GALLOTANNIC ACID 289 



2. Chinese galls, produced by the burrowing of Aphis 

 chinensis in the leaf-stalks of young twigs of Rhus semialata. 

 These galls may contain up to 70 per cent of gallotannic acid. 



Gallotannic also occurs in sumach {Rhus Coriaria), in tea, 

 and in many other plants. 



Extraction of Gallotannic Acid. 



Gallotannic acid is best prepared by extracting finely- 

 powdered gall nuts with a mixture of 12 parts of ether with 

 3 parts of alcohol ; 12 parts of water are then added and, 

 after shaking, the lower aqueous layer is run off from below 

 and evaporated. The resulting tannic acid may be decolorized 

 by boiling with animal charcoal. 



Pelouze recommends the following method : The pow- 

 dered material is heated under a reflux condenser with a 

 mixture of 30 parts of ether, 5 parts of water, and 2 parts of 

 alcohol. On cooling three layers of liquid are formed, of which 

 the lowest contains 33 per cent, the middle 8 per cent, and 

 the top 2 per cent of the tannic acid present in the substance. 



Gallotannic acid forms an amorphous powder * which, 

 when pure, is almost colourless ; it is readily soluble in water, 

 forming a solution with an astringent taste and which reacts 

 acid to litmus ; it dissolves also in alcohol or glycerol, but is 

 only sparingly soluble in ether and is insoluble in chloroform, 

 benzene, ligroin or carbon disulphide ; it is also insoluble 

 in hydrochloric or sulphuric acids and is precipitated by these 

 substances from its aqueous solutions ; it is soluble in alkalis, 

 and the solution, as in the case of gallic acid or of pyrogallol, 

 rapidly absorbs oxygen from the air and darkens in colour. 



When boiled with 2 per cent hydrochloric acid for some 

 time, gallotannic acid is broken up into gallic acid. 



If heated slowly from 160 to 215° and kept at the higher 

 temperature for thirty minutes, carbon dioxide, water, pyro- 

 gallol and metagallic acid are produced. The pyrogallol vola- 

 tilizes and condenses in the cooler part of the vessel. 



* What is known as " Crystal tannin " in commerce is not really 

 crystalline ; it is made by drawing a syrupy solution into threads and 

 breaking these up after drying. 



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