338 AGRICULTURAL ANALYSIS 



strutted on this supposition. In point of fact even in the strong- 

 est alkali solution there is a certain amount of tension but this 

 is so slight as only to affect the results in the second place of 

 decimals. Since, as a rule, only a few analyses are made by this 

 method, it will be found safer to use a caustic alkali solution of 

 given strength and to calculate the results from the tables of 

 aqueous tensions given above. 



298. The Soda-Lime Process. This process originally per- 

 fected by Varrentrap and Will, and improved by Peligot, was 

 used very extensively by analysts until within the last two decades 

 for the determination of nitrogen not existing in the nitric form. 

 It is based on the principle that when nitrogen exists as a salt 

 of ammonia, or as an amid, or as proteid matter, it is con- 

 verted into gaseous ammonia by combustion with an alkali. 

 This ammonia can be carried into a set solution of acid by a stream 

 of gas free of ammonia and the excess of acid remaining after 

 the combustion is complete can be determined by titration against 

 a standard alkali solution. The results under proper conditions 

 are accurate even when a small quantity of nitric nitrogen is 

 present. When, however, there is any considerable quantity of 

 this compound in the sample the method becomes inapplicable 

 by reason of non-reduction of some of the nitrogen oxids pro- 

 duced by the combustion. 



In bodies very rich in nitrogen, such as urea, all the nitrogen 

 is not transferred directly into ammonia at the commencement 

 of the combustion. A portion of it may unite with a part of the 

 carbon to form cyanogen, which may unite with the soda to 

 form sodium cyanid. With an excess of alkali, however, and 

 prolonged combustion, this product will be finally decomposed 

 and all the nitrogen be secured as ammonia. 



The nascent hydrogen which unites with the nascent nitrogen 

 during the combustion is also derived from the organic matter 

 which always contains enough carbon to decompose the water 

 formed in order to be oxidized to carbon dioxid. While at first, 

 therefore, during combustion, the hydrogen may unite with the 

 oxygen, it becomes again free by the oxidation of the carbon and 

 in this condition unites with the nascent nitrogen to form ammo- 



