GALLOTANNIC ACID 211 



acid to litmus; it dissolves also in alcohol or glycerine, but is 

 only sparingly soluble in ether and is insoluble in chloroform, 

 benzene, ligroin or carbon disulphide ; it is also insoluble 

 in hydrochloric or sulphuric acids and is precipitated by these 

 substances from its aqueous solutions ; it is soluble in alkalis, 

 and the solution, as in the case of gallic acid or of pyrogallol, 

 rapidly absorbs oxygen from the air and darkens in colour. 



When boiled with 2 per cent hydrochloric acid for some 

 time, gallotannic acid is broken up into gallic acid 



If heated slowly from 1 60 to 215 and kept at the higher 

 temperature for thirty minutes, carbon dioxide, water, pyro- 

 gallol and metagallic acid are produced. The pyrogallol vola- 

 tilizes and condenses in the cooler part of the vessel. 



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

 solving I gram of tannin in 5 c.c. of glycerine, 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 produces a blue-black colour or precipi- 

 tate. 



3. A dilute solution of iodine in potassium iodide 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. (N.B. Neither pyrogallol nor gallic acid is precipi- 

 tated 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. 



