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. C 6 H 4 (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 C 6 H 4 (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 Id&a. 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. 



