NITROGEN BY KJELDAHDS PROCESS 



II 



taken that the neck of the flask is dry, to prevent 



any particles of the substance adhering thereto, and 



25 c.c. concentrated sulphuric acid 



free from nitrogen compounds are 



added. The flask is then placed 



in an inclined position and heated 



gently over a naked flame, in a 



fume cupboard, excessive frothing 



(due to the evolution of COg and 



SO2) being avoided. 



When the frothing has to a large 

 extent subsided, 10 grams of dry 

 powdered KgSO^ are added, and the 

 heating is then continued. The ob- 

 ject of adding the K^SO^ is to raise 

 the boiling-point of the acid, and 

 thereby ensure a more rapid oxida- 

 tion of the carbon present. The liquid 

 which was at first black, gradually 

 becomes lighter in colour and finally 

 quite colourless, at which stage the 

 heating is stopped. If this change 

 takes place only slowly, the addition 

 of a small crystal of copper sulphate will hasten the 

 oxidation. 



The contents of the flask are then allowed to cool, 

 and will solidify to a colourless mass of crystals consist- 

 ing largely of potassium hydrogen sulphate. Sufficient 

 water is added to dissolve the crystals, and the contents 

 of the flask are thoroughly rinsed out into the distilling 

 apparatus shown in Figs. 2 or 3. A few drops of a 

 solution of methyl orange are added and a pinch of 

 zinc dust or a piece of porous tile to ensure even boil- 

 ing, the cork fitted, and a very concentrated (syrupy) 

 solution of caustic soda run in from the tap funnel with 



Fig. I.— Kjeldahl 

 digestion flask. 



