174 NON-METALS AND THEIR COMBINATIONS. 



2. To the same amount of the solution, add some acidified solution 

 of potassium permanganate, which is decolorized at once. What 

 becomes of the nitrite ? 



3. Carry out the directions of Test 1, under Hydrogen Dioxide, 

 using a few drops of the nitrite solution in place of the hydrogen 

 dioxide. Note that the blue color is not developed until an acid is 

 added. Only free nitrous acid acts on potassium iodide : 



HN0 2 + HI == H 2 + NO + I. 

 This is a very delicate test, but not decisive alone. 



4. Dilute 1 drop of the nitrite solution in half a beakerful of 

 water, add 1 c.c. of meta-phenylene-diamine reagent (see Nitrous Acid, 

 under Water Analysis, at end of chapter 38). A yellow to dark 

 brown color is produced, according to the proportion of nitrite. The 

 test is used only for very small amounts of nitrite. 



Test 1 is usually sufficient to recognize a nitrite. 



Nitric acid, Acidum nitricum, HNO 3 ; NO 2 OH, = 62.57 (Aqua 

 fortis). Nitrogen pentoxide, N 2 O 5 , a white, solid, unstable compound, 

 is of scientific interest only. Whjsn brought in contact with water it 

 readily combines with it, forming nitric acid : 



N 2 O 5 + H 2 = 2HNO 3 . 



The usual method for obtaining nitric acid is the decomposition of 

 sodium nitrate by sulphuric acid : 



NaNO 3 + H 2 SO 4 == HNO 3 + HNaSO 4 ; 



Sodium 

 bisulphate. 



or 



2NaNO 3 + H 2 SO. = 2HNO 3 + Na^SO,. 



Sodium 

 sulphate. 



At the present time nitric acid is produced also from the atmosphere by 

 causing the nitrogen and oxygen to unite under the influence of electric dis- 

 charges. The nitrogen tetroxide formed, when dissolved in water, gives nitric 

 acid and nitric oxide : 



3N0 2 + H 2 == 2HN0 3 + NO. 



The NO unites with oxygen to give N0 2 , which is again dissolved. The com- 

 mercial success of this method depends upon the cost of electric power. It is 

 interesting as offering a source of nitrates when the native supply shall have 

 been exhausted. 



Experiment 10. Prepare an apparatus as shown in Fig. 40. Heat in a retort of 

 about 250-c.c. capacity a mixture of about 50 grammes of potassium nitrate 



