VII] OXIDIZING ENZYMES 91 



leather. As examples may be taken species of Caesalpinia, Spanish 

 Chestnut (Castanea,), Eucalyptus, Oak {Quercus), Mangrove {Rhizophoru), 

 Sumac {Rhus). Tannins also occur in quantity in galls, especially on 

 species of Quercus. 



As a class, the tannins are non-crystalline and exist in the colloidal 

 state in solution. They have a bitter astringent taste. They have 

 certain properties and reactions in common, i.e. they precipitate gelatine 

 from solution, are themselves precipitated from solution by potassium 

 bichromate, and give either blue or green colorations with solutions of 

 iron salts. Many tannins occur as glucosides but this is by no means 

 always the case. 



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

 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. 89. 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 ui) 

 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 

 impurities. 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 ferric chloride solution. A deep blue-black colour is 

 produced. 



{b) 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. 



(f) 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 lead acetate 

 solution. The tannin will be precipitated. 



(e) Melt a little of a 10% 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 

 precipitated. 



For the above tests, in addition to galls, the bark stri])i)ed from two to 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. 



