Purification of solids by crystallisation

The purification of solids by crystallisation is based upon differences in their solubility in a given solvent or mixture of solvents. In its simplest form, the crystallisation process consists of (i) dissolving the impure substance in some suitable solvent at or near the boiling point,... 

Crystallisation is the simplest and, perhaps, the oldest method of purifying solids. Because of the effectiveness of the method, crystallisation finds extensive use for the purification of solids. Appreciable difference of solubility of the given solid in a suitable solvent at the boiling point of the solvent and at room temperature forms the basis of this purification technique. The amount of impurities present in the solid is, of course, much less than that of the principal substance and it is unlikely that they will saturate the solution at room temperature. Therefore, these impurities remain in the mother liquor in the end. The steps involved in crystallisation are (i) Preparation of a clear saturated, or nearly saturated, solution of the solid, (ii) Cooling the solution, (iii) Collecting the solid by filtration and removing the residual solvent from the solid. 

Solid organic compounds when isolated from organic reactions are seldom pure they are usually contaminated with small amounts of other compounds ( impurities ) which are produced along with the desired product. Tlie purification of impure crystalline compounds is usually effected by crystallisation from a suitable solvent or mixture of solvents. Attention must, however, be drawn to the fact that direct crystallisation of a crude reaction product is not always advisable as certain impurities may retard the rate of crystallisation and, in some cases, may even prevent the formation of crystals entirely furthermore, considerable loss of... 

Acids that are solids can be purified in this way, except that distillation is replaced by repeated crystallisation (preferable from at least two different solvents such as water, alcohol or aqueous alcohol, toluene, toluene/petroleum ether or acetic acid.) Water-insoluble acids can be partially purified by dissolution in N sodium hydroxide solution and precipitation with dilute mineral acid. If the acid is required to be free from sodium ions, then it is better to dissolve the acid in hot N ammonia, heat to ca 80°, adding slightly more than an equal volume of N formic acid and allowing to cool slowly for crystallisation. Any ammonia, formic acid or ammonium formate that adhere to the acid are removed when the acid is dried in a vacuum — they are volatile. The separation and purification of naturally occurring fatty acids, based on distillation, salt solubility and low temperature crystallisation, are described by K.S.Markley (Ed.), Fatty Acids, 2nd Edn, part 3, Chap. 20, Interscience, New York, 1964. 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

Simple substances, and in particular those which are also readily volatile, such as hydrocarbons, alcohols, esters, the lower acids and amines, are distilled under atmospheric pressure. All substances which decompose easily, and those which have especially high boiling points, are distilled under reduced pressure. In general solid substances should only be distilled when purification by crystallisation has been unsuccessful on account of too great solubility or for other reasons. Naturally in each case the possibility of distillation (without decomposition) must be established in advance. 

Fractional solidification and its applications to obtaining ultrapure chemical substances, has been treated in detail in Fractional Solidification by M.Zief and W.R.Wilcox eds, Edward Arnold Inc, London 1967, and Purification of Inorganic and Organic Materials by M.Zief, Marcel Dekker Inc, New York 1969. These monographs should be consulted for discussion of the basic principles of solid-liquid processes such as zone melting, progressive freezing and column crystallisation, laboratory apparatus and industrial scale equipment, and examples of applications. These include the removal of cyclohexane from benzene, and the purification of aromatic amines, dienes and naphthalene. 

Solid organic compounds when isolated from organic reactions are seldom pure they are usually contaminated with small amounts of other compounds (impurities) which are produced along with the desired product. The purification of impure crystalline compounds is usually effected by crystallisation from a suitable solvent or mixture of solvents. 

The choice of the appropriate method, or sometimes two or more alternative methods for the separation of a particular mixture into its constituents, will most often depend upon the physical state (gaseous, liquid or solid) in which those constituents are present at room temperature. Crystallisation, for example, will not normally be contemplated for effecting the separation of the components of a mixture if they are gaseous or liquid, nor would purification of a solid by distillation be attempted normally. 

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