474 AGRICULTURAL ANALYSIS 



this solution to 90 cubic centimeters of pure sulfuric acid. 

 This secures a normal solution of nitrosylsulfuric acid, of which 

 each cubic centimeter corresponds to o.oi milligram of nitrogen. 



In the other cylinder is placed one cubic centimeter of the 

 solution to be examined, and the contents of both cylinders are 

 well mixed so that the nitrous acid in a nascent state may act on 

 the reagent. The colors are compared after any convenient 

 period, but, as a rule, after five minutes. 



The chief improvement made by Lunge and Lwoff on the 

 method of Griess is in keeping the reagent in a mixed state 

 ready for use, by means of which any nitrous impurities in the 

 components thereof are surely indicated. Its advantage over 

 the method of Ilosvay consists in using the comparative normal 

 nitrite solution as nitrosylsulfuric acid, in which state it is much 

 more stable. 91 



406. Estimation of Nitrous Acid with Starch as Indicator. 

 The method of procedure, depending on the blue color pro- 

 duced in a solution of starch in presence of a nitrite and zinc iodid, 

 when treated with sulfuric acid, is not of wide application on ac- 

 count of the interference produced by organic matter. The soil 

 or fertilizer extract or water is treated in a test-tube, with a few 

 drops of starch solution and some zinc iodid, to which is added 

 some sulfuric acid. The decomposition of the nitrite is attended 

 with the setting free of an equivalent amount of iodin which gives 

 a blue coloration to the starch solution. The depth of the tint is 

 imitated by treating a standard solution of nitrite in a similar 

 way until the proper quantity is found, which gives at once the 

 proportion of nitrite in the sample examined. This process, how 

 ever, is scarcely more than a qualitative one. 



407. Estimation of Nitrites by the Method of Chabrier. In 

 order to make the estimation of the evolved nitrous acid more 

 definite by the iodin method, Chabrier has elaborated a plan for 

 titrating it with a reducing agent. 92 



The substance chosen for this purpose is sodium hyposulfite. 

 In point of fact, it is not the nitrous acid which is attacked by 



91 Bulletin de la Soci^te" chimique de Paris, 1894, [3], 11 : 218. 

 91 Encyclopedic chimique, 1888, 4 : 262. 



