202 TANNINS 



Reactions. 



1. Pyrocatechol is precipitated from aqueous solution by 

 lead acetate. (Distinction from resorcin and hydroquinone.) 



2. With ferric chloride it gives a green colour which is 

 changed to violet on the addition of sodium acetate. 



3. Like pyrogallol it reduces silver nitrate in the cold and 

 has therefore been used as a photographic developer. 



4. It reduces Fehling's solution on warming. 



RESORCINOL. C6H4(OH)2. 



This is isomeric with pyrocatechol (for constitutional formula 

 see page 200) ; it does not generally occur in tannins * but in 

 certain resins, notably galbanum resin and asafoetida. 



It is used commercially in the manufacture of dye-stuffs, 

 and when heated with sodium nitrite gives the indicator 

 known as Lacmoid. 



Resorcinol crystallizes from benzene in colourless needles 

 and melts at 1 19° ; it is somewhat soluble in water, the solution 

 having a sweetish taste. 



Reactions. 



1. It is not precipitated from solution by lead acetate. 



2. With ferric chloride it gives a dark violet colour which 

 is destroyed by the addition of sodium acetate. 



3. It reduces ammoniacal silver nitrate or Fehling's solution 

 on warming. 



HYDROQUINONE. 

 This third isomer of the formula C6H4(OH)2 likewise is 

 not found in tannins, but occurs combined with glucose in the 

 glucoside arbutin and uncombined in the leaves and flowers of 

 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. 



* According to Nierenstein, it is produced together with protocatechuic acid 

 and phloroglucinol from quebracho tannin by potash fusion. 



