PREPARATION OF PURE COMPOUNDS 25 



V. SEPARATION OF SOLIDS. 



It is almost impossible to formulate a general scheme for the 

 separation of a mixture of solids. The separation depends to a very 

 large extent upon whether all the constituents, or whether only one 

 constituent, or whether one or more groups of compounds in the 

 mixture, is to be investigated ; it depends also upon the nature of 

 the constituents in the mixture and of those requiring separation. 



A scheme of extraction with various solvents, such as the following, 

 may be adopted for the separation of the constituents in a natural 

 product. 



Extraction with Solvents. 



The solvents most commonly used in separating one or a group of 

 constituents from plant or animal material are : 



(1) Ether, chloroform, petroleum ether, acetone, methyl alcohol. 



(2) Dilute mineral acid (HC1). 



(3) Dilute alkali sodium carbonate and sodium hydroxide. 



(4) Water or glycerol. 



When solvents such as ether, petroleum ether, etc., which are not 

 miscible with water are used as the first solvent, the material is gener- 

 ally dried preparatory to their use. 



The extraction of the material can be carried out in several 

 ways : 



(1) If ether, etc., be used, the extraction is effected in a Soxhlet 

 apparatus (p. 178), or, more simply, by hanging up the material in a 

 paper or linen bag in a wide tube so that condensed liquid falls upon 

 the material and from the material back to the extracting solvent 



(2) In a percolator. 



(3) Or simply in a glass or metal vessel. The mass is frequently 

 stirred with the solvent and filtered off and the residue pressed out. 



Drying of Material. 



Plant and animal tissues are composed to a very large extent of water, 

 which varies from 10-75 P er cent - m amount, and in order to ensure proper 

 contact of solvent, which is immiscible with water, with the material it is 

 necessary to dry it before extraction. 



Plant tissues are generally more easily dried than animal tissues ; the 

 material is brought into a fine state of division by chopping, or grinding, or 

 some other process, by hand or by machinery, and it is exposed to temperatures 

 ranging from 40-100, depending on the constituent required. 



Animal tissues which consist mainly of protein are more troublesome to 

 obtain in a dry state. They are more difficult to bring into a fine state of 



