Crystallisation

Crystallisation

This is the process most commonly employed for the purification of solid organic compounds. The impure salt is dissolved in the minimum volume of a suitable solvent. The soluble impurities pass into solution while the insoluble ones left behind. The hot solution is then filtered and allowed to cool undisturbed till crystallization is complete. The crystals are then separated from the mother liquor by filteration and dried. The efficiency of the process of crystallization depends on the skill with which the following steps are carried out :

(1) Choice of solvent. For a judicious choice of the solvent, about 50 mg of the solid is taken in a test tube and shaken with a few drops of the solvent. The solvent is unsuitable if whole of the solid goes into solution at room temperature. If the solid dissolves upon heating and throws out maximum crystals on cooling, the solvent is suitable. This process is repeated with the solvents such as water, benzene, ether, ethyl alcohol, acetone, etc., till the most satisfactory one is sorted out.

(2) Preparation of solution. A suitable quantity of the solid substance is taken in a conical flask fitted with a reflux condenser. A small volume of the solvent selected in step(1) is also placed in the flask. The quantity of the solvent should be just enough to dissolve the whole solid on boiling. The heating may be done on the water bath or wire gauge according as the solvent is low boiling or high boiling.

(3) Filtration of hot solution. The hot solution obtained above is then filtered through a fluted filter paper placed in an ordinary glass funnel. If the quantity of the solution is large, it takes longer and the crystals may form in the funnel during filteration. To prevent this, a hot-water funnel may be used.

(4) Crystallization. The hot filtrate is then allowed to cool undisturbed in a beaker. The pure solid substance separates as crystals. When a rich crop of crystals has been obtained, crystallization is complete. Sometimes crystallization does not occur even after cooling the filtrate for a sufficiently long time. In such a case the crystallization is induced either by scratching the walls of the beaker with a glass rod or by adding a few crystals of the pure compounds to the solution.

(5) Separation and drying of crystals. The crystals are separated from the mother liquor by filtration. The filtration is generally effected under reduced pressure using a buchner funnel as shown in fig. 2.1 . When the whole of the mother liquor has been drained into the filtration flask, the crystals are washed with small quantities of the pure cold solvent to remove adhering impurities. The crystals are then dried by pressing between pads of filter papers, in an oven, or in a vacuum desiccator (Fig 2.2).