The obtaining of crystals by cooling the hot saturated solution leaving behind mother-liquor is called Crystallization.
The preparation of a chemical compound usually affords a crude product and there is an need to purify it by crystallization from a suitable solvent. The basic principle of crystallization is the fact that the solute should be soluble in a suitable solvent at high temperature and the maximum amount of the solute is thrown out as crystals when it is cooled.
Crystallization involves following steps.
- Choice of a Solvent
- The solvent should dissolve a large amount of the substance at its boiling point and only a small amount at the room temperature.
- It should not react chemically with the solute.
- It should either not dissolve the impurities or the impurities should not crystallize from it along with the solute.
- On cooling it should deposit well-formed crystals of the pure compound. (v) It should be in expensive, safe to use and easily removable.
Commonly used solvents are water, rectified spirit (95% ethanol), absolute ethanol, diethyl ether, acetone, chloroform, carbon tetrachloride, acetic acid and petroleum ether. If none of the solvents is found a combination of immiscible solvents may be employed. If the solvent is inflammable then precaution should be taken while heating the solution so that it does not catch fire. In such cases, water bath is used for heating purpose.
2. Preparation of the Saturated Solution
The substance is dissolved in minimum amount of the suitable solvent. Then it is heated directly or on water bath with constant stirring. More solvent is added to the boiling solution if necessary until all the solute has dissolved.
Insoluble impurities in the saturated solution are then removed by filtering the hot saturated solution, through a normal or a fluted filter paper. This avoids the premature crystallization of the solute on the filter paper or in the funnel stem. If necessary hot water funnel should be used for this purpose.
To prepare medium sized crystals, hot filtrate is cooled at a moderate rate. If cooling is carried slowly then bigger crystals are formed which are likely to include considerable amounts of solvent carrying impurities with it which complicates the drying process.
5. Collecting the Crystals
When the crystallization is complete, the mixture of crystals and mother-liquor is filtered through a Gooch crucible using vacuum pump. Full suction is applied in order to drain the crude mother-liquor from the crystals as effectively as possible. The crystals are then washed with a small portion of cold solvent and the process is repeated several times. The crude mother-liquor is quite often concentrated by evaporation and cooled to obtain a fresh crop of crystals.
6. Drying of the Crystallized Substance
The pure crystals obtained are then dried in an oven provided the substance does not melt or decompose on heating at 100°C.
Vacuum desiccator is used for the purpose as a reliable method of drying crystals. In this process the crystals are spread over a watch glass and kept in a vacuum desiccator for several hours. The drying agents used in desiccator are CaCl2, silica gel or phosphorous pentaoxide.
7. Decolourization of Undesirable Colors.
To remove colouring matter from a substance, it is boiled in solvent with finely powdered animal charcoal. The colouring matter adsorbed by the charcoal is removed by filtration. Then the pure decolorized substance is crystallized out by cooling the filtrate.