22 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(f) Crystallisation from a Mixture of Solvents. 



A solid may be too readily soluble in some solvents, but insoluble in other 

 solvents, and consequently its recrystallisation from a single solvent may be 

 difficult. Crystallisation may be then effected by using a mixture of solvents. 

 A concentrated solution is made in the hot solvent and the other solvent is 

 added whilst the first is still hot until the mixture becomes turbid, when it is 

 allowed to cool. Sometimes it is better to add the second solvent to a strong 

 solution until it is turbid, heating until the turbidity has gone, and adding, if 

 necessary, more of the first solvent. On cooling crystals may appear. 



Alcohol and water, acetone and water, benzene and ligroin, chloroform 

 and ligroin, alcohol and ether are mixtures of solvents frequently used. 



Trials should be made to ascertain the best mixture of solvents. 



(d) Crystallisation by Evaporation. 



Another possibility is that the substance is easily soluble in all solvents. 

 Crystallisation is then effected by partial evaporation. The filtered solution is 

 placed in a shallow dish, i.e. a crystallising dish ; it is covered with a funnel or 

 clock glass which is raised up so as to leave an air space and the dish is pre- 

 ferably placed on a clock glass in case the solution creeps, so that the crystals 

 can be scraped up without contamination from the bench. Crystals gradually 

 form and they are removed before complete evaporation has occurred. Crusts 

 are removed and treated as described above ; the crystals are filtered off, 

 washed with very little solvent and dried. The mother liquor in this case will 

 contain a large amount of solid. 



The evaporation need not be conducted in the air ; it may be hastened 

 by placing the solution in a desiccator over sulphuric acid, etc., and under 

 reduced pressure, or by evaporation by distilling the solution ; if aqueous, 

 evaporation over a flame or on a water-bath. 



(e) Solids Soluble with Difficulty in all Solvents. 



If a solid is almost insoluble in all solvents recrystallisation is effected, after 

 preliminary testing, by boiling the solid for some time with the best solvent, 

 filteiing off the insoluble portion, and evaporating the filtrate to a small 

 volume. The crystals which separate are treated in the way described 

 above. 



Operations with Small Quantities. 



Filters of small sizes are obtainable to retain the solid. Small filter flasks 

 to contain the mother liquor are obtainable in the shape of test ttfbes, or a 

 test tube may be placed inside the filter flask so that the mother liquor collects 

 in it. 



When there is too little solid and mother liquor for filtration, the crystals 

 and mother liquor are placed upon a piece of porous earthenware or between 

 sheets of filter paper, so that the liquid is absorbed. 



Recrystallisation of the solid may be effected in a small test tube, and 

 the mother liquor may be dissolved out of the filter paper or earthenware, if 

 required. 



