hen considering HNO3 manufacture, it is important to consider that Nitric Acid is a strong mineral acid and one of the mostly widely used acids both in industry and in the laboratory. It has a wide range of industrial uses which include the manufacture of fertilisers and the production of explosives. Nitric Acid is also commonly known as Spirit of Nitre, Aqua Fortis and its chemical name is HNO3. When HNO3 is in a concentration of over 86%, it is known as 'Fuming' Nitric Acid, 'Fuming' Spirit of Nitre, 'Fuming' Aqua Fortic or 'Fuming' NHO3. Nitric Acid can be formed naturally, in the laboratotry and on an industrial scale.
Nitric Acid manufacture occurs in the natural world where Nitric Acid contributes to the acidity of rainwater from the presence of Nitric Oxide (NO) in the atmosphere at a concentration of about 0.01 ppm. During lightning storms, Nitrogen and Oxygen react to form Nitric Acid which in turn is oxidised to Nitrogen Dioxide, NO2. This then reacts with the rainwater to produce Nitric Acid, HNO3. This process is expressed with the following chemical formulations:-
N2 + O2 > 2NO, where electricity is present
NO + 1/2O2 > NO2
3NO2 + H2O > 2HNO3 + NO
The pH of the water depends on the concentration of H+ (Hydrogen ions) in the solution. When Nitric formed as above dissociates into Hydrogen ions and Nitrate ions, it lowers the pH so increasing acidity. Human intervention from combustion processes also contribute to atmospheric NO levels e.g. car engines.
Nitric Acid manufacture in the laboratory requires a basic reaction of Sulphuric Acid and a Nitrate salt for this process to take place. Sulphuric Acid is used because it is stronger and less volatile than HNO3.
A typical reaction would be:-
Copper Nitrate + Sulphuric Acid > Copper Sulphate + Nitric Acid
Cu(NO3)2 + H2SO4 > CuSO4 + 2HNO3
For a laboratory preparation, an easily available and relatively cheap salt would be used such as Potassium or Sodium Nitrate. Distillation apparatus is required for the reaction and should be made entirely of glass due to the 'Corrosive' nature of Nitric Acid. Rubber or cork bungs would not be acceptable as they would corrode.
Sulphuric Acid is added to the Nitrate Salt contained in the distillation flask which is heated to approximately 200 degrees C. The resulting acid vapours are condensed and collected in a sealed flask which has to be cooled by running water. During the reaction, reddish brown fumes are produced due to Nitrogen Dioxide gas from the hot conditions.
4HNO3 > 2H2O + 4NO2 + O2
The resulting acid will be a yellowish brown colour as the Nitrogen Dioxide dissolves in the acid vapours. The colour can be removed by bubbling air or Carbon Dioxide through the warm acid or by dilution with water.
Nitric Acid manufacture takes place on an industrial scale by Ostwald's Process. This reaction involves one volume of dry Ammonia and 10 volumes of air. Initially, catalystic oxidation of Ammonia using a Platinum catalyst at 700 - 800 degrees C forms Nitric Oxide in a catalytic chamber.
4NH3 + SO2 > 4NO + 6H2O, where Pl is used at 700 - 800 degrees C
The resulting Nitric Oxide passes into an oxidising chamber where it is reacted with Oxygen at 50 degrees C to produce Nitrogen Dioxide.
2NO2 + O2 > 2NO2, at 50 degrees C
Finally, the Nitrogen Dioxide passes into an absorption chamber where it is absored into water to form Nitric Acid.
4NO2 + 2H2O + O2 > 4HNO3
When the final reaction is carried out in the absence of air (O2), Nitric Oxide is produced which can be recycled back into the process.
3NO2 + H2O > 2HNO3 + NO
The first stage of the reaction process can involve an unwanted side reaction where Nitrogen is reverted back into N2. To minimise, reduce the contact time of gases with the catalyst.
4NH3 + 6NO > 5N2 + 6H2O