TANNINS 275 



This constitution not only allows for a large number of varieties of tannins 

 where t is the same throughout the molecule, but also where t is different. 1 



In addition to the interest attached to the syntheses of a tannin there is 

 another interest. Some of the synthetical tannins possess very high molecular 

 weights; one of them has a higher molecular weight, 2051, than any other 

 known synthetical coinpound, and has largely exceeded the figure of 1213, 

 which is the molecular weight of the synthetical octadecapeptide (p. 363). 



Oak-gall Tannin. 



Tannin is prepared from finely powdered gall nuts by extraction with a 

 mixture of 1 2 parts of ether and 3 parts of alcohol ; 1 2 parts of water are 

 added and the mixture thoroughly shaken. The lower aqueous layer is 

 separated and evaporated. The tannin may be purified by boiling with char- 

 coal. Or the powdered gall nut is heated under a reflux condenser with a 

 mixture of 30 parts of ether, 5 parts of water and 2 parts of alcohol. 

 The lowest layer contains about 30 per cent, of tannin which is obtained 

 as above. 



Tannin is an amorphous powder ; the pseudo-crystalline appearance of 

 some kinds of tannin results from the method of its preparation, a syrupy 

 solution being drawn up into threads, which are dried and broken. It is 

 almost colourless when pure. It dissolves easily in water and its solution is 

 acid in reaction to litmus and has an astringent taste. It is soluble in alcohol 

 and glycerol, but very little soluble in ether, chloroform, ligroin. It is pre- 

 cipitated from solution by hydrochloric acid and sulphuric acid ; it is soluble 

 in alkalies and the alkaline solution turns brown on exposure to the air. 



It is decomposed on heating at 160-215 mto pyrogallol, gallic acid and 

 other products. 



Reactions. 



(1) With ferrous sulphate containing no ferric salt there is no change, 

 but on exposing to the air the solution darkens. 



(2) With ferric chloride a blue-black colour or precipitate is formed (ink). 



(3) It is precipitated from solution by gelatin, hide powder (leather and 

 other proteins). 



(4) It precipitates alkaloids from solution. 



(5) Dilute iodine solution gives a pink colour. 



(6) Potassium cyanide gives a reddish- brown colour, which changes to 

 brown ; on shaking with air the red tint appears again. 



(7) Lime water gives a grey precipitate. 



(8) It is precipitated by lead acetate or by lead nitrate. 



1 A summary of the researches on tannins is given in the " Ber. deutsch. Chem. Ges.," 

 . 46, 3253- 



