TANNINS 97 



CLASSES OF TANNINS 



The tannins are divided into two general classes, known 

 respectively as the pyrogallol tannins and the catechol tannins. 

 These differ in their characteristic reactions as follows: 



Pyrogallol variety Catechol variety 



Ferric salts Dark blue Greenish black 



Bromine water No precipitate Yellow or brown precipitate 



Leather Produce a " bloom " No " bloom " 



Cone, sulfuric acid Yellow or brown Red or pink 



Lime water Gray or blue ppte. Pink to brown ppte. 



Pyrogallol tannins contain approximately 52 per cent of carbon; 

 while the catechol tannins usually contain 59 per cent to 60 per 

 cent, the difference being due to the absence of glucose from the 

 molecule in the latter types. 



The two types are distributed in plants as follows: pyrogallol 

 tannins in oak-galls, oak wood, sumac, chestnut, divi-divi, and 

 algaro billa; catechol tannins in the barks of pines, hemlocks, 

 oaks, acacias, mimosas, cassia, and mangrove, in quebracho wood, 

 canaigre roots, cutch ,and gambier. The so-called " pseudo- 

 tannins " (i.e., compounds which do not tan leather but possess 

 other properties like tannins) are found in hops, tea, wine, fruits, 

 etc. 



SOME COMMON TANNINS 



Ordinary commercial " tannin," or " tannic add," is a com- 

 pound of one molecule of glucose with five of digallic acid. It is 

 found in many plants, and is prepared commercially from the 

 Turkish oak-galls and the Chinese sumac-galls. It exhibits all 

 the characteristic properties which have been listed above for 

 tannins in general and responds to all the characteristic reactions 

 of a pyrogallol tannin. It is extensively used for the manufacture 

 of blue-black ink, and in many technical processes. 



Catechu tannin and catechin are compounds of the catechol 

 tannin type. The latter is obtained from acacia wood, mahogany 

 wood, mimosa wood, etc. It is not a true tannin, since it does not 

 convert hide into leather; but when heated to 120 or above, it is 

 easily dehydrated, forming catechu tannin which is identical with 

 that which is obtained directly from gambier and Bombay cutch 

 (products made by evaporating water extracts from the bark of 



