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No. 29. 

 IMPROVED METHODS OF EVAPOEATION IN THE LABORATORY. 



By H. G. BECKER, A.R.C.Sc.I., A.I.C., 

 Demonstrator in Chemistry, College of Science, Dublin. 



(Read June 26. Printed August 15, 1923.) 



Owing to the frequency with which the operation occurs in chemical work, it is 

 desirable that the methods of evaporation used in the laboratory should be as 

 efficient as possible. On the industrial scale evaporation has been brought to a 

 high degi-ee of efficiency, largely owing to the fact that in this case it is not 

 necessary to distinguish between true evaporation and ebullition. On the 

 laboratory scale, however, the problem is complicated by the requirement that the 

 liquid must vaporise without ebullition in order to avoid loss by spirting, and 

 this has led to the usual method of heating the liquid to a low temperature on 

 a water bath, and allowing the evaporation to proceed slowly over long periods 

 of time. 



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Fig. 1. 



The object of the work here described was to determine experimentally to 

 what extent this quiet evaporation could be hastened by allowing it to take 

 place under the most favourable conditions, and whether the saving in time 

 effected would justify the use of the more complicated apparatus which might be 

 required. 



For practical purposes the determining factors in the vaporisation of liquids 

 are (1) the rate of heat transmission to the liquid (which is determined by the 

 temperature of the source of heat and the conductivity of the containing vessel), 

 and (2) the rate at which the vapour is removed (which may be fixed by diffusion, 

 a current of air, or by the removal of atmospheric pressure). The ratio between 

 the two rates determines the temperature of the liquid, and also whether 

 vaporisation takes place with or without ebullition. From the point of view of 

 quiet evaporation the removal of the vapour is the more important of the two, 

 since the rate at which the vapour can escape from the free surface of an 

 adequately heated liquid is the determining factor in preventing the temperature 

 rising to the boiling point. The most favourable conditions for rapid, quiet 

 evaporation are therefore that the rate of removal of the vapour should be as 



SCIENT. FROO. R.D.S., VOL. XVII, NO. 29. 2 Y 



