90] The Soda Lime Process 61 



Since the sulphuric acid solution is seminormal, one litre 



will neutralise - ' grams of NH 3 , or one c.c. will neutralise 

 2 



I Z = -oo85 gram of NH 3 , which is equivalent to — - 



= •007 gram N, If, therefore, we multiply the number of 

 c.c. neutralised by ammonia by '007, we obtain the weight of 

 N in the urea. From this the percentage of nitrogen may be 

 calculated. 



THE SULPHURIC ACID METHOD 



This method was due to Kjeldahl^ but the more accurate 

 modification here described is known as Gunnings process. 



90. Substance used. — Since urea does not require the 

 full working of this process, a sample of linseed cake whose 

 percentage of N is known should be used. 



Method employed. — The substance is decomposed by 

 hot strong sulphuric acid, whose boiling-point is increased by 

 the addition of potassic sulphate. The ammonia so formed is 

 expelled by means of caustic soda, and absorbed and estimated 

 as before. 



The general principle of this method was invented by 

 Kjeldahl, and since the. time when it was first made public 

 a large number of modifications have been used. The 

 original method decomposed any organic matter by means of 

 sulphuric acid, mercuric oxide, and potassium permanganate. 

 One writer recommends the use of manganese dioxide in place 

 of permanganate. Others, again, have shown that these 

 powerful oxidising agents occasionally oxidise some of the 

 nitrogen, which thus escapes as an oxide of nitrogen. The 

 method here described is used in some of the principal 

 agricultural laboratories. 



