14 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Frothing During Evaporation in l/acuo. 



Aqueous solutions of extracts of plant or animal matter have a great 

 tendency to froth during concentration in vacua. 



The froth can be broken by allowing drops of alcohol to fall upon it 

 from a tap funnel inserted into the distilling flask. The alcohol vapour helps 

 in the evaporation when the vacuum has been established. \ 



The liquid to be evaporated can be allowed to flow into the exhausted 

 vessel through a spiral tube at such a rate that no large volume is ever present 

 in the flask. 



A contrivance to catch the froth and return it to the flask has been devised 

 by Davis and Daish. It has been very useful for evaporating plant extracts. 

 It is shown in Fig. 9. 



To Water Pump 



FIG. 9. From the Journal of Agricultural Science, Vol. V, p. 435 (Cambridge Uni- 

 versity Press). 



. Any froth which is formed is broken by the piece of copper gauze in B 

 and the liquid runs back into the flask A. The vapours are condensed by 

 the condenser D and collected in G. The glass piece E is introduced so as 

 to allow of the emptying of G : the tap is turned and the screw clamp on 

 the pressure-tubing at S closed ; liquid collects in E whilst G is removed, 

 emptied, replaced and evacuated. The tube M leads to a manometer, the 

 bottles P and H and the valve J serve to regulate inequalities of pressure. 



