198 TANNINS 



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. 

 Moeller recommends the use of a solution of anhydrous ferric 

 chloride in anhydrous ether. 



6. A solution of ammonium molybdate in a strong solu- 

 tion of ammonium chloride gives a copious yellow precipitate 

 with many tannins ; when added to digallic acid a red colora- 

 tion results. According to Gardiner* this reagent affords a 

 means of distinguishing glucoside tannin from tannic acid. 



The red yellow colour obtained by adding ammonium 

 molybdate to tannic acid is destroyed by oxalic acid. 



7. Lime water gives a white precipitate which turns red, 

 brown or blue. 



8. Aqueous solutions of various organic bases such as 

 caffeine and antipyrin precipitate the tannins. 



Van Wisselinghf recommends i per cent aqueous solu- 

 tions of antipyrine and of caffeine. 



It must be remembered that several other substances be- 

 sides tannins are precipitated by these reagents. 



9. Pfeffer has drawn attention to the fact that tannins are 

 precipitated by methylene blue without prejudice to the vi- 

 tality of the cells. The stain must be used in very dilute 

 solutions (i pt. in 500,000 of water), and the tissue under in- 

 vestigation must remain in a large quantity of the solution for 

 several hours. Van Wisselingh's experience is contrary to 

 Pfeffer's, for ne finds that even very dilute solutions of 

 methylene blue are harmful to Spirogyra, the plant used by 

 Pfeffer, and after treatment for several days only a little of the 

 tannin was precipitated. 



10. On the addition of a solution of gelatine a dirty white 

 precipitate is formed. 



♦Gardiner: " Proc. Camb. Phil. Soc," 1883, 4, 387. 



+ Van Wisselingh : " Konin. Akad. v. Wetensch., Amsterdam," 1910, 685, 



