212 TANNINS 



Detection of Gallic Acid in Presence of Gallotannic Acid. 



Gallic acid may be detected in the presence of gallotannic 

 acid by dissolving the mixture in water and extracting the 

 solution with ether ; the ethereal extract on evaporation yields 

 gallic acid which may be identified by the usual tests. 



Gallotannic acid may also be separated from gallic acid by 

 adding a solution of lead acetate strongly acidified with acetic 

 acid ; under these circumstances lead tannate is precipitated 

 while lead gallate remains dissolved. 



Similarly tannic acid is precipitated by many alkaloids and 

 basic substances which have no action on gallic acid. 



THE CONSTITUTION OF GALLOTANNIC ACID. 



The close relationship subsisting between gallotannic and 

 gallic acids was first observed by Scheele, who, by allowing an 

 infusion of gall nuts to undergo fermentation, obtained gallic 

 acid. Within recent years this change has been studied anew 

 by Fernbach,* who isolated a tannin splitting enzyme, tannase, 

 from Penicillium, and also by Pottevin,f who isolated a similar 

 enzyme from the mould Aspergillus. 



This change, which may be represented by the equation 



C 14 H 10 9 + H 2 = 2 C 7 H 6 5 



may be effected more rapidly by boiling gallotannic acid with 

 dilute sulphuric acid. 



When, therefore, it was found by Schiff j that gallic acid 

 could be converted back into the anhydride by means of phos- 

 phorus oxychloride it was assumed that this substance, which 

 was called digallic acid, was identical with natural gallotannic 

 acid or " tannin ". 



This view came to be generally accepted, although objec- 

 tions were raised from time to time on the ground that the 

 physical constants, such as electrical conductivity and optical 

 activity of natural tannin and synthetic digallic acid were 

 different. 



Until 1912 there was considerable uncertainty as to whether 

 tannin occurred in the plant combined with glucose in the 



* Fernbach : " Compt. rend.," igoo, 131, 1214. 

 fPottevin: id., 1900, 131, 1215. 

 J Schiff: " Ber. deut. chem. Gesells.," 1871, 4, 232. 

 Walden : id., 1897, 30, 3 I 5 1 5 l8 98, 31, 3167. 



