428 AGRICULTURAL ANALYSIS 



centimeters of pure hydrochloric acid; the whole is then heated. 

 As the color of the indigo disappears more is added. When the 

 color ceases to fade, the liquid in the test-tube is concentrated 

 by boiling. If concentration fail to destroy the blue or green 

 color, another one-half cubic centimeter of hydrochloric acid is 

 introduced. The reaction is completed when neither concentra- 

 tion nor fresh addition of hydrochloric acid destroys the excess 

 of indigo present. The color produced by a small excess of in- 

 digo is a bright green ; this tint is the final reaction sought. The 

 small excess of indigo necessary to produce a green color is de- 

 ducted in every experiment. 



When more than mere traces of organic matter are present, 

 Boussingault advises that the nitric acid be first separated by 

 distillation and then reduced by the indigo solution. For this 

 purpose the concentrated solution of the nitrate, two or three 

 cubic centimeters, is placed in a small tubulated retort with two 

 grams of manganese dioxid in fine powder. The retort is next 

 half filled with fragments of broken glass, over which is poured 

 one cubic centimeter of concentrated sulfuric acid. The retort 

 is heated carefully by means of a small flame, which is kept in 

 motion so as to successively come in contact with all parts of 

 the bottom of the retort. The distillate is received in a graduated 

 test-tube which is kept cool. The distillation is continued until 

 the vapors of sulfuric acid begin to appear. The apparatus is 

 allowed to cool, the stopper of the retort removed, two cubic cen- 

 timeters of water introduced, and the distillation repeated until 

 fumes of sulfuric acid are again seen. The distillation with water 

 is made twice in order to remove every trace of nitric acid from 

 the retort. The distillate is neutralized with a solution of potas- 

 sium hydroxid and concentrated to two cubic centimeters, and 

 the nitric acid estimated in the manner already described. The 

 manganese dioxid used should be previously well washed and 

 the sulfuric must be free of nitric acid. 



Preparation of the Indigo Solution. Fifty grams of. indigo, in 

 fine powder are digested for 24 hours at 40 in a liter of distilled 

 water. The water is poured off and replaced with a fresh sup- 



