66 The Estimation of Nitrogen [96-98 



The calculation is exactly the same as in the soda lime 

 process. 



96. Comparison of the Two Methods.— The soda 

 lime process has the advantage of requiring little or no appara- 

 tus but what may be found in any laboratory, but it possesses 

 the disadvantage that the acid in the bulb is often much 

 discoloured during the operation, thus rendering accurate 

 titration very difficult. Another disadvantage which is not at 

 all shared by the acid process is that the substance for analysis 

 must be very finely divided. When such materials as horn and 

 dried blood have to be analysed, fine grinding is next to im- 

 possible, and very erroneous results are often obtained. In all 

 cases where grinding is difficult, the substance should, be 

 ground finely enough to enable the operator to weigh out a 

 good average sample, and the nitrogen estimated by the acid 

 method. 



ESTIMATION OF NITROGEN IN PRESENCE OF 

 NITRATES 



97. Substance used. — A mixture of three parts starch 

 and one part ammonium nitrate, NH 4 N0 3 . 



Method employed.— The substance is decomposed by 

 sulphuric acid and salicylic acid, the N 2 5 being reduced mean- 

 while to ammonia with zinc-dust. The ammonia is separated 

 and estimated as before. 



98. Apparatus. — This is exactly the same as is used in 

 paragraphs 92-96. 



Chemicals. — Sulphuric and salicylic acids, prepared by 

 dissolving 2 grams of salicylic acid in 30 c.c. of sulphuric acid. 

 Mercury. 

 Sodium sulphide solution, 80 grams per litre. 



