204 TANNINS 



PYROGALLOL OK PYROGALLIC ACID. C8H^(OH)3. 



This substance is so called because it is formed by heating 

 gallic acid according to the reaction — 



C8H,(OH)3COOH = C6H3(OH)3 + CO^ 

 Gallic acid Pyrogallol 



It is also formed by fusing haematoxylin with caustic potash. 



Pyrogallol crystallizes in colourless needles or plates melting 

 at 132" and is soluble in water; its solution, in excess of 

 caustic alkali, absorbs oxygen with great avidity, turning brown 

 and forming carbon dioxide, acetic acid and other substances. 



Pyrogallol reduces salts of silver, mercury, or gold to their 

 respective metals. 



Reactions. 



1. Pyrogallol is precipitated from solution by lead acetate 

 but not by lead nitrate. 



2. It gives a blue colour with ferrous sulphate and a red 

 colour with ferric chloride. 



3. Aqueous or alcoholic solutions of pyrogallol, in common 

 with those of gallic acid or tannic acid, are coloured purple 

 by iodine. 



4. Lime water added to an aqueous solution of pyrogallol 

 produces a purple colour which rapidly becomes brown. 



5. Solutions of pyrogallol give no precipitate with gelatine. 



6. Potassium cyanide gives a reddish-brown coloration, 

 which turns brown, but the red tint becomes apparent again 

 on shaking. 



PHLOROGLUCINOL. C6H3(OH)3. 



Phloroglucinol, which is isomeric with pyrogallol, is pro- 

 duced by fusing a number of resins, such as catechin, kino, 

 dragon's blood, etc., with potash, and it occurs naturally, in a 

 number of glucosides, such as phloretin, quercetin, hesperidin, 

 etc. It crystallizes with two molecules of water, but loses 

 them if heated to 100°, and melts at 218°; it dissolves readily 

 in water, forming a sweet solution, and is readily soluble in 

 alcohol or ether. 



