PREPARATION OF PURK COMPOUNDS 17 



IV. PURIFICATION OF SOLIDS BY CRYSTALLISATION. 



The majority of solid organic compounds are crystalline, but 

 many of the complex solids, such as starch, glycogen, various pro- 

 teins, which belong to the class of substances termed colloids, have 

 not yet been prepared in a crystalline form and are only known in an 

 amorphous state. Others, such as the fats, though obtainable in 

 a crystalline form, are mixtures of closely related substances and it 

 is extremely difficult, or almost impossible, to separate them into 

 individual compounds. 



The separation of a solid in a crystalline form is essential to its 

 preparation in a pure condition. Its recrystallisation, when once 

 obtained in a crystalline form, will lead to its preparation in a state 

 of purity. 



The criterion of the purity of a crystalline solid compound is its 

 melting-point. A pure solid organic compound melts sharply at a 

 definite temperature. An impure organic compound does not melt 

 sharply and it melts at too low a temperature. The knowledge of 

 the melting-point of a compound helps in its identification. 



(a) Choice of Solvent for Crystallisation. 



The purification of a solid by crystallisation depends very largely 

 upon the choice of a suitable solvent. The best condition for puri- 

 fication is very slight solubility in the cold solvent and ready solubility 

 in the boiling solvent. A hot saturated solution of the solid will de- 

 posit the greater part of the solid in a crystalline condition on cooling, 

 and if the solution be allowed to cool slowly the crystals which are 

 deposited will be more regular than if the solution be cooled rapidly. 



In order to ascertain the solubility of a substance in a solvent the 

 substance must be in a fine state of division. A small quantity of 

 the substance is finely powdered in a watch glass with a glass rod, or 

 better in a small agate mortar with a pestle. 



A few milligrams of the substance are then placed in a small test 

 tube, a few drops of solvent are added and the solid well stirred or 

 shaken with it. If the solid is apparently not soluble in this amount 

 of solvent more is gradually added, and so it can be determined 

 whether the substance is easily soluble, moderately soluble, or in- 

 soluble in the cold liquid. 



Substances which are readily soluble in cold solvent are not 

 usually recrystallised from this medium since there will be no great 

 difference in solubility in the hot and cold liquid, and a large quantity 

 of solid will remain in solution even when crystals are obtained, 



