4 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



I. PURIFICATION OF A LIQUID BY DISTILLATION- 

 DETERMINATION OF THE BOILING-POINT. 



A liquid is purified by distillation. The criterion of the purity of 

 a liquid is its boiling-point A pure liquid has a constant boiling- 

 point 



A. Drying and Cleaning of Apparatus. 



Since organic liquids are very frequently not miscible with water 

 all the apparatus which is used with them must be dry. 



The apparatus may be dried in an oven for some time, but on 

 removal to prevent deposition of aqueous vapour a current of air is 

 blown through them whilst they are hot and during cooling. The 

 current of air is most conveniently got from bellows, or by suction with 

 a pump. The tubing, or if the glass vessel be narrow, a glass tube 

 inserted in the tubing, is placed inside the vessel so that the farthest 

 extremity is dried and cooled first. 



More usually since the vessels are wet with water, they are rinsed 

 with alcohol after draining as much water away as possible, then with 

 ether after again draining. A current of air from the bellows or water- 

 pump is drawn through to evaporate the ether. The vessel may be 

 warmed in a luminous flame, when no more ether is present and 

 the vessel is not quite dry, and air driven through as above. It is im- 

 portant that all the ether be evaporated, since it may become ignited 

 or form an explosive mixture inside the vessel. 



Apparatus which contains charred matter may be cleaned by 

 oxidising it away with potassium bichromate and sulphuric acid, or 

 by heating in it a mixture of concentrated sulphuric and nitric acids, 

 washing with water and proceeding as above to dry it. 



B. Distillation of a Liquid and Determination of its Boiling- 

 point. 



The liquid is placed in a clean, dry fractionating or distilling flask 

 a round-bottom flask with a side tube in its neck 1 of suitable size 

 so that only about half or at most two-thirds of the space is filled. 

 Some small pieces of unglazed porcelain, or porous earthenware, or 

 pieces of platinum, are added to ensure steady boiling without bump- 

 ing. The neck of the flask is closed with a well-fitting cork 2 which 



1 With liquids of high boiling-point the flask should have the side tube low down so 

 as to prevent decomposition by the high temperature. 



Rubber corks are dissolved by many organic liquids and consequently are not used 

 except in special cases. 



