PREPARATION OF PURE COMPOUNDS 



B. Fractional Distillation. 



A mixture of two or more miscible liquids may be separated from 

 one another by fractional distillation if their boiling-points differ by 

 20-30. 



The mixture is distilled as described previously (p. 4), preferably 

 from a flask with its side tube high in the neck and the thermometer is 

 carefully watched. The first portion which distils will consist mainly 

 or entirely of the more volatile constituent which has a higher vapour 

 pressure ; the last portion will consist of the less volatile constituent 

 having a lower vapour pressure or higher boiling-point. Between 

 these portions there may be a small intermediate fraction consisting 

 of a mixture of the liquids. The three portions or fractions are 

 collected in separate receivers. 



Eg.- 



A mixture of equal volumes of chloroform and aniline will give on 

 fractional distillation a fraction boiling at 61 (almost pure chloroform), 

 an intermediate fraction, and a final fraction boiling at 183 (almost 

 pure aniline). During the distillation of such a mixture when the 

 boiling-point of a constituent exceeds 120 the water 

 should be run out of the condenser. 



Redistillation of the first and last fractions will 

 give each of them in a state of purity. 



If a mixture of liquids contains constituents which 

 have boiling-points fairly close to one another, the 

 fractionation must be repeated several times until each 

 fraction is found to have a constant boiling-point. 



The separation of such a mixture is greatly facili- 

 tated by the use of a fractionating column or still-head. 

 This is simply a device to lengthen the neck of the 

 distilling flask so that the higher boiling fractions are 

 exposed to the air and condensed before they reach 

 the condenser and run back into the flask. Numerous 

 forms have been invented ; two efficient forms are 

 those of Hempel and of Young (pear still-head, Fig. 4). 



The former consists of a glass tube filled with 

 glass beads and a side tube. The latter consists of a 

 piece of glass tubing upon which are blown 2, 3, 4 or 

 more bulbs of a pear shape, and a side tube. The 

 liquid is placed in a round-bottom flask, the fractional 

 column inserted and this in turn connected by iN side tube to the 

 condenser. The mixture in the flask, to which several small pieces of 



FIG. 4. 



