

Some times in the gravimetric filtration the fine solid may also come down along with the filtrate.

c) We need to ensure whether the suction is working properly or not and also to fix the filter paper firmly to the Buchner funnel before filtration, we need to use the suction, or else the solid compound may also come down alond with the filtrate. Sometimes we can use even two to three filter papers also to avoid the fine particles of solid in the filtrate.