PREPARATION OF PURE COMPOUNDS V) 



(iii) By making alkaline. Aqueous and acid extracts may contain 

 bases ; when made alkaline with sodium carbonate or sodium hydrox- 

 ide, the base or bases may be precipitated. 



(iv) If no precipitate has been obtained in (ii.) or (iii.) the solution 

 may be submitted to steam distillation. Volatile acids and bases 

 distil over. 



(v) Acids may be precipitated from the exactly neutral solution 

 as insoluble salts of heavy and other metals by adding lead acetate, 

 mercurous nitrate, calcium chloride, etc. 



(vi) Bases may be precipitated by adding to the previously acidi- 

 fied solution phosphotungstic acid, or other reagents used for precipitat- 

 ing alkaloids from solutions. 



(vii) Bases may be precipitated (especially from alcoholic solution) 

 as double salts with mercuric chloride, gold chloride, platinum chloride. 



B. Extraction. 



The acid or alkaline extract may be extracted with an immiscible 

 solvent by repeated shaking or by means of an extractor (p. 600). 



C. Fractional Crystallisation. 



The solution on evaporation may deposit crystals. On filtering 

 and evaporating further, another crop of crystals may be obtained. 



D. Salting Out. 



Proteins, polysaccharides, soaps and other complex compounds 

 separate out when their aqueous solutions are saturated with sodium 

 chloride, magnesium, zinc and ammonium sulphates. 



E. Dialysis. 



Mixtures of colloids and crystalloids are separated by dialysis. 



F. By Preparing Suitable Chemical Derivatives. 



Some knowledge of the chemical nature of the compound is 

 essential before a derivative can be prepared. The principal reactions 

 of the various groups o,f componnds are given under the separate 

 headings. 



Two other procedures are occasionally employed in separating solids : 

 (i) Sublimation. On carefully heating a mixture in a basin, one or 



more solids may be volatilised. Tiie vapours are condensed on a cool clock 



glass or funnel and the substance is thus obtained. 



(ii) Sedimentation. A mixture of solids of different specific gravity 



may be separated by shaking with a liquid of suitable density. 



Separation may not be effected by applying any one of the above 

 methods. Usually only an incomplete separation is made and further 

 separation is carried out by using another method. 



