28o TANNINS 



Vaccinium Vitis Idcea. Hydroquinone crystallizes from water 

 in colourless prisms and melts at 169-170°. 



Reactions. 



1. It gives no precipitate with lead acetate. 



2. Ferric chloride gives no colour but oxidizes it to 

 quinone. 



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. 

 OH 

 OH 



COOH 

 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 : — 



CgH3(OH)2COOH = CeH,(OH)2 -f COj 

 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, 



* Maclurin, formerly called moritannic acid, is a colouring matter of 

 fustic ; its constitution is represented by the formula 



OH OH 



HO / ^— CO— (^ >OH 



OH 



