2i 4 TANNINS 



ELLAGITANNIC ACID. 



This tannin, which is commonly found together with 

 gallotannic acid, is important as being the mother substance 

 of ellagic acid, which is responsible for the bloom characteristic 

 of pyrogallol tannins. The quantity of this substance present 

 in different plants varies considerably ; it is greatest in divi- 

 divi. Amongst the other tannins giving ellagic acid bloom 

 may be mentioned algarobilla, myrobalans, chestnut tannin, 

 pomegranate tannin, valonia, etc. 



Ellagitannic acid, unlike ellagic acid (p. 205), is soluble 

 in water or alcohol ; .prolonged boiling with water converts it 

 into ellagic acid. It has been variously described by different 

 authors as a glucoside, as a hydrated soluble form of ellagic 

 acid, or as a condensation product of ellagic acid with gallic acid.* 



PYROCATECHOL TANNINS. 

 CATECHU TANNIC ACID. 



Catechu tannic acid is the name given to the tannin con- 

 tained in gambier catechu and in Bombay catechu or cutch,f 

 a substance obtained by evaporating an aqueous extract of 

 the bark of various trees (see below). A similar tannin is 

 also contained in kino. 



Little is known as to its constitution, but it is believed to 

 be an anhydride of catechin. 



CATECHIN. 



This substance, which is obtained from Acacia catechu, 

 Ouroparia catechu, mahogany wood, Mimosa, and pyrocatechol 

 tannins in general, is not in itself a tannin since it does not 

 precipitate gelatine, but it is converted into a tannin, namely 

 catechu tannic acid, by loss of water, a change which may be 

 rapidly brought about by heating to 120 or above. 



Catechin may be prepared by extracting powdered catechu 

 with ether ; the crude material obtained on evaporating off the 

 ether may be purified by crystallization from water. 



*Cf. Nierenstein: " Ber. deut. chem. Gesells.," 1907, 40, 4575; X 9 9 42, 

 353; 1910,43, 1257. 



f This substance is used largely for dyeing. 



