106 AROMATIC COMPOUNDS [ch. 



It is possible to classify the tannins into two groups according to 

 whether they are complexes derived from protocatechuic acid or gallic 

 acid: 



1. The pyrogallol tannins. These give a dark blue colour with ferric 

 chloride solution, and no precipitate with bromine water. 



2. The catechol tannins. These give a greenish-black colour with 

 iron salts, and a precipitate with bromine water. 



Expt. 100. Reactions of tannins. Take three oak galls (the brown galls formed by 

 species of Cynips on the Common Oak) and pound them finely in a mortar. Boil up 

 the powder well with a small amount of water in an evaporating basin and let stand 

 for a short time. Then filter. The filtrate will contain tannin together with im- 

 purities. Make the following tests with the extract : 



(a) Put 2 c.c. of the tannin extract into a small evaporating dish, dilute with 

 water, and add a drop or two of 5 % ferric chloride solution. A deep blue-black 

 colour is produced. 



(6) Put 2 or 3 drops of the tannin extract into a small evaporating dish, and 

 dilute with water: add a little dilute ammonia and then a few drops of a dilute 

 solution of potassium ferricyanide solution. A red coloration will appear. 



(c) To 5 c.c. of the tannin solution in a test-tube add some strong potassium 

 dichromate solution. The tannin will be precipitated. 



{d) To about 5 c.c. of the tannin extract in a test-tube add a little 5 ^(q lead acetate 

 solution. The tannin will be precipitated. 



(e) Melt a little of a 10 ^j^ solution of gelatine by warming gently and then pour 

 drop by drop into a test-tube half full of tannin extract. The gelatine will be pre- 

 cipitated. 



For the above tests, in addition to galls, the bark stripped from two or three year 

 old twigs of Quercus may also be used, and will give the same reactions. The bark 

 should be cut into small pieces for extraction. 



It should be noted that although many tannins give the above 

 reactions, it does not necessarily follow that all tannins will give all the 

 reactions. 



Expt. 101. To demonstrate the existence of pyrogallol and catechol tannins. The 

 existence of a pyrogallol tannin 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 Coriaria) 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 Hippocastanum). Cut or tear the bark into 

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

 the filtrate with ferric chloride solution as in Expt. 101. 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. 101 (c) and (e) should also be made on the extracts, in order to 



