202 TANNINS 



3. It reduces ammoniacal silver nitrate and Fehling's solu- 

 tion. 



4. It turns brown in alkaline solution when exposed to 

 the air ; its powerful reducing properties enable it to be used 

 in photography as a developer. 



PROTOCATECHUIC ACID. 



Protocatechuic acid is closely related to pyrocatechol, 

 differing from this substance only by one atom of carbon 

 and two of oxygen which it loses when heated above its 

 melting point (199), thus: 



C 6 H 3 (OH) 2 COOH = C 6 H 4 (OH) 2 + CO 2 

 Protocatechuic acid Pyrocatechol 



It rarely occurs uncombined except, for example, in the 

 fruits of Illicium religiosum ; in combination, it is found in 

 such substances as Catechin and Maclurin,* both of which give 

 protocatechuic acid on potash fusion ; it may further be ob- 

 tained by a similar process from many resins such as gum 

 benzoin, asafoetida, myrrh and also from kino. 



Finally its dimethyl ether, known as veratric acid, 

 C 6 H 3 (OCH 3 ) 2 COOH, occurs together with the alkaloid 

 veratrine in the seeds of Veratrum sabadilla. 



Protocatechuic acid is soluble in water and melts at 199. 



Reactions. 



1. Aqueous solutions of protocatechuic acid are precipitated 

 by lead acetate. 



2. It gives a green colour with ferric chloride ; on addition 

 of very dilute sodium carbonate the green colour changes first 

 to blue and then to red. 



3. Ferrous salts produce with protocatechuic acid a viol< 

 colour. 



* Maclurin, sometimes also called moringatannic acid, is a colouring mattt 

 of fustic ; its constitution is represented by the formula 



OH OH 



OH 



