52 PROXIMATE CONSTITUENTS OF PLANTS 



81. Qualitative Examination of Tannin 



(Ci,Hi„0,+2H,0). 



Tannin is generally obtained as a light brown semi- 

 crystalline solid, which is white when pure. It dissolves 

 readily in water, giving a solution with an intensely 

 astringent taste ; it also dissolves readily in aqueous 

 alcohol, but is insoluble in absolute alcohol or ether. 



The following tests should be applied with tannin,, 

 and also with the solution obtained by extracting a 

 few powdered oak-galls with water, filtering, and con- 

 centrating the solution. 



(i.) An alkaline solution of tannin (alkaline gallo- 

 tannate) when exposed to the air turns brown, owing 

 to oxidation. 



(ii.) On adding a solution of ferric chloride to a 

 solution of tannin, a black or greenish black coloration 

 or precipitate is produced. This colouring matter forms 

 the basis of ordinary writing (not printer's) ink. 



(iii.) Tannin forms a buff-coloured flocculent pre- 

 cipitate with a cold solution of gelatine. A similar 

 action takes place in hides during the process of 

 tanning. 



(iv.) Lead acetate gives a white precipitate of lead 

 tannate (gallotannate). The precipitate is decomposed 

 by HgS to give lead sulphide and tannin, and this 

 reaction can hence be used for the preparation of the 

 latter substance in a pure condition. 



Resins. 



The resins are amorphous substances occurring in plants, and 

 are frequently associated with essential oils, from which they 

 appear to be formed by oxidation processes — common "rosin," 

 for example, being produced in such a way from the terpenes 

 present in pine trees. 



Many resins, such as guaiacum, gamboge, asafoetida, opoponax. 



