206 



TANNINS 



from which it will be seen that it may be considered to be pro- 

 duced by the abstraction of two molecules of water from two 

 molecules of gallic acid, with simultaneous oxidation or removal 

 of two atoms of hydrogen. 



Synthetically it may be prepared by the oxidation of gallic 

 acid by means of arsenic acid, or better by oxidizing gallic 

 acid in acetic acid solution with potassium persulphate and 

 sulphuric acid.* 



Whether or not this substance occurs free in nature is not 

 definitely established ; certain it is, however, that ellagic acid 

 can be readily obtained by the hydrolysis of ellagitannic acid, 

 a substance which almost invariably accompanies gallotannic 

 acid in the numerous vegetable products in which this latter 

 occurs ; it also occurs in conjunction with tannins of the pyro- 

 gallol class, and constitutes the bloom which is produced on 

 leather by" this type of tannin. 



The most convenient natural sources are " divi-divi " (Ccesal- 

 pinia coriaria), " algarobilla " (C&salpinia brevifolia)> "myro- 

 balans" (Terminalia Chebula), etc. Aqueous extracts of these 

 fruits on long standing frequently deposit ellagic acid, most 

 probably by the action of a ferment contained in the plant ; it 

 is, however, prepared by pouring a hot concentrated alcoholic 

 extract of divi-divi into cold water ; the acid is thereby pre- 

 cipitated, and may be filtered and purified. 



Ellagic acid is a yellow microcrystalline solid which is 

 very slightly soluble in water, and therefore readily separates 

 from aqueous solutions in which it is formed ; it is also very 

 slightly soluble in alcohol or ether, but dissolves somewhat 

 readily in boiling pyridine. The dried substance treated with 

 1-2 drops of nitric acid gives, on dilution with 10-20 drops of 

 water, a blood-red colour (Griessmayer's reaction). 



Ellagic acid is used to some extent as a dye-stuff, being 

 sold under the name of " Alizarine yellow in paste," for use 

 with chromium mordants. 



Catellagic, Metellagic, and Flavellagic acids are the names 

 given by Perkin to artificially synthesized acids obtained by 

 him. They are closely related to ellagic acid, but have not, 

 as yet, been found to occur naturally. 





* Perkin and Nierenstein : " J. Chem. Soc., Lond.," 1905, 87, 1415. 





