Nitric Acid: Properties, Uses & Lab Safety Guide

Nitric Acid is a highly corrosive and strongly oxidizing mineral acid that plays a central role across analytical chemistry, materials science, and industrial synthesis.

Chemical Formula

HNO₃

Molecular Weight

63.01 g/mol

Physical State

Colorless to pale yellow fuming liquid

Boiling Point

~83 °C (azeotrope at ~68% w/w)

pKa

–1.4 (strong acid, fully dissociates in water)

Key Hazard

Corrosive, strong oxidizer; reacts violently with organic materials

CAS Number

7697-37-2

Chemical Properties and Grade Overview

Nitric acid (HNO₃, CAS 7697-37-2) is a strong mineral acid with a molecular weight of 63.01 g/mol. In its pure form it is a colorless, fuming liquid that is fully miscible with water. At concentrations above approximately 86%, it is referred to as fuming nitric acid and exhibits a reddish-brown color due to dissolved nitrogen dioxide (NO₂).

As a strong acid, HNO₃ dissociates completely in dilute aqueous solution (pKa ≈ –1.4). Its most distinctive chemical characteristic is its powerful oxidizing capacity, which derives from the high oxidation state of nitrogen (+5). This property allows nitric acid to dissolve many metals—including copper and silver—that are resistant to non-oxidizing acids such as hydrochloric acid.

Reagent-grade nitric acid is available in a range of concentrations, commonly 65–70% (w/w) for general laboratory use and higher concentrations for specialized applications. Purity grades include ACS reagent, trace-metal grade, and ULSI/semiconductor grade, each with defined limits for metallic and non-metallic impurities.

Key Laboratory and Industrial Applications

Analytical Chemistry: Nitric acid is indispensable in sample preparation for trace elemental analysis. It is used for acid digestion of biological tissues, environmental samples, and geological materials prior to analysis by ICP-MS, ICP-OES, or atomic absorption spectroscopy (AAS). Its strong oxidizing character ensures complete mineralization of organic matrices.

Aqua Regia Preparation: Combined with hydrochloric acid in a 1:3 (v/v) ratio, nitric acid forms aqua regia, the only acid mixture capable of dissolving noble metals such as gold and platinum. This mixture is widely used in precious-metal recovery and catalyst recycling.

Nitration Reactions: In organic synthesis, a mixture of concentrated nitric acid and sulfuric acid (mixed acid) is the classical reagent for aromatic nitration, introducing the nitro (–NO₂) group into organic substrates. This reaction underlies the production of pharmaceuticals, dyes, and energetic materials.

Surface Treatment and Passivation: Nitric acid is used industrially to passivate stainless steel and other alloys, forming a protective chromium oxide layer that enhances corrosion resistance. It is also employed in semiconductor fabrication for wafer cleaning and oxide etching.

Fertilizer Production: On an industrial scale, nitric acid is a precursor to ammonium nitrate, one of the world's most widely used nitrogen fertilizers, produced via the Ostwald process.

Safe Handling, Storage, and Disposal

Personal Protective Equipment (PPE): Always wear chemical-splash goggles, a face shield, acid-resistant gloves (butyl rubber or neoprene), and a lab coat or chemical-resistant apron when handling nitric acid. Contact with skin causes immediate yellow staining (xanthoprotein reaction) followed by painful chemical burns.

Ventilation: All work with concentrated nitric acid must be performed in a certified fume hood. Nitrogen oxide fumes generated during reactions are acutely toxic and can cause delayed pulmonary edema.

Storage: Store nitric acid in a cool, well-ventilated area away from heat sources, direct sunlight, and incompatible materials. Keep segregated from organic solvents, flammable materials, reducing agents, and bases. Use corrosion-resistant secondary containment (e.g., HDPE trays). Do not store in metal containers.

Incompatibilities: Nitric acid reacts violently with organic compounds (acetone, alcohols, acetic acid), strong reducing agents, and finely divided metals. Contact with paper, wood, or clothing can cause ignition.

Spill Response: In the event of a spill, evacuate the area, ventilate, and use appropriate acid-neutralizing absorbents (e.g., sodium bicarbonate-based spill kits). Do not use organic-based absorbents.

Disposal: Neutralize dilute solutions with dilute sodium hydroxide or sodium carbonate before disposal. Dispose of all nitric acid waste in accordance with local, state, and federal regulations for corrosive and oxidizing hazardous waste.

Regulatory Classification and SDS Highlights

Nitric acid is classified as a hazardous material under multiple international regulatory frameworks:

• GHS/UN Classification: Oxidizing liquid (Category 1 at ≥65%; Category 3 at lower concentrations), Acute Toxicity (inhalation), Skin Corrosion Category 1A, Eye Damage Category 1.
• DOT (US): Hazard Class 8 (Corrosive) with an Oxidizer subsidiary risk; UN number UN2031 (nitric acid >70%) or UN2031/UN2032 depending on concentration.
• OSHA PEL: 2 ppm TWA for nitrogen dioxide generated from nitric acid fumes; refer to current OSHA standards for updated exposure limits.
• REACH/CLP (EU): Listed as a substance of very high concern for skin and eye corrosion; must be accompanied by a compliant SDS.

Always consult the current Safety Data Sheet (SDS) for the specific product lot and concentration before use. The SDS provides authoritative first-aid measures, firefighting procedures, and exposure control information.

Frequently asked questions

What is the difference between concentrated and dilute nitric acid?

Concentrated nitric acid typically refers to solutions of 65–70% (w/w) or higher, which are strongly oxidizing and fuming. Dilute nitric acid (below ~30%) is less oxidizing but still corrosive. Importantly, concentrated nitric acid passivates iron and aluminum by forming a dense oxide layer, while dilute nitric acid can actively dissolve these metals.

Can nitric acid be stored with other laboratory acids?

No. Although nitric acid is an acid, its strong oxidizing nature means it must be stored separately from other mineral acids such as hydrochloric acid and sulfuric acid, and completely segregated from organic solvents, flammables, and reducing agents. Use dedicated corrosive-storage cabinets with secondary containment.

Why does skin turn yellow after contact with nitric acid?

The yellow discoloration is caused by the xanthoprotein reaction, in which concentrated nitric acid nitrates aromatic amino acid residues in skin proteins, producing yellow-colored nitro derivatives. This color change is not merely a stain—it indicates chemical denaturation of proteins and underlying tissue damage that requires immediate medical attention.

What grade of nitric acid should I use for trace metal analysis?

For trace elemental analysis by ICP-MS or ICP-OES, trace-metal grade (also called ultra-pure or sub-boiling distilled) nitric acid is required. This grade has extremely low metal impurity levels (typically parts-per-trillion range) to avoid contaminating samples. Standard ACS reagent grade contains metal impurities at concentrations that would interfere with ultra-sensitive measurements.

References

1. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997; Chapter on Nitrogen.
2. Occupational Safety and Health Administration (OSHA). Nitric Acid Chemical Sampling Information. U.S. Department of Labor.
3. National Institute for Occupational Safety and Health (NIOSH). Pocket Guide to Chemical Hazards: Nitric Acid. CDC/NIOSH.
4. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley-Interscience: New York, 1992; pp 501–505 (Nitration).

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