93] 



The Sulphuric Acid Method 



63 



minutes) it will be quite black and impervious to the light, but 

 also quite free from clots of carbonaceous matter. It is now 

 necessary to clear the solution. Place the flask upright for 

 a moment, throw into it about 8 grams of dry powdered 

 K 2 S0 4 , and immediately return to the inclined position. The 

 potash salt will very considerably raise the boiling-point of the 

 acid, and at this higher temperature the carbonaceous matter 

 in solution is readily oxidised, with evolution of S0 2 . As the 

 liquid becomes clearer the light of the Argand begins to pene- 



trate it, illuminating the white fumes in the flask. This makes 

 the interior of the flask appear first dark red, which changes 

 gradually to yellow and finally to white. It is then allowed to 

 cool. In the figure the flask f shows the beginning of the 

 operation, e is clearing, and d is cooling. 



93. The Distillation.— The whole of the nitrogen being 

 now in the shape of sulphate of ammonia, the next process 

 consists of distilling off the ammonia after setting it free by 

 caustic soda. 



Fig. 29 shows a very convenient form of apparatus. 



