PREPARATION OF PURE COMPOUNDS 



.CONDENSER 



21 



VAPOUR JAGt 



c 



VACUUM 



A TOB = 8 INCHES 



E TO 0=54 

 B TO F= 3 



TO FLASK 



FIG. 13. 



(v) Mother Liquor. 



The mother liquor generally contains some dissolved solid, which 

 should be recovered. The impure crusts, if any, are added to the 

 liquid and the liquid is concentrated by distilling or evaporating (p. 13) 

 until crystals begin to separate. The solution is poured or filtered into 

 a beaker and allowed to cool ; a second crop crystallises out and is 

 treated as above. A third and more crops may be obtained on further 

 concentration. These are not so pure as the original crop, but may 

 be recrystallised and obtained pure. 



(vi) Decolorising Solutions. 



Substances containing tarry or resinous impurities or colouring 

 matter cannot sometimes be freed from them by simple recrystallisation. 

 During recrystallisation and while the solid is in solution (especially 

 aqueous or alcoholic), the solution is boiled for 2-5 minutes or longer 

 with a small quantity of blood charcoal which is removed by filtration. 

 The first portions of filtrate generally require filtering again through 

 the same paper as the finely divided charcoal passes through at first. 

 To remove colouring matter from a solution which should be colour- 

 less, prolonged boiling with several quantities of charcoal is sometimes 

 necessary. 



The purification by crystallisation of about 5 gm. benzoic acid, 

 oxalic acid and succinic acid from water and of urea from alcohol serve 

 as simple examples of the method of recrystallisation. 



