270 TANNINS 



I 



plants. Thus, Levi and Wilmer * found that in the case of 

 the horse-chestnut, Aesculus Hippocastanum, the youngest 

 leaves were richest in tannin, the minimum amount obtained 

 in June, whilst in August the quantity rapidly rose until the 

 original value was reached ; finally a diminution of tannin 

 occurred just before leaf-fall. Weekly analyses of leaves were 

 made from the opening of the buds to the fall of the leaves in 

 September. The obtained percentages of tannin were: 6-5, 

 3-3, 3-5, 2-8, 37, 3-2, 1-9, 2-8, 3-5, 3-6, 3-4, 5-1, 3-i, 5-3, 4-4, 

 4-3, 3-4, 6-2, 6-6, 5-2, 6-i, 6-5, 4-5 per cent. 



These variations in the tannin-content of parts of plants 

 are of great interest ; the value, however, of such estimations 

 would be greatly enhanced if estimations were carried out at 

 the same time to see whether, for instance, there is any 

 obvious relationship between the tannin-content of leaves and 

 of other parts of the plants such as the periderm. 



MICROCHEMICAL REACTIONS OF TANNINS. 



Before passing on to the detailed examination of the 

 various tannins, the following microchemical tests may be 

 mentioned, but it must be borne in mind that these reactions 

 do not enable one to distinguish between the various tannins. 



1. Tannins reduce Fehling's solution. 



2. They are precipitated by basic lead acetate and the 

 salts of many other metals ; thus uranium acetate gives a 

 brown precipitate or a brown or brown-red coloration, and an 

 aqueous solution of copper acetate gives a brown precipitate. 



3. Potassium bichromate in a strong aqueous solution or 

 a I per cent solution of chromic acid gives brownish-coloured 

 precipitates. 



4. A red-brown to brown coloration is obtained by the 

 use of a dilute ammoniacal solution of potassium ferricyanide. 

 This test is very delicate, and the reagent must be used spar- 

 ingly since the coloration is destroyed by an excess. 



5. The addition of a neutral solution of ferric chloride 

 gives a blue-black or greenish coloration or precipitate. 



* Levi and Wilmer : " Hide and Leather," 1905. 



