1 8 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The solvent in those cases where the substance is slightly soluble 

 or insoluble is now heated ; if the solid dissolves easily more is 

 added until the solution is saturated ; if not, more solvent is added 

 so as to bring, if possible, the solid into solution. The solution is 

 cooled by holding under running water and it is noticed how much 

 of the solid crystallises out. If a considerable quantity separates 

 out, the solvent will probably be suitable for recrystallising larger 

 quantities. 



Sometimes crystallisation does not occur spontaneously on cooling, 

 but it may be started by scratching the sides of the test tube or by 

 adding a crystal of the solid 



The following solvents are most frequently used : 



(1) water 



(2) alcohol 



(3) acetone 



(4) benzene 



(5) chloroform 



(6) ligroin 



(7) glacial acetic acid 



(8) methyl alcohol 



The following substances cane sugar, oxalic acid, benzoic acid, 

 urea and succinic acid may be taken as examples for observing the 

 difference in solubility in water and alcohol, and for the choice of 

 solvent for crystallisation. 



(b) Recrystallisation. 



If the suitable solvent has been found to be water, or glacial 



acetic acid or a liquid which is not 

 inflammable and boils at a fairly 

 high temperature, the recrystallisa- 

 tion may be carried out in a beaker 

 heated over a gauze. 



If the suitable solvent has been 

 found to be alcohol, acetone, 

 ligroin, benzene liquids which are 

 volatile and inflammable the re- 

 crystallisation must be carried out 

 in a flask to which is attached a 

 reflux or inverted condenser, as in 

 Fig. 12. 



Solvents boiling below 90 are 

 heated on the water-bath, above 

 100 over a flame, .through a wire 

 gauze and with an air condenser (inner tube of condenser or a tube 

 about 80 cm. long by "5-1 cm. in diameter) as reflux. 



FlO. 12. 



