What is nitric acid?
Nitric acid, also known as hydrogen nitrate and more historically as aqua fortis and spirit of niter, usually has a clear-to-yellowish colour, depending on concentration and age. A yellow tint can develop over time due to the release of nitrogen dioxide (NO₂) gas, especially with exposure to light or air. Its odour is incredibly sharp and acrid.
As an oxidising agent, nitric acid acts as an electron acceptor (that is, it removes electrons from other substances). This alters and breaks down materials, which aids in reactions like metal cleaning. This oxidative quality makes nitric acid highly corrosive – it reacts aggressively with many substances and must be handled with extreme care.
As a nitrating agent, together with sulphuric acid, nitric acid generates nitronium ions – a reactive species that attaches to organic molecules. This introduces nitro groups (-NO₂) into these molecules – essential in the manufacture of explosives, dyes and drug intermediaries.
How is nitric acid made?
Modern day, large-scale production of nitric acid is based on the Ostwald process, which involves high-temperature catalytic oxidation of ammonia.
- Ammonia oxidation: Ammonia (NH₃) is passed over a platinum-rhodium catalyst at high temperatures (around 800–900°C). This creates an exothermic (heat-releasing) oxidation reaction which forms nitric oxide (NO).
- Nitrogen dioxide formation: At a lower temperature (around 250°C), nitric oxide then reacts with oxygen in the air to create nitrogen dioxide (NO₂).
- Absorption in water: In an absorption tower, the resulting nitrogen dioxide is absorbed in water, which creates nitric acid, nitrogen and more nitric oxide, which can be recycled back into the process.
Efficient and scalable, this process typically yields concentrations of nitric acid in the 60% region. This can be referred to as technical grade nitric acid, suitable for industrial applications such as fertiliser production and metal treatment.
However, a number of industrial applications may require a higher purity or a lower concentration.
Distilling nitric acid
Nitric acid has an azeotropic limit of 68% – this means that at this concentration, it forms a constant-boiling mixture with water. It cannot be further concentrated by simple distillation under normal atmospheric pressure.
To achieve higher concentrations, water can be removed by using vacuum distillation or by using dehydrating agents like sulfuric acid. This produces fuming nitric acid (90%-99%), which has specialised applications in rocket propellants, etching, and nitration processes.
Diluting nitric acid
Many applications require a lower concentration, such as metal pickling (10%-20%), laboratory use (6%-12%) and cleaning and etching (10-30%). Nitric acid is diluted by specialists who add concentrated nitric acid to water.
What is nitric acid used for?
The largest consumption of nitric acid is in fertiliser production, especially for manufacturing ammonium nitrate. Other major industrial applications include explosives, metal processing and chemical synthesis.
Agriculture
- Production of nitrate-based fertilisers, including ammonium nitrate and calcium nitrate
- Function: supplies nitrate ions essential for plant nutrition
Explosives and propellants
- Synthesis of explosives and military grade materials, such as nitroglycerin, RDX, trinitrotoluene (TNT) and nitrocellulose
- Rocket propellants, where fuming nitric acid is used for aerospace fuel
- Function: acts as a strong nitrating agent to form highly energetic compounds
Dyes and pigments
- Synthesis of dyes and nylon precursors
- Function: adds nitro groups to aromatic compounds, creating ingredients used in dyes
Electronics
- Cleaning and etching of semiconductor components
- Function: powerful oxidiser for removing impurities and surface layers
Metal treatment
- Pickling stainless steel (removing oxides and scale), etching and refining metals, surface finishing and cleaning
- Function: oxidises and dissolves surface oxides for cleaning and finishing
Pharmaceuticals and organic synthesis
- Synthesising intermediates (chemicals produced during a larger synthesis process) for drugs, dyes, and other organic compounds
- Function: nitrating and oxidising power enables the formation of reactive intermediates
Laboratory and research
- Reagent in analytical chemistry, especially for sample digestion and trace metal analysis
- Function: strong acid and oxidiser used to break down samples for analysis
Water treatment
- Corrosion inhibition, disinfection, cleaning and descaling
- Function: oxidises organic matter and dissolves scale
- pH control and alkalinity adjustment
- Function: strong acid used to neutralise alkaline water
How does nitric acid compare to other mineral acids?
Nitric acid’s industrial use is defined by its oxidising and nitrating power – these unique qualities set it apart from other strong mineral acids. However, it does share some similarities with…
- Sulphuric acid
- Nitric acid and sulphuric acid are similar in industrial scale and their versatility – both are strong and corrosive.
- Both acids play a role in fertiliser production. They are also used together in explosives production.
- However, sulphuric acid does not have nitrating qualities – it facilitates nitration by producing nitronium ions with nitric acid. Instead, its major uses are in petroleum refining and lead-acid battery manufacture.
- Hydrochloric acid
- Nitric acid and hydrochloric acid share some niche uses, specifically in water treatment and metal pickling (for removing rust and scale). Both can also be used for pH control.
- However, HCl is not a strong oxidising agent. As well as steel production and water treatment, it is used in inorganic chemical production and food processing.
- Phosphoric acid
- Nitric acid and phosphoric acid have application in agriculture – both play a big role in production of fertilisers.
- While nitric acid is used in nitrate-based fertilisers (such as ammonium nitrate), phosphoric acid is used to manufacture phosphate fertilisers (such as monoammonium phosphate (MAP) and diammonium phosphate (DAP)).
- Phosphoric acid is milder than nitric acid and is not oxidising. It is sometimes used in rust removal and metal surface preparation in mild industrial cleaning products, unlike nitric acid which has a more aggressive action.
Airedale Group: suppliers and distributors of nitric acid to industry
Nitric can be purchased by businesses and professionals with appropriate documentation after due diligence checks, including a customer statement.
Our Functional division boasts over 50 years’ experience across high-demand industrial chemistry, covering applications in agriculture, chemical synthesis, I&I cleaning, water treatment and many more.
In addition to over 200 other chemical solutions, we supply nitric acid in a range of pack sizes to suit your requirements, from 25L up to bulk transfer. Visit our nitric acid product page to request a SDS.
Nitric Acid
