KJELDAHL PROCESS 341 



may in some cases require prolonged heating, but the decom- 

 position is usually accelerated by (a) adding potassium sulphate 

 to the sulphuric acid in order to raise its boiling point, or (d) 

 adding a catalytic agent in the form of a globule of mercury or 

 some copper sulphate, or (c) adding an oxidizing agent, such 

 as potassium permanganate. 



When the decomposition is complete, the acid is cooled 

 and diluted largely with water and finally made alkaline and 

 distilled, the liberated ammonia being absorbed in a known 

 quantity of standard acid ; by determining the amount of free 

 acid remaining, the amount neutralized by the ammonia is 

 calculated, and hence the amount of nitrogen contained in the 

 original substance may be deduced. 



The actual experiment is carried out as follows : — 



From 0-3 to 07 gram * of substance is weighed out into 

 a 250 c.c. hard glass round-bottomed flask ; 10 grams of pure 

 potassium sulphate and a small globule of mercury, or a small 

 quantity of mercuric sulphate, are added, and the mixture 

 covered with 20 c.c. of pure concentrated sulphuric acid free 

 from nitrogen ; a small funnel is placed in the neck of the 

 flask, which is then clamped in an inclined position and heated 

 over an iron wire gauze in a fume cupboard. When the liquid 

 is only faintly coloured, it is cooled and diluted with 150 c.c. 

 of water and transferred to a 750 c.c. flask (Fig. 4); the small 

 flask is then thoroughly rinsed, in small portions at a time, 

 with another 150 c.c. of water, all of which are carefully poured 

 into the large flask without loss. 



Fifty c.c. of a standard acid, approximately decinormal, 

 are now delivered into the flask R, a couple of drops of 

 methyl orange are added, and the flask is stood in a vessel of 

 cold water. 



About 10 c.c. of a 10 per cent solution of potassium sulphide 

 together with a few lumps of granulated zinc f and two or 

 three pieces of litmus paper are next added to the diluted 

 solution in the flask F. Sufficient 50 per cent caustic soda 

 (usually about 50 c.c.) is now added to render the thoroughly 

 mixed contents alkaline to the litmus paper; the flask is then 



* Sufficient substance should be taken to contain about 0-05 gram of nitrogen. 

 + This when heated in the presence of alkali gives off a steady stream of 

 hydrogen, and so prevents bumping. 



