210 TANNINS 



is to be hoped that the synthesis* of "tannin" itself may 

 soon be effected. 



EXTRACTION OF GALLOTANNIC ACID. 



Gallotannic acid is best prepared by extracting finely- 

 powdered gall nuts with a mixture of twelve parts of ether 

 with three parts of alcohol ; twelve 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 thirty parts of ether, five parts of water, and two 

 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 -j- 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 glycerine, 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. 



The action of heat on tannins may also be studied by dis- 

 solving I gram of tannin in 5 c,c. of glycerine, heating slowly 

 to 210° and maintaining the liquid at this temperature for half 



*See also Fischer and Strauss : id., 1912, 45, 2467. 



t 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. 



