Crystallization

CRYSTALLIZATION, the obtaining (formation) of a substance in a crystalline form. Of the three principal cases of crystal formation—upon sublimation (see), from a molten state, and from solutions—the last is of greatest importance. Usually one employs: 1. Slow evaporation of the solvent. 2. Addition of a third substance, miscible with the solvent and decreasing the solubility therein of the crystallizable substance; to the solution (usually hot) an precipitant is added until turbidity appears and the mixture is allowed to stand; for example, water is added to an alcoholic solution, petroleum ether to an ethereal solution, alcohol to phenol, and so on. 3. Cooling of a saturated hot solution; the substance is dissolved in a suitable solvent with heating and stirring, taking only slightly more solvent than is needed for dissolution, and filtering hot (preferably through a heated funnel); upon cooling crystals are deposited. Rest and slow cooling favor the growth of crystals, however the size of the crystals also depends on the nature of the substance. If it is desired to obtain a quantity of crystals greater than is possible upon cooling to room temperature, a cooling mixture is used, but in this case it is necessary to employ as solvents liquids that do not freeze at low temperatures, for example carbon disulfide, alcohol, ether, petroleum ether. It is often possible to induce the crystallization of a substance that has been released in the form of oil by introducing into it a small crystal of that substance ("infecting" it), and sometimes even a substance close to it in chemical structure. Rubbing the walls of the vessel with a glass rod also accelerates or induces crystallization. Crystallization is applied with the aim of purifying a substance or obtaining it freshly crystallized with a certain amount of "crystallization" solvent—water, alcohol, chloroform, etc. In some cases, to isolate a substance in a chemically individual form, recourse is had to the crystallization of its well-crystallizing simplest derivatives: salts, acetyl, benzoyl, and other derivatives. The formation of a well-crystallizing double compound of an indifferent organic substance, for example the compound of glucose and sodium chloride, is very rare: 2C₆H₁₂O₆ + NaCl + H₂O. Some substances, for example proteins, can be obtained in a crystalline form by salting out (see). In purifying a substance by crystallization (often repeatedly) one starts from the assumption that the crystallizing substance can be separated from an impurity due to its different solubility in a suitable solvent. In some cases it is possible to obtain a pure substance only by fractional crystallization. Cases of mixtures inseparable by crystallization and the formation of mixed crystals are observed. Crystallization is usually carried out in crystallizers—thin-walled low glasses—or in cups. The obtained crystals are freed from the mother liquor by washing on a Buchner funnel or they are laid out on an unglazed porcelain plate or filter paper that absorbs the mother liquor, and, if necessary, are pressed between leaves of filter paper. Upon concentrating the mother liquor or upon adding a precipitant to it, or upon the combined action of both, further portions of crystals can be obtained. In choosing a solvent it is necessary to bear in mind that it should not act chemically on the substances to be crystallized and should not contain crystallization-harming impurities, and that in the case of crystallization by cooling a hot solution, the solubility of the substance in the hot solvent should differ sufficiently sharply from the solubility in the cold solvent. The most common solvents are water, ethyl, methyl, and amyl alcohols, ether, benzene, chloroform, acetone, acetic acid, petroleum ether, phenol, pyridine, carbon disulfide, H₂SO₄ and others. For microscopic examination the substance is crystallized on a slide, since even with careful transfer to a slide the crystals are damaged. A drop of concentrated solution of the test substance is placed on a slide, covered with a cover glass, and left in the air or, if the substance easily melts, in a desiccator, and the resulting crystals are examined under a microscope.