Crystallization

Crystallisation is the single most widely used method for purifying a SOLID organic compound, and it proceeds through four distinct stages. (i) Selection of solvent: because most organic substances, being covalent, do not dissolve well in a polar solvent like water, the first practical task is simply finding a solvent that works -- the powdered solid is placed in a test tube and a candidate solvent added little by little, with constant stirring and gentle heating, just until enough has been added to dissolve the solid; if the solid dissolves fully on heating and then throws out a generous crop of crystals again on cooling, that solvent is judged suitable, and if not, the whole process is simply repeated with a different candidate solvent (benzene, ether, acetone and alcohol are the ones specifically named) until a suitable one is found. (ii) Preparation of the solution: the substance is dissolved in the smallest practical quantity of the chosen solvent; a small amount of animal charcoal can optionally be stirred in first to decolorise any coloured impurity present, and the heating itself is done either over a plain wire gauze or over a water bath, depending on whether the solvent in use is low-boiling or high-boiling. (iii) Filtration of the hot solution: the hot solution is filtered, WHILE STILL HOT, through a fluted filter paper set in a funnel -- filtering hot avoids the dissolved product itself crystallising out prematurely on the filter paper. (iv) Crystallisation itself: the hot filtrate that comes through is then simply allowed to cool; most impurities remain behind on the filter paper from the previous step, and the pure solid compound separates back out of solution as crystals as the filtrate cools -- crystallisation is judged complete once a good, copious crop of crystals has formed, and if the process is proceeding too slowly, it can be sped up (induced) either by scratching the inside wall of the beaker with a glass rod, or by 'seeding' the solution with a few crystals of the already-pure compound. Finally, isolation and drying: the newly f …