34 Biological Chemistry. 



wherever it is applicable. It has the further advantage in 

 that several analyses can be carried out at the same time. 

 It depends upon the principle that when a nitrogenous 

 organic substance is incinerated with concentrated sulphuric 

 acid, the nitrogen is reduced to ammonia, which combines 

 with the acid to form ammonium sulphate. On adding 

 excess of alkali to the incineration mixture, the ammonia 

 is set free, and the amotmt formed can be estimated by 

 distilling it off into an excess of standard acid, which 

 excess can afterwards be determined by titration with 

 standard alkali. 



The amount of substance to be taken for analysis will 

 vary with its nitrogen content. When this is small, 

 relatively large amounts should be incinerated to obtain 

 correct results, but usually not more than 1 gramme is 

 necessary even for nitrogen-poor products. The weighed 

 amount is introduced into a round-bottomed flask with 

 some water and about 10 c.c. of concentrated sulphuric 

 acid. The mixture is heated, gently at first, but after- 

 wards more vigorously, and the operation is continued 

 until, at a high temperature, the contents of the flask are 

 colourless (that is, contain no more organic matter). Care 

 must be taken not to allow the liquid completely to 

 evaporate ; if too much of the liquid has disappeared 

 before incineration is complete, more acid (another 10 c.c.) 

 must be added after the contents of the flask have been 

 allowed partially to cool. At the completion of the de- 

 struction of the organic matter, the contents of the flask 

 are diluted with about 200 c.c. water, and then 40 per cent, 

 sodium hydroxide solution (600 c.c. for each 10 c.c. sul- 

 phuric acid used) is added. The ammonia set free is then 

 distilled off into standard acid. The apparatus employed 

 for this purpose (when six analyses are carried out at 

 once) is illustrated (Fig. 14), the standard acid being 



