PREPARATION OF PURE COMPOUNDS 19 



(i) Solution. 



The substance to be recrystallised is powdered finely and placed in 

 a flask or beaker with a small quantity of solvent. Excess of solvent 

 must be avoided as the object is to prepare a hot saturated solution. 

 The solvent is boiled. If, after boiling for some time, a consider- 

 able amount of solid remains, more solvent is cautiously added 

 (through the condenser) and the boiling continued ; solvent is added until 

 the whole of the solid, except insoluble matter, is dissolved. It should 

 be noted that thu j last portions of a solid often only dissolve with 

 difficulty and fresh solvent should not be added too soon. 



(ii) Filtration. 



Particles of insoluble impurity are now filtered off by rapidly filter- 

 ing the hot solution through a pleated filter paper. Very frequently 

 the solution is very concentrated and begins to crystallise immediately 

 filtration is commenced. To avoid this a funnel with a very short 

 stem or without stem is used, and it is previously heated in an oven or 

 by passing through a flame ; the solution is filtered whilst it is still hot. 



When the tendency to crystallise immediately is very pronounced the filtra- 

 tion must be carried out through a funnel heated by steam. The funnel 

 may be surrounded by coils of metallic piping through which steam from a 

 generator is passed, or it may be enclosed in a larger metal funnel with two 

 walls between which there is water and from the outer of which there is a 

 projection for heating the contents by a flame. Care must be taken that 

 inflammable liquids do not become ignited if this form of hot-water funnel be 

 used. The water can be raised to boiling and the flame removed. 



If crystallisation of solid should commence dur-ngthe filtration, the 

 funnel and paper are placed over the flask, the paper pierced and the 

 particles washed with a little solvent into the flask and the solution 

 again boiled up. 



The filtrate is collected in a beaker of such a size that it is not 

 filled more than two-thirds. 



Some liquids " creep," and if the vessel be filled too full or if 

 shallow dishes be used the solution will creep over the edge and de- 

 posit crusts of impure crystals over the edges. 



If crystals begin to separate before the filtration is completed, it is 

 best to heat the filtrate until solution is again effected. To exclude dust 

 and prevent evaporation, the beaker is covered with a clock glass with 

 its convex side uppermost. Condensed drops of solvent will then run 

 towards the side and not drop into the liquid and disturb the formation 

 of the crystals. The solution is set aside in a cool place to crystallise. 



Crystallisation may be complete as soon as the solution is cold, or 

 it may take several hours, or days. 



