204 



TANNINS 

 Reactions. 



1. Phloroglucinol is precipitated from solution by lead 

 acetate. 



2. It gives with ferric chloride a bluish-violet colour. 



3. A solution of phloroglucinol in hydrochloric acid pro- 

 duces a red colour on a pine wood shaving ; this reaction can 

 also be made use of for detecting lignified cell walls (p. 164). 



4. It is a reducing agent, and reduces Fehling's solution. 



In addition to the above-mentioned phenols, which are 

 products of the decomposition of tannins by heat or by fusion 

 with alkalis, there are other important substances produced 

 by acid hydrolysis, namely, gallic and ellagic acids and the 

 phlobaphenes. 



GALLIC ACID. C 7 H 6 O 5 . 



Gallic acid, C 6 H 2 (OH) 3 COOH, was first prepared by Scheele 

 in 1786 by leaving an aqueous extract of gall nuts to stand in 

 a warm place, and from time to time removing the layer of 

 mould which formed on it ; the crystalline precipitate which de- 

 posited from the solution was purified by recrystallization from 

 water. Gallic acid, besides occurring in gall nuts, both free 

 and in the form of its anhydride tannic acid, is also found free 

 in sumach, divi-divi, the fruits of Cczsalpinia coriaria^ in the 

 leaves of Arctostaphylos Uva-ursi, and in tea and wine. It 

 may be prepared from tannic acid by acid hydrolysis. 



Gallic acid crystallizes in silken needles, and melts at 

 220, forming pyrogallol and evolving carbon dioxide; it is 

 sparingly soluble in cold water, but dissolves readily in hot 

 water and in alkalis ; alkaline solutions, like those of pyro- 

 gallol, absorb oxygen from the air, becoming brown in colour ; 

 they also reduce metallic solutions of silver or gold and 

 Fehling's solution. 



Gallic acid is converted into its anhydride digallic acid by 

 heating with phosphorus oxychloride to 130 or by boiling 

 with arsenic acid : 



2C 6 H 2 (OH) 3 COOH = C 14 H 10 9 + H 2 O 

 Gallic acid Digallic acid 



This digallic acid precipitates gelatine from solution, and for 



