ESTIMATION OF NITROUS ACID 477 



cubic centimeters of the iodin solution is a convenient quantity 

 for the titration and the thiosulfate should be diluted by adding 

 to 10 cubic centimeters of the solution mentioned above, 990 

 cubic centimeters of water. Each liter of this dilute solution 

 contains two and a half grams of the sodium thiosulfate. 



Example. Suppose that it has required 21.3 cubic centimeters 

 of thiosulfate to absorb 10 cubic centimeters of the iodin solu- 

 tion; further, that 10 liters of water have been evaporated and 

 titrated as described above, and that the volume of thiosulfate 

 employed is 13.8 cubic centimeters. From this is derived the 



following formula : - = 6.48 milligrams of nitrous acid ; 



or 0.648 milligram per liter. 



408. Estimation of Nitrous Acid by Coloration of Solution of 

 Ferrous Salt. This method, due to Piccini, is based on the pro- 

 duction of the well-known brown color formed by the action of 

 nitric oxid on a ferrous salt. 93 The nitrite is decomposed by 

 heating with acetic acid while nitrates thus treated do not develop 

 the reaction. The tint produced is imitated, as above, by testing 

 against a standard solution of nitrite. Ferrous chlorid is to be 

 preferred to other ferrous salts for the above purpose. The pro- 

 cess should be carried on in solutions free of air. 



VOLUMETRIC METHOD FOR NITROUS ACID 



409. Estimation of Nitrous Acid by Decomposition with .Potas- 

 sium Ferrocyanid. The method of Schaeffer was first described 

 in 1851, but little attention has been paid to it since. The method 

 was brought into notice again by Deventer: 94 



The reaction depends upon the decomposition of nitrous acid 

 by potassium ferrocyanid in the presence of acetic acid with the 

 formation of potassium ferricyanid and acetate, and nitric oxid. 

 The reaction is expressed by the following equation: 



2K 4 FeCy -f2HNO 2 +2C 2 H 4 O 2 

 =K 6 Fe 2 Cy 12 +2KC 2 H 3 O 2 +2NO+2H 2 O. 

 A eudiometer with a glass stop-cock is arranged as shown in 



93 p^Hgot, Trait^ de Chimie analytique applique & 1' Agriculture, 1883 : 

 261. 



94 Berichte der deutschen chemischen Gesellschaft, 1893, 26 : 589. 



