The process for obtaining crystals of a substance by cooling its hot saturated solution is called crystallization.
Crystallization is used to purify the crude solid product.
Its principle is that ” Solute is highly soluble in a given solvent at high temperature and an excess amount of the solute comes out as crystals when this solution is cooled”.
It involves following the steps
(1) Choice of solvent (2) Preparation of saturated solution
(3) Filtration (4) Cooling
(5) Collecting the crystals (6) Drying the crystals
(7) Decolourization of undesirable colour
(1) Choice of Solvent
The solvent is chosen on a hit and trial basis. Many solvents are tried.
Properties (Characteristics) of an Ideal Solvent.
● It should dissolve a large amount of solute at its boiling point, and a very small amount at room temperature.
● It should not react chemically with the solute
● It should not dissolve impurities, or impurities should not crystallize out along with the solute.
● On cooling, it should give well-formed crystals of the pure compound
● It should be cheap
● It should be safe to use
● It should be easily removable.
Solvents for Crystallization
● Commonly used solvents for crystallization are water, rectified spirit ( 95 % ethanol ), absolute ethanol, diethyl ether, acetone, chloroform, CCI4, acetic acid and petroleum ether. ·
• If no solvent is found suitable, then a combination of two or more solvents is used
● If the solvent is inflammable then heating is done on a water bath.
(2) Preparation of Saturated Solution
The substance is dissolved in a minimum amount of the selected solvent.
It is then heated directly or in a water bath with constant stirring.
More solvent may be added to the boiling solution until all the solute is dissolved.
The insoluble impurities are removed, by filtering hot saturated solution.
The solution is filtered hot to avoid premature crystallization of the solute on the filter paper or in the stem of the funnel.
A hot water funnel may be used for this purpose.
The hot filtered solution is cooled at a moderate rate to obtain medium-sized crystals.
Slow cooling gives bigger-sized crystals, which usually contain solvents with impurities. Such crystals are difficult to dry.
(5) Collecting the Crystals
When crystallization is complete, the mixture of crystals and mother liquor is filtered through the Gooch crucible using a vacuum pump ·
Full suction is applied to remove maximum liquor from the crystals.
If the filter cake is hard, it is pressed firmly with a cork to remove the remaining mother liquor.
• Crystals are then washed with a small amount of cold solvent several times
The mother liquor is often concentrated by evaporation. It is then cooled to obtain a fresh new crop of crystals.
The efficiency of the crystallization process depends upon the percentage of pure material obtained from crude substances.
(6) Drying the Crystallized Substance
There are three methods for drying crystals
(1) Filter Paper:
• Crystals may be dried by pressing between several folds of the filter paper. This method has two disadvantages
(a) The crystals are crushed to a fine powder and
(b) The fibres of filter paper often contaminate the product.
Drying can be done in an oven if the substance does not melt or decompose on heating at 100 ° C.
(3) Vacuum desiccator:
A vacuum desiccator is a much better method. In this, crystals are spread on a watch glass and kept in a vacuum desiccator for some time.
The drying agents used in a desiccator are CaCl₂, Silica gel or P2O5 (7)
Decolourization of Undesirable Colours
During the preparation of a crude substance, sometimes colouring materials or resinous products make the substance coloured.
Colouring impurities are removed by boiling the substance in the solvent with animal charcoal.
This solution is then filtered. The coloured impurities are adsorbed with animal charcoal.
Pure decolourized substance crystallizes out from the filtrate on cooling.
Read more about Filtration.