MICROCHEMISTR F. 1 1 7 



<5. Osmic Acid . 



203. Osmic acid is rapidly reduced by tannic acids and 

 very soon forms with them a solution sometimes bluish and 

 sometimes brownish, and finally a black precipitate. For 

 the reaction a \% solution should be used. The osmic acid 

 is regenerated and the preparation wholly decolorized by 

 peroxide of hydrogen. 



If hydrochloric acid be first added to the preparation and' 

 then \% osmic acid, there appears, according to Dufour 

 (I, 3 of separata), in a few minutes a blue color, and soon, if 

 much tannic acid be present, a blue precipitate. 



As Overton (II, 5) has shown, albuminoids saturated with 

 tannins are browned ; thus he obtained a beautiful brown 

 coloring of protein crystalloids on leaving sections from the 

 endosperm of Ricinus, from which the fat had been removed,, 

 for about ten minutes in a dilute solution of tannin, and 

 transferring them, after careful washing, to 2% osmic acid, 



e. Ammonium Molybdate. 



204. Gardiner (III) used for the recognition of tannins a 

 concentrated solution of ammonium molybdate in a saturated 

 ammonium chloride solution. This gives with most tannins, 

 a yellow precipitate, with digallic acid a red one, and with 

 gallic acid a compound soluble in ammonium chloride solu- 

 tion. 



C. Sodium Tungstate. 



205. Bramer lately recommends (I) for the recognition of 

 tannin a solution of I gram of sodium tungstate and 2 grams- 

 of sodium acetate in 10 ccm. of water. This should give a 

 brown precipitate with gallic acid, and a reddish-yellow one 

 with gallotannic acid. But the presence of tartaric or citric 

 acid hinders the reaction. 



77. Alkaline Carbonates. 



206. Alkaline carbonates cause the precipitation, in cells, 

 containing tannin, of globular or rod-shaped bodies which,, 

 when freshly precipitated, may be redissolved on washing; 



