94 ORGANIC AGRICULTURAL CHEMISTRY 



colored or colorless liquid remains (usually 2-5 hours' heating is re- 

 quired). Cool the contents of the flask. Add quickly about 200 

 c.c. water and then about 50 c.c. concentrated (50 per cent) sodium 

 hydroxide solution and connect the flask at once with a condenser. 

 The receiver end of the condenser should dip below the surface of the 

 liquid in the receiving flask. This liquid is water plus a known 

 amount of standard {N/io usually) hydrochloric acid suflicient to 

 more than neutralize all ammonia distilled over. Distill the con- 

 tents of the digestion flask long enough to drive over all ammonia 

 (usually I to I hour). Titrate back excess of acid and calculate 

 the amount of ammonia, or of nitrogen, obtained from the original 

 protein substance used. 



(5) Reactions of Kjeldahl Determination of Nitrogen in Protein. 

 The Kjeldahl method for determining nitrogen is applicable to all 

 protein substances and has been modified in various ways to adapt 

 it to various nitrogen compounds such as nitrates, etc. It is the 

 universal method for determining nitrogen in practically all agricul- 

 tural analysis of such materials as plant and animal substances, fer- 

 tilizers, soils, etc. The details in regard to the method and its 

 modifications will be found in any book on analytical chemistry or 

 agricultural analysis. 



The modification given in this study is known as the Gunning 

 modification. The principle of the method is in general as follows: 

 When an organic nitrogen-containing compound, such as a protein, is 

 heated to the boiling point with concentrated sulphuric acid, the pro- 

 tein is completely decomposed and all carbon and hydrogen are oxidized. 

 All of the nitrogen is converted into ammonia, which in the presence 

 of the sulphuric acid forms ammonium sulphate, which is non-volatile 

 and is thus not lost in the digestion. The potassium sulphate added 

 to the sulphuric acid in the Gunning modification is for the purpose 

 of raising the boiling point of the acid, thus increasing the oxidation 

 power or its rapidity. When the digestion begins much carbon is 

 set free which gradually disappears as oxidation proceeds. When 

 the digested material is colorless or straw color, the carbon is all 

 oxidized and usually all nitrogen compounds have by this time been 

 converted into ammonia and the digestion is complete. At the com- 

 pletion of digestion, therefore, all of the protein nitrogen is in the 

 digestion liquid in the form of ammonium sulphate. After cooling 

 and dilution, strong sodium hydroxide is added suflScient to more 



