COMPOSITION OF ORGANIC COMPOUNDS 35 



(ff) Kjeldahl's Method. The principle of this method consists in 

 oxidising the substance with concentrated sulphuric acid ; the nitrogen 

 is converted into ammonia. The solution is made alkaline with 

 caustic soda and distilled. The ammonia is evolved and is collected 

 in excess of standard acid ; on subsequent titration with standard 

 alkali the amount evolved is given by difference. 



This method is much simpler to carry out than the Dumas 1 method, 

 but it cannot be employed for all nitrogen- containing compounds. 



Dakin and Dudley l have found that pyrrole and its derivatives, piperidine 

 and some of its derivatives give satisfactory results if the heating of the sub- 

 stance about -15 gm. with sulphuric acid be continued for at least four 

 hours after the solution becomes clear. Pyridine, quinoline, pyrazole and 

 their derivatives do not give satisfactory results. \ 



Of other groups of compounds, nitro-, nitroso-, azo-, diazo-, hydrazo-, 

 aminoazo- compounds, also compounds of nitric and nitrous acid, i.e. those 

 compounds containing nitrogen joined to oxygen or another nitrogen atom, 

 give satisfactory results if they are previously reduced with tin. Osazones do 

 not give satisfactory results. 



On account of its simplicity this method has found extensive use 

 in biological chemistry. The large group of compounds the pro- 

 teins all contain nitrogen ; the amount of protein in a solution is 

 estimated by determining the nitrogen content (see below) ; and the 

 amount of nitrogen in urine is a factor of importance in studying 

 metabolism. 



In most laboratories there is an apparatus in which six determina- 

 tions can be carried out at the same time as in Fig. 20, p. 37. 



Example: Estimation of Nitrogen in Egg-white Solution. 



A known volume, say 5 c.c., of the egg-white solution is placed 

 with a pipette into a clean round-bottom Jena glass flask of 700 cc. 

 capacity. I o or 20 c.c. of pure concentrated sulphuric acid and a crystal 

 of copper sulphate, about the size of a pea and weighing about 0^25 gm., 

 which helps in the oxidation, are added. (l gm. of potassium sulphate 

 is also sometimes added for this purpose, as it raises the temperature.) 

 The flask is heated in a fume-cupboard until the liquid, which at first 

 becomes brown from charring, becomes quite or nearly colourless, a pro- 

 cess which takes about an hour. The flask is allowed to cool and is 

 half-filled with distilled water. By means of a special distillation tube 

 it is connected to a condenser set up in a vertical position as in Fig. 

 19, No. I. 



*J. Biol. Chem., 1914, 17, 275. 



