Nitric Acid

Nitric acid (chemical formula HNO₃) is a highly corrosive, strong mineral acid and powerful oxidizing agent. It is one of the most important industrial inorganic chemicals.

Physical Properties

• Appearance: Pure nitric acid is a colorless liquid, but commercial samples often appear yellow or reddish due to dissolved nitrogen oxides (especially in fuming forms). It has a characteristic acrid, suffocating odor.
• Molar mass: 63.01 g/mol.
• Density: ~1.51 g/cm³ (pure); ~1.41 g/cm³ for the common 68% aqueous solution.
• Melting point: -42 °C (-44 °F).
• Boiling point: 83 °C (pure); the common 68% azeotrope boils at ~121 °C.
• Solubility: Completely miscible with water. It fumes in moist air.

Commercially, nitric acid is most often supplied as a 68% (by weight) aqueous solution, known as concentrated or azeotropic nitric acid. Solutions above ~86–90% are called fuming nitric acid (which can be white fuming or red fuming depending on the NO₂ content). Concentrations over 68% cannot be obtained by simple distillation of dilute acid due to the formation of a maximum-boiling azeotrope with water.

Chemical Properties

Nitric acid is a strong acid (pKa ≈ -1.37) that fully dissociates in water:

HNO₃ → H⁺ + NO₃⁻

It is also a strong oxidizing agent, especially when concentrated or hot. It can oxidize many metals (even those below hydrogen in the reactivity series, like copper), non-metals, and organic compounds. Reactions often produce nitrogen oxides (NO, NO₂) as byproducts rather than hydrogen gas.

The molecule has a planar structure with the nitrogen atom bonded to three oxygen atoms (one via a hydroxyl group in the molecular form) and exhibits resonance.

Production (Ostwald Process)

Most industrial nitric acid is produced via the Ostwald process (developed in the early 1900s), which starts from ammonia (usually made via the Haber-Bosch process):

1. Catalytic oxidation of ammonia: 4NH₃ + 5O₂ → 4NO + 6H₂O (using Pt-Rh catalyst at high temperature ~800–900 °C).
2. Oxidation of nitric oxide: 2NO + O₂ → 2NO₂.
3. Absorption in water: 3NO₂ + H₂O → 2HNO₃ + NO (with recycling of NO).

This process yields acid that is typically concentrated to the azeotropic 68% level. It is highly efficient and supplies the vast majority of global production.

Major Uses

• Fertilizers: ~75–80% of production is used to make ammonium nitrate (NH₄NO₃) by neutralizing nitric acid with ammonia. Ammonium nitrate is a key nitrogen fertilizer.
• Explosives: Production of nitroglycerin, TNT, nitrocellulose, and other nitro compounds.
• Metal processing: Etching, dissolving, and passivating metals (e.g., stainless steel, copper, silver, gold refining). It is used in aqua regia (mixture with hydrochloric acid) to dissolve noble metals.
• Chemical synthesis: Precursor for dyes, pharmaceuticals, nylon intermediates (e.g., adipic acid), and other organic nitrates.
• Laboratory and analytical: Digestion of samples for metal analysis (ICP, AAS), cleaning glassware, and as a reagent in titrations or oxidations.
• Other: Rocket propellants, semiconductor manufacturing, photoengraving, and wood aging (for furniture).

Safety and Hazards

Nitric acid is extremely hazardous and must be handled with great care:

• Corrosive: Causes severe chemical burns to skin, eyes, and mucous membranes on contact. Burns can appear yellow due to xanthoproteic reaction with proteins.
• Toxic fumes: Releases irritating and toxic nitrogen oxide gases (NO₂, etc.), which can cause delayed pulmonary edema (fluid in lungs) even hours after exposure.
• Oxidizer: Enhances combustion of flammable materials; can react violently with organics, reducing agents, or metals. Not flammable itself, but reactions can produce heat and gases leading to explosions or fires.
• Other effects: Inhalation irritates respiratory tract; ingestion causes severe internal burns; chronic exposure can erode teeth.

Handling precautions:

• Always use in a certified fume hood with good ventilation.
• Wear appropriate PPE: chemical-resistant gloves (e.g., butyl, neoprene, Viton), face shield/goggles, lab coat/apron, and respirator if needed.
• Store in cool, well-ventilated areas in compatible containers (glass, certain plastics, or stainless steel for dilute forms). Keep away from organics, bases, metals, and combustibles.
• Never add water to concentrated acid (add acid to water slowly if diluting).

First aid (seek immediate medical attention):

• Skin/eyes: Flush with large amounts of water for at least 15–30 minutes; remove contaminated clothing.
• Inhalation: Move to fresh air; monitor for delayed lung effects (24–48 hours).
• Ingestion: Do not induce vomiting; rinse mouth and get help.

Exposure limits (OSHA PEL, ACGIH TLV): 2 ppm (8-hour TWA), with short-term limits around 4 ppm.

Nitric acid demands respect—small spills or exposures can escalate quickly. Always consult the Safety Data Sheet (SDS) for your specific concentration and follow local regulations.