^90 TANNINS' 



The action of heat on tannins may also be studied by dis- 

 solving I gram of tannin in 5 c.c. of glycerol, heating slowly 

 to 210° and maintaining the liquid at this temperature for half 

 an hour. The liquid is then cooled and shaken with 20 c.c. 

 of ether ; after the addition of water the ethereal solution is 

 separated and evaporated ; the residue contains pyrogallol. 



Reactions. 



1. Ferrous sulphate, free from ferric salts, produces at first 

 no change, but on exposure to air the solution darkens from 

 oxidation. 



2. Ferric chloride, or better, iron alum, produces a blue- 

 black colour or precipitate. 



3. A dilute solution of iodine in potassium iodide, added 

 to a faintly alkaline solution, gives a transient pink colour, as 

 in the case of gallic acid. 



4. Gallotannic acid is precipitated from solution by gela- 

 tine, and similarly combines with hide powder converting it 

 into leather. (Distinction from gallic acid.) 



5. Gallotannic acid precipitates proteins, alkaloids, and 

 many other organic substances from solution. 



6. Lead nitrate or lead acetate gives precipitates of lead 

 tannate. (Neither pyrogallol nor gallic acid is precipitated by 

 lead nitrate, though both give precipitates with lead acetate.) 



7. Potassium cyanide gives a reddish-brown colour which 

 changes to brown, but the red tint reappears on shaking with 

 air. 



8. Lime water gives a grey precipitate. 



Detection of Gallic Acid in Presence of Gallotannic Acid. 



Gallic acid may be detected in the presence of gallotannic 

 acid by dissolving the mixture in water and extracting the 

 solution with ether ; the ethereal extract on evaporation yields 

 gallic acid which may be identified by the usual tests. 



Gallotannic acid may also be separated from gallic acid by 

 adding a solution of lead acetate strongly acidified with acetic 

 acid ; under these circumstances lead tannate is precipitated 

 while lead gallate remains dissolved. 



