HNO₃ — Nitric Acid

Nitric acid (HNO₃) is a strong, highly corrosive mineral acid and a key chemical in both industrial and laboratory chemistry. It is one of the most important inorganic acids used globally for the production of fertilizers, explosives, dyes, and plastics. In its pure form, nitric acid is a colorless liquid, but over time or upon exposure to light, it acquires a yellowish tint due to the decomposition into nitrogen dioxide (NO₂).

Nitric acid’s high oxidizing and nitrating properties make it extremely valuable in organic synthesis and metallurgy. It can react violently with many substances, releasing toxic fumes. In laboratories, it is commonly used for nitration reactions, dissolution of metals, and preparation of nitrates.

Nitric acid consists of one hydrogen atom, one nitrogen atom, and three oxygen atoms. The nitrogen atom is at the center, forming one double bond with an oxygen atom and two single bonds with the other oxygen atoms—one of which carries a hydroxyl group (–OH). The molecular structure can be represented as:

The nitrogen atom exhibits a +5 oxidation state, making the molecule highly oxidizing. Nitric acid molecules form hydrogen bonds due to the presence of the –OH group. In aqueous solution, nitric acid completely dissociates to form hydronium (H₃O⁺) and nitrate (NO₃⁻) ions:

This complete ionization makes HNO₃ a strong monoprotic acid—capable of donating one proton per molecule.

Industrially, nitric acid is prepared by the Ostwald Process, which involves the catalytic oxidation of ammonia (NH₃). The process consists of three main steps:

1. Oxidation of Ammonia: Ammonia is oxidized to nitrogen monoxide (NO) in the presence of a platinum-rhodium catalyst at 900°C and 9 atm pressure.

2. Oxidation of Nitric Oxide: Nitric oxide is further oxidized to nitrogen dioxide (NO₂).

3. Absorption of Nitrogen Dioxide: Nitrogen dioxide reacts with water to form nitric acid and nitric oxide, which is recycled back into the process.

This method produces nitric acid with a concentration of about 68%, which can be further concentrated by distillation with sulfuric acid to produce fuming nitric acid (above 90% concentration).

• Color and Appearance: Pure nitric acid is a colorless liquid that turns yellow or brown upon decomposition due to nitrogen dioxide formation.
• Acidic Nature: It is a strong monoprotic acid that ionizes completely in aqueous solution.
• Oxidizing Property: Nitric acid acts as a strong oxidizer and reacts with metals and nonmetals.
• Decomposition: On exposure to light or heat, it decomposes into water, nitrogen dioxide, and oxygen.

\(4HNO_3 \rightarrow 2H_2O + 4NO_2 + O_2\)

• Reaction with Metals: It reacts differently with metals based on concentration.
• Dilute nitric acid produces nitric oxide (NO).

\(3Cu + 8HNO_3 (dil.) \rightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O\)

• Concentrated nitric acid produces nitrogen dioxide (NO₂).

\(Cu + 4HNO_3 (conc.) \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O\)

• Reaction with Nonmetals: Acts as a powerful oxidizer, forming corresponding oxides or acids. For example:

\(C + 4HNO_3 \rightarrow CO_2 + 2H_2O + 4NO_2\)

• Nitrating Agent: Used to introduce nitro groups (–NO₂) into aromatic compounds in the presence of sulfuric acid (H₂SO₄) in nitration reactions.

• Fertilizer Industry: Used in the manufacture of ammonium nitrate (NH₄NO₃), a major nitrogen fertilizer.
• Explosives: A key raw material in the production of explosives such as TNT (trinitrotoluene), nitroglycerin, and RDX.
• Metallurgy: Used for refining metals and etching steel and copper.
• Chemical Manufacturing: Acts as an intermediate for the production of dyes, plastics, and organic nitrates.
• Laboratory Reagent: Commonly used in acid digestion and preparation of nitrate salts.
• Rocket Propellant: In fuming form, it is used as an oxidizer in liquid rocket fuels.
• Cleaning Agent: Used for cleaning glassware and removing organic residues due to its strong oxidizing power.

Nitric acid is extremely corrosive and can cause severe burns on contact with skin. Inhalation of vapors or fumes (mainly nitrogen oxides) can damage respiratory tissues and cause pulmonary edema. Prolonged exposure may lead to chronic lung issues.

Always handle nitric acid in a well-ventilated area or under a fume hood. Use protective gloves, goggles, and lab coats. Spills should be neutralized with a weak base like sodium bicarbonate (NaHCO₃) and cleaned with water.

Concentrated nitric acid reacts violently with organic materials, reducing agents, and combustible substances. It should be stored in glass containers away from heat and direct sunlight.