VIII] AROMATIC COMPOUNDS 107 



confirm the presence of tannin, since other substances, such as flavones, may give a 

 green colour with iron salts (see p. 111). 



Some of the individual tannins will now be considered. 



Gallotannic (or tannic) acid is one of the most important of the 

 pyrogallol tannins. It occurs in Oak galls and Oak wood, in tea, in the 

 Sumac (Rhus Coriaria), etc. According to recent investigations (Fischer 

 and Freudenburg, 8) tannic acid may be regarded as a compound of one 

 molecule of glucose with five molecules of digallic acid in which five 

 hydroxyls of the sugar are esterified by five molecules of acid: 

 CH2(0X) • CH(OX) • CH • CH(OX) • CH(OX) • CH(OX) 



1-^ o ! 



where 



X= —CO • C6H2(OH)2 • O • CO • C6H2(OH)3 



Tannic acid is an almost colourless amorphous substance. It has an 

 astringent taste, is soluble in water and alcohol, only slightly soluble in 

 ether, and insoluble in chloroform. It" is decomposed, by boiling with 

 2 Yo hydrochloric acid, into gallic acid. 



Expt. 102. Extraction and reactions of tannic {or gallotannic) add. By a crude 

 method a solution of gallotannic acid can be obtained from tea. About 5 gms. of the 

 residue, after the extraction with ether in Expt. 100, is again extracted with ether 

 once or twice which will remove all but traces of gallic acid. Boil up the residue from 

 ether with a little water and filter. With the filtrate make the following tests which 

 differentiate between gallic and gallotannic acid : 



(a) To about 10 c.c. add a little IO^Iq gelatine. The gelatine is precipitated. 



(6) To a little of the filtrate add a few drops of 5 % lead nitrate solution. The 

 tannic acid is precipitated. 



The remaining tests are given in common with gallic acid. If the extract is too 

 coloured, dilute with water. 



(c) Dilute a few drops of the filtrate with water in a porcelain dish and add a 

 drop of 5 % ferric chloride solution. A blue-black colour is given. 



{d) Dilute a few drops of the filtrate with water in a porcelain dish and add a 

 drop or two of iodine solution. A transient red colour is formed. 



(e) To a little of the filtrate in a test-tube add a few drops of 1 7o potassium 

 cyanide solution. A reddish-brown colour is formed which changes to brown but 

 becomes red again on shaking with air. 



In addition to tannic acid, a great many other tannins are known, 

 but their constitution is obscure. 



Expt. 103. To demonstrate that in tannin-containing plants the tannin may he 

 also present in the leaves. Take about two dozen leaves of the Common Oak {Quercus 

 Rohur) and pound them in a mortar. Then boil the crushed mass in an evaporating 

 dish with a little water. Filter, and with the filtrate make the tests for tannin. 

 Leaves of other trees also may be used, e.g. the Wig Tree {Rhus Cotinus), Sweet 

 Chestnut {Castanea vulgaris). 



