270 TANNINS AND [CH. XII 



from aqueous solution by any immiscible solvent it must 

 be precipitated by some appropriate reagent and the 

 excess of reagent removed. 



Before examination, the original extract must be 

 evaporated till all alcohol is completely driven off and the 

 residue taken up with water, and filtered if necessary. 

 Any residue insoluble in water may be assumed to be 

 resins, etc., arid neglected. 



If the solution is acid it should be exactly neutralised 

 with dilute soda before testing. 



Qualitative tests for Tannins. 



Test portions of the neutral aqueous solution with : 



(1) A few drops of ' neutral ' ferric chloride (large 

 excess must be carefully avoided). 



Blue-black or dull green coloration shows tannins. 



(2) A few drops of solution of potassium ferri- 

 cyanide and ammonia. 



Reddish-brown coloration changing to brown shows 

 tannins. 



(3) Gelatin solution. 



Dirty white precipitate shows tannins. 



(4) Lime-water (Ca(OH) 2 ). 



Blue, brown, or red colour or precipitate shows 

 tannins. 



(5) Uranium acetate. 



Brown precipitate, or reddish-brown or brown colour 

 shows tannins. 



