4-66 AGRICULTURAL ANALYSIS 



In this manner the content of nitric acid in the liquid under 

 examination can be compared directly with the solution of potas- 

 sium nitrate of known strength. The coloration is distinctly pro- 

 duced with o.oi milligram in 50 cubic centimeters of liquid, at 

 least three-fourths of which must be sulfuric acid. 



399. Piccini Process. The method proposed by Piccini may 

 also be used. 85 



About five cubic centimeters of the nitrite solution are placed 

 in a small beaker, some pure urea dissolved therein and a few 

 drops of sulfuric acid, and then held in boiling water for three 

 minutes. The nitrous acid is thus completely destroyed. Ammo- 

 nium chlorid may be substituted for urea. The reaction is repre- 

 sented by the equation, NH 4 NO 2 =N 2 -}-2H 2 O. The nitric acid 

 present is then determined by diphenylamin or other suitable re- 

 agents. Diphenylamin reacts both with nitrous and nitric acids, 

 producing a blue or violet tint. Warington calls attention to a 

 slight difference, however, in its deportment with these two acids. 

 When the solution of the reagent is not too strong a drop of it 

 produces but little turbidity when added to water or to a solu- 

 tion containing nitric acid. When, however, nitrous acid is pres- 

 ent, a cream-colored turbidity is produced. The violet color also 

 appears at once on adding sulfuric acid when a nitrite is present, 

 while in the case of nitrates, more sulfuric acid is required, ex- 

 cept when the solution is very strong. In this connection, it 

 must not be forgotten that in heating nitrites with urea or am- 

 monium chlorid in the presence of a slight excess of sulfuric acid, 

 a trace of nitric acid may be formed. 



400. Colors Produced by Diphenylamin. In some descriptions 

 of the color reactions produced by diphenylamin in the presence 

 of nitrates and nitrites and sulfuric acid, the distinctive color 

 produced is described as blue. This color is actually produced in 

 certain conditions, which are not always easy to secure. Atten- 

 tion is called to this variation in tint in Volume I, page 532. With 

 nitrites, the blue color is rather easily produced with varying 

 proportions of nitrous acid, but such is not the case with nitrates 



85 Journal of the Chemical Society, 1891, 59 : 489. 

 Gazzetta chimica italiana, 1879, 9 : 395. 



