288 TANNINS 



completely precipitated when these solutions are boiled with 

 formaldehyde and hydrochloric acid, whereas pyrogallol tannins 

 are only incompletely precipitated, if at all. 



To carry out this test 50 c.c. of the tannin solution (0-5 per 

 cent) are boiled for half an hour under a reflux condenser with 

 25 c.c. of a mixture of 100 c.c. of concentrated hydrochloric 

 acid (diluted with an equal volume of water) and mixed with 

 150 c.c. of 40 per cent formaldehyde ; 10 c.c. of filtrate from 

 the above, mixed with 10 drops of i per cent iron alum and 

 I gram of solid sodium acetate, should give no colour with 

 a pyrocatechol tannin, whereas a blue or violet colour results in 

 the case of a pyrogallol tannin. 



PROPERTIES AND DESCRIPTION OF INDIVIDUAL 



TANNINS. 



As already stated, the term Tannin is merely a generic 

 name for the whole group of substances, though it has been, 

 and still is, frequently used to mean a particular tannin, namely 

 that contained in oak galls. This substance is, however, 

 better named gallotannic acid, as it is customary to name the 

 tannins after the source from which they are obtained ; thus 

 quercitannic acid indicates the tannin of oak bark, sumac- 

 tannin that derived from sumac, and so on. 



PYROGALLOL TANNINS. 



Owing to lack of space it is only proposed to describe two 

 tannins belonging to this group, namely gallotannic and 

 ellagitannic acid. 



GALLOTANNIC ACID. CjeH5204«. 

 (Syn. Tannic acid, or merely " Tannin.") 



This substance, as its name implies, is the tannin contained 

 in galls, and it is important to remember that oak gall tannin 

 is entirely distinct from either oak wood or oak bark tannin, 

 the latter of which is a pyrocatechol tannin. 



The two chief commercial sources of gallotannic acid are — 

 I. Turkish or Aleppo galls, produced by the gall wasp 

 Cynips gallcE, which lays its eggs in the buds of Quercus infec- 

 toria. These contain from 50-60 per cent of gallotannic acid. 



