20 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(iii) Collecting of Pure Crystals. 



Any crusts of crystals which may have formed on the sides and 

 edges of the vessel by evaporation must be removed before the re- 

 mainder of the crystals are collected, as they are impure. They are 

 carefully scraped off with a spatula and collected and returned to the 

 mother liquor after filtration. 



The pure crystals are collected on a filter plate or on a Buchner 

 funnel (p. 16). The vessel is rinsed out with a little solvent which is 

 used also for washing the crystals, and washing is done two or three 

 times. 



(iv) Drying of Pure Crystals. 



The crystals are left to drain as completely as possible in the 

 funnel and are then transferred to either 



(1) several thicknesses of filter paper and the liquid pressed 



out ; 



(2) a piece of unglazed porous plate, carefully dusted before 



use; 



(3) a watch or clock glass and dried in the air. 



To keep out dust they are covered over by a clock glass or funnel 

 which is raised up about 2 cm. on supports so that an air space is 

 left for the solvent to evaporate ; this may take 24 hours ; or they 

 are dried by placing them in a vacuum desiccator over sulphuric acid, 

 soda lime, etc. 



Sometimes the crystals, if placed on a watch glass and when they are 

 nearly free from solvent, are dried by putting the glass containing them 

 on a boiling water-bath. The crystals in this case should not melt 

 below 1 00 or contain sufficient solvent that they dissolve in it on 

 warming. They are cooled by placing in a desiccator. 



Some substances, e.g. carbohydrates, are very difficult to dry completely, 

 but can be obtained anhydrous by keeping them in vacua at 100 or 130 in 

 presence of phosphorus pentoxide for a few hours. A convenient form of ap- 

 paratus for drying such compounds is shown in Fig. 13. The substance is put 

 into a tube or boat and placed in the central vacuum vessel. This is connected 

 by a ground joint to a bulb containing P. 2 O 5 in the neck of which there should 

 be some glass wool to prevent 1 './)., from entering the vacuum tube. It is ex- 

 hausted and heated by inserting in a jacket tube, which is kept hot by the 

 vapour of a liquid boiled in a flask below and condensed above. 



