Nitric acid is highly corrosive mineral acid and commonly referred as the spirit of niter. Nitric acid is considered as one of the most important chemical products because of its commercial applications. The different forms of nitric acid are available in the marketplace which are fuming nitric acid and anhydrous nitric acid.
Nitric acid is a strong oxidizing agent. It ionizes readily in solution, forming a good conductor of electricity. It reacts with metals, oxides, and hydroxides, forming nitrate salts. The main application of nitric acid is in the manufacture of fertilizers (mainly ammonium ad calcium ammonium nitrate) and for the production of nitro-containing organic intermediates. About 80% of nitric acid is used for fertilizer products, while the remaining 20% is used in industrial purposes.
The greatest chemical end uses for nitric acid is in the polymer industry. Nitric acid is used to manufacture hexanedioic acid (adipic acid), TDI (toluene diisocyanate or methylbenzene diisocyanate) and dinitrobenzene. Hexanedioic acid is used to make polyamides, while TDI and dinitrobenzene are used in polyurethane production. Nitrobenzene is also used for aniline, which is a key reagent in dye making. Other applications also include metallurgy, ore flotation, etching steel, photoengraving, and reprocessing of spent nuclear fuel.
General properties of Nitric Acid
- Nitric acid is a strong corrosive acid at ambient temperature and strong oxidizing agent.
- Nitric acid is commercially available in an azetrope composition with the water molecule.
- Nitric acid is colorless and tends to acquire a yellow color due to decomposition into oxides of nitrogen and water.
- Pure form of nitric acid is not stable and even at ordinary temperature it undergoes slight decomposition.
Ostwald process is a chemical process for commercial nitric acid preparation and it is developed by the Wihelm Ostwald in 1902. The Ostwald process is one of the most important process because it is an easy way to produce nitric acid, which has high demand as a raw material for the fertilizer industry. Ammonia (NH3) is used as a raw material for the production of Nitric acid in Ostwald process and it’s obtained from the Haber’s process. Catalyst such as Platinum combined with 10% Rhodium is used in the Ostwald process.
The reaction takes place via 3 steps:
4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2O (g)
Ammonia is heated with oxygen to produce nitric oxide (NO) and water.
2 NO (g) + O2 (g) → 2 NO2(g)
The nitric oxide from the first step combines with oxygen to create nitrogen dioxide (NO2)
3 NO2 (g) + H2O (l) → 2 HNO3 (aq) + NO (g)
In this final step, the nitrogen dioxide is absorbed with water to create an aqueous solution of nitric acid, and also yields nitrogen oxide.
Aqueous HNO3 obtained from the Ostwald process can be concentrated by distillation up to 68% and further concentration to 98% can be achieved by dehydration with concentrated H2SO4.
Thermal decomposition of cupric nitrate:
Nitric acid is made by laboratory synthesis via thermal decomposition of cupric nitrate. Cupric nitrate (commonly known as Cupper (II) Nitrate) is heated, and decomposes to copper oxide while producing nitrogen oxide and oxygen. Nitrogen oxide gases are then passed through the water to produce nitric acid and byproduct as sodium sulphate. The 2 step process is represented by the reaction as shown below:
2 Cu(NO3)2 → 2 CuO (s) + 4 NO2 (g) + O2 (g)
4 NO2 (g) + O2 (g) + 2 H2O (l) → 4 HNO3 (aq)
Single displacement synthesis
A simple and most used preparation of nitric acid in labs is by reaction of highly concentrated sulfuric acid with dry nitrate salt, usually sodium or potassium nitrate. If concentrated sulfuric acid is not available, concentrated phosphoric acid works equally well. The heat stable nonvolatile acid acts as a displacement reagent that displaces the nitric acid from the nitrate.
The reagents are heated at around 830C whereby high purity, red fuming nitric acid can be obtained from distillation. The red fuming nitric acid can be used for oxidizing non-metals such as iodine, antimony, etc. The reaction equation can be represented as shown below:
Market for Nitric Acid
The annual production of nitric acid was estimated to be 62.2 million metric tons in 2016, making it one of the largest commodity chemicals in the world. The United States is the world’s fourth-largest producer and consumer of nitric acid. The top three being China, Western Europe and Eastern Europe. As of 2016, the U.S. accounted for 13.0% of total world production and 12.9% of consumption of nitric acid.