92 AROMATIC COMPOUNDS AND [CH. 



Expt. 90. To demonstrate the existence of pyrogallol and catechol tannins. The 

 existence of a pyrogallol tanniii which gives a blue reaction with iron salts has 

 been illustrated in the last experiment on the Oak galls and the bark from Oak twigs. 

 The bark of the Sumac (Rhus Coriarid] and the fruit pericarp, leaves and bark of the 

 Sweet Chestnut (Castanea vulgaris} may be used as additional material for pyrogallol 

 tannins. 



For an iron-greening tannin strip off the outer bark from two to three year old 

 twigs of the Horse Chestnut (Aesculus Hippocastanuni). Cut or tear the bark into 

 small pieces and boil well with a little water in an evaporating dish. Filter and test 

 the nitrate with ferric chloride solution as in Expt. 89. A green coloration will be 

 given. Iron-greening tannins may also be extracted from the bark of twigs of the 

 Walnut (Juglans regia] and of the Larch (Larix europaea}. 



In the case of both classes of tannins, in addition to the ferric chloride reaction, 

 the tests of Expt. 89 (c) and (e) should also be made on the extracts, in order to 

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

 green colour with iron salts (see p. 94). 



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 Freudenberg, 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 : 

 CH 2 (OX) ' CH (OX) CH CH (OX) CH (OX) CH (OX) 



o 1 



t 



where X= CO -C 6 H2(OH) 2 "O CO C 6 H 2 (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 /o hydrochloric acid, into gallic acid. 



Expt. 91. Extraction and reactions of tannic (or gallotannic) acid. 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. 88, 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 10 % gelatine. The gelatine is precipitated. 



(6) To a little of the filtrate add a few drops of 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 ferric chloride solution. A blue-black colour is given. 



