The Purification of Solid Organic Compounds by Crystallization

Introduction. Compounds of carbon, whether obtained from natural sources or by means of a reaction, are seldom pure. Organic compounds from plants or animals usually occur in mixtures with related compormds those obtained through a cHemical reaction are contaminated with small amormts of other compounds which are produced along with the desired product. It is necessary, therefore, to consider in detail the methods used for the purification of both solid and liquid compounds of carbon. In the present experiment the purification of solids will be considered. 

Generally solids are divided into two clas.ses, crystalline and amorphous. The former are characterized by arrangement of particles according to a definite pattern, while the latter, as, for example, glass, do not have a crystalline structure. Most organic solid compounds are crystalline. The purification of impure crystalline compounds is usually accomplished by crystallization from an appropriate solvent. The property which is most frequently observed in order to determine the purity of a crystalline compound is the melting point, as discussed in the next experiment on page 41. 

Crystallization. In the purification of organic sohds by crystallization, the first step is to select a solvent which will dissolve the crude solid readily when hot, but only sparingly when cold. The crystalline substance is then dissolved in the hot solvent so as to form a nearly saturated solution. If the solution is colored, a small amoxmt of powdered charcoal is added to adsorb and remove the traces of coloring matter the solution is then filtered to remove the charcoal and insoluble impurities. On cooling, a certain amount of the dissolved substance separates out as crystals, leaving the greater part of the impurities in solution. The crystals are removed from the solution (often called mother liquor ), by filtration. The resulting crystalline solid is tested for purity, and if found impure 

The removal of impurities by crystallization is based upon the following considerations. Assume that in a reaction a crude compound A is obtained which contains 5 per cent of another substance B produced in the same reaction. In most cases the solubilities of A and B are different for any particular solvent. Let us assume for the first case that in a solvent Si, at 20° B dissolves to the extent of 2 g per 100 ml of solvent and A to the extent of 1 g per 100 ml of solvent. If 50 g of the crude (which is composed of 47.5 g of A and 2.5 g of B) is dissolved in 100 ml of solvent Si while hot, and the solution is allowed to cool, the mother liquor at 20° will contain 1 g of A and 2 g of 5. Therefore, the crystals (47 g) which separated on cooling contain 0.5 g of B. If the crystalhzation is repeated using 100 ml of fresh solvent Si, the second mother liquor will contain 1 g of A and 0.5 g of B. In this manner 45.5 g of A can be obtained in the pure state. 

In the above case it was assumed that the impurity is more soluble in a given solvent than the compound which is being purified. For the second case assume that the impurity is less soluble in a given solvent than the compound which is being purified. For example, 100 ml of solvent Sj dissolves at 20° 10 g of A and 3 g of. B. If 50 g of the impure compound is dissolved in 100 ml of solvent S2 while hot and the solution is allowed to cool, the mother liquor at 20° will contain 2.5 g of B (that is, all of B), and 10 g of A. In this manner 37.5 g of A will be obtained in the pure state. This discussion illustrates the general principle used in the purification of solids. The effect of each of the compounds on the solubility of the other, however, was not taken into consideration. In dealing with very small quantities this effect must be taken into accoxmt. For example, the solubility of a compound in alcohol may be 0.5 g per 100 ml but in the presence of another compound it may rise to 1.5 g per 100 ml. 

---