TANNINS. 



The various kinds of tannin which can be extracted from gall nuts, 

 sumach, pomegranate, oak bark, kino, etc., seem to belong to two main groups 

 according to the reactions which they give with ferric chloride, bromine 

 water, etc, They appear to contain a pyrogallol nucleus, or a catechol 

 nucleus, thus : 



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. H 2 SO 4 Dark red ring at junction of liquids. 



Some of the tannins of the catechol variety, gambier and cutch, appear 

 to contain a phloroglucinol nucleus, since a pine shaving moistened with an 

 extract and treated with concentrated hydrochloric acid gives a red or purple 

 stain which is characteristic of phloroglucinol. 



Constitution. 



As yet the constitution of the various tannins has not been definitely 

 determined, but it appears from the work of Emil Fischer and his pupils that 

 the tannins are esters of glucose with gallic acid, digallic acid and other 

 complex phenolic acids. 



Fischer and his pupils have definitely shown that the tannin from 

 Aleppo, Chinese and other galls contains 7-8 per cent, of glucose. The 

 tannin was carefully purified ; the purified product was hydrolysed by 

 dilute sulphuric acid and the glucose identified and estimated. If the 

 former supposition that tannins were glucosides were correct, a larger 

 amount of glucose would result. The yield of glucose corresponds very 

 closely to that which would be obtained if tannin were the pentadigalloyl 

 ester of glucose. This constitution, though not absolutely proved, has been 

 made extremely probable by synthesis. Fischer has prepared esters of glucose 

 with a series of these phenolic acids and they possess the main properties of 

 the tannins. 



Tannins would therefore be constituted thus : 



mo 1 



where t represents a complex acid like tannic acid or digallic acid. 



These compounds correspond to the fats in which glycerol is the basis 

 and various fatty acids are combined with it. 



274 



