PREPARATION OF PURE COMPOUNDS 



C. Distillation in Vacuo. 



Liquids which boil at high temperatures and decompose on distillation 

 under atmospheric pressure can frequently be distilled under reduced pressure. 



The liquid is distilled from a fractionating flask, the side tube of which is 

 inserted in another fractionating flask, or other stout vessel with a side tube, 

 which acts as receiver. This is kept cold by allowing a stream of cold water 

 to run over it ; the water is collected in a funnel, which serves as a support 

 to the receiver and it runs thence to the waste. The vacuum is produced by 

 connecting the side tube of the receiver with a water pump or a mechanical 

 pump. A gauge is in connection between the apparatus and the pump by a 

 T piece so that the pressure can be ascertained. 



To ensure continuous ebullition without bumping a slow stream of air, 

 or carbon dioxide from a Kipp apparatus if the liquid tends to oxidise or de- 

 compose in the air, is passed through the liquid by inserting in the cork a tube 

 with a long capillary which reaches almost to the bottom. The supply of air 

 is regulated by means of a screw, pinch on a piece of pressure-tubing placed 

 on the end of the glass tube. The apparatus is shown in Fig. 2. 



TO PUMP 



FIG. 2. 



A distilling flask with a double neck as in Fig. 7 (p. n) is more convenient 

 than a simple distilling flask ; the neck with the side tube carries the thermo- 

 meter, the other neck the capillary. 



The heating of the flask during distillation in vacua is best effected by 

 means Of a bath ; overheating is prevented, and the flask and its contents can 

 be completely immersed which ensures a uniform temperature. 



Rubber corks are used in vacuum distillation as they more easily prevent 

 leakage in of air. If they are attacked by the vapours of the liquid they 

 may be protected by placing a piece of cork between their ends and the 

 vapours. 



