t 

 352 AGRICULTURAL ANALYSIS 



every possible error is thereby excluded. That the solution once 

 decolorized within a short time again assumes a deep blue color, 

 is a matter of no concern, inasmuch as both solutions are added 

 in such a manner that the end reaction lies exactly at the point 

 when the starch iodid reaction distinctly disappears. 



310. Theory of the Reactions. As has been seen above, the 

 final product of heating a nitrogenous organic compound with 

 sulfuric acid and an oxidizing body is ammonium sulfate. The 

 various steps by which this is obtained have been traced by 

 Dafert. 98 



1 i ) The sulfuric acid abstracts from the organic matter the 

 elements of water, 



(2) The sulfur dioxid produced by the action of the residual 

 carbon on sulfuric acid exercises a reducing effect on the nitrog- 

 enous bodies present. 



(3) From the nitrogenous bodies produced by the above re- 

 duction ammonia is formed by the action of an oxidizing body. 



(4) The ammonia formed is at once fixed by the acid as am- 

 monium sulfate. According to the theory of Asboth, the hydro- 

 gen which is formed during the action of sulfuric acid on organic 

 matter, when in a nascent state, also aids greatly in the produc- 

 tion of ammonia. This idea is based on the fact that with those 

 bodies which afford a deficit of hydrogen the formation of am- 

 monia is imperfect." 



311. Preparation of Reagents. (i) Pure Sulfuric Acid. As 

 is well known, the so-called pure sulfuric acid in the market 

 usually contains ammonia, a fact which compelled Kjeldahl to 

 determine the quantity of nitrogen in the acid in every instance, 

 and to make correction for the same in the analysis. An acid 

 absolutely free from this impurity may, however, readily be pre- 

 pared by the distillation of the commercial article in a small glass 

 retort holding easily about 400 cubic centimeters. To conduct 

 this operation without danger it is only necessary to arrange the 

 apparatus so that the heavy fluid is heated to boiling, not from 

 the bottom of the retort, but from its sides, and that the upper 



98 Zeitschrift fiir analytische Chemie, 1885, 24 : 455- 

 w Chemisches Central-Blatt, 1886 : 165. 



