SECTION V. 



TANNINS. 



The term Tannin is variously employed by different writers, 

 sometimes to denote a particular substance better described as 

 gallotannic or digallic acid, and sometimes as a collective term 

 for a whole group of substances having certain characteristics 

 in common. In order to prevent confusion it is proposed here 

 to use the word tannin only in the latter sense. 



The properties of the tannins may be summarized as 

 follows : — 



1. They are mostly uncrystallizable colloidal substances 

 with astringent properties. 



2. They precipitate gelatine from solution and form in- 

 soluble compounds with gelatine yielding tissues, a property 

 which enables them to convert hide into leather.* 



For this purpose add 2 c.c. of 0-5 per cent of the tannin 

 solution to an equal volume of 0-5 per cent gelatine ; an im- 

 mediate precipitate or turbidity should appear. 



3. They all give blackish-blue or blackish-green colours 

 with ferric salts, a fact which is made use of in the manufac- 

 ture of ink. 



. This test is best carried out by adding 3-5 drops of i per 

 cent iron alum to 3 c.c. of a 0-5 per cent neutral solution of 



* According to some authors, this property is not an essential charac- 

 teristic of tannins ; on the other hand, Dekker prefers to regard those sub- 

 stances which do not give this reaction as pseudo-tannins and includes 

 under this heading the tannins of Portlandia grandiflora, Asperula odorata, 

 Rubia tinctorum, Scrophularia nodosa, Humulus Lupulus, etc. Similarly, 

 Procter points out that such substances as moritannic acid, or maclurin, 

 and lupulotannic acid, are more closely related to the colouring matters 

 than to the tannins. In the opinion of Freudenberg. the conditions favour- 

 ing the formation of a precipitate with gelatine are the possession of a 

 sufficiency of hydroxyl groups coupled with a sparing solubihty of the 

 crystallised tannin in cold water. 



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