

ORGANIC ACIDS AND THEIR SAT.73 



M 



grapes are made into wine, crude acid potassium tartrate, 

 called "argol," is precipitated in the vats. When purified it is 

 called "cream of tartar." If it is treated with sulphuric 

 acid and recrystallized, tartaric acid is produced. The neutral 

 calcium salt is insoluble in water. The acid potassium salt 

 is soluble in water but practically insoluble in alcohol. 

 Tartaric acid crystallizes in large monoclinic prisms, soluble 

 in water and in alcohol. The sodium potassium tartrate is 

 Rochelle salts, and potassium antimonyl tartrate is tartar 

 emetic, both of which are used in medicine. 



(e) Tannic Acid, commonly known as tannin, is extracted 

 from gall nuts (Section 89, i), and forms usually a light, 

 yellowish-buff, amorphous powder or small scales, soluble in 

 water and in alcohol, insoluble in ether, and of acid reaction. 

 It has an astringent, sour taste, precipitates some proteins, 

 notably gelatine (Section 230), and forms a blue-black pre- 

 cipitate with ferric salts. Its formula is: 



HO 



HO 



O 



C— O— 



OH 



/\ 



OH 



digallic acid 



HO 



COOH 



On boiling with dilute mineral acids it hydrolyzes to 

 two molecules of gallic acid: 



COOH 



/\ 

 / \ 



HO 



OH 



OH 



Tannic acid is used in making inks with ferric salts and as 

 an astringent in medicine. 



89. Tannins. — These are a group of compounds found in 

 various plants, «and in all parts of plants, namely, roots, 



