Crystallization is the removal of a solid from solution by increasing its concentration above the saturation point in such a manner that the excess solid seperates out in the form of crystals.
The preparation of a chemical compound usually affords a crude product and there is a 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 excess amount of the solute is thrown out as crystals when it is cooled. The process of crystallization involes the following steps.
Choice of a solvent
The solvent is chosen on hit and trial basis and it is necessary totry a number of solventsbefore arriving at a conclusion. An ideal solvent should have the following features.
- It 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.
- It should be inexpensive.
- It should be safe to use and should be easily removable.
The solvents which are mostly used for crystallization are, water, rectified spirit (95% ethanol), absolute ethanol, diethyl ether, acetone, chloroform, carbon tetrachloride, acetic acid and petroleum ther. If non of the solvents is found suitable for crystallization, a combination of two or more miscible solvents may be employed. If the solvent is inflamable 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.
Preparation of the saturated solution
After selectiong a suitable solvent, the substance is then dissolved in a minimum amount of solvent and is heated directly or on a water bath with constant stirring. Add more solvent to the boiling solution if necessary untill all the solute has dissoled.
The 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 premaure crystallization of the solute on the filter paper or in the funnel stem. If necessary hot water funnel should be used for this purpose.
The hot filetred solution is then cooled at a moderate rate so that medium sized crystals are formed. Slow cooling yields bigger crystals which are likely to include considerable amount of solvent carrying impurities with it and complicating the drying process.
Collecting the crystals
When the crystallization is complete, the mixture of cystals and the mother liquor is filtered through a gooch crucible using a vacume pump. Full suction is applied in order to drain the mother-liquor from the crystals as effectively as possible. When the filter cake is rigid enough it is pressed firmly with a cork to drain the left over liquid.
The crystals are then washed with a small portion of cold solvent and the process is repeated several times. The mother liquor is quite often concentrated by evaporation and cooled to obtain a freshcrop of crystals. The process of crystallization appears to be very simple yet the success of operation lies in the amount or the percentage of crystallized product obtained from the cryde substance.
Drying of the crystallized substance
Pressing it between several folds of filter papers and repeating the process several times dries the crystallized substance. This process has the disadvantage that the crystals are crushed to a fine powder and sometimes the fibres of filter paper contaminate the product. Alternatively, the crystals are dried in an oven provided the substance does not melt or decompose on heating at 100˚C.
A safe and reliable method of drying crystals in through a vaccum desiccator. In this process the crystals are spread over a wash glass and kept in a vaccum desiccator for several hours. The drying agents used in a desiccator are CaCl2 , silica gel or phosphorus pentaoxide.
Decolourization of undesireable colours
Sometimes during the preparation of a crude substance, the colouring matter or resinous products affect the appearance of product and it may appear coloure. Such impurities are conveniently removed by boiling the substance in the solvent with the sufficient quantity of finely powdered animal charcol and then filtering the hot solution. The coloured impurities are absorbed by animal charcol and the pure decolourized substance crytallizes out from the filterate on cooling.
It is a process in which a solid, when heated, vapourizes directly without passing through the liquid phase and these vapours can be condensed to form the solid again. It is frequently used to purify a solid. Examples of such solids are ammonium chloride, iodine, naphthalene, benzoic acid etc. To carryout the process, the sunstance is taken in a watch-glass covered with an inverted funnel. The substance is then heated slowly over a stand bath and the funnel is cooled with wet cotton. The pure solid deposits on the inner side of the funnel.