MICROCHEMICAL REACTIONS 271 



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 Wisselingh t recommends i per cent aqueous solutions 

 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 

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

 solutions (i part in 500,000 of water), and the tissue under 

 investigation must remain in a large quantity of the solution 

 for several hours. Van Wisselingh's experience is contrary to 

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

 methylene blue are harmful to Spirogyra, the plant used by 

 PfefTer, 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. 



11. A brilliant red colour, even when the tannins are in 

 a very dilute solution, results from the addition of an aqueous 

 solution of iodine in potassium iodide mixed with a little 

 10 per cent, ammonia. 



The following are microchemical tests for gallic acid : — 

 I. The rapidity of the reaction with potassium chromate 

 may provide a means of distinguishing gallic acid from tannic 



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

 t Van Wisselingh : " Konin. Akad. v. Wetensch Amsterdam," igio, 

 685. 



