What is Nitric Acid (HNO3)?

Nitric acid (HNO3) is a highly corrosive, fuming mineral acid essential to metal finishing, laboratory synthesis, and industrial manufacturing. Unlike the weaker, unstable nitrous acid, nitric acid is a powerful oxidizing agent. When combined with hydrochloric acid, it forms a solution capable of dissolving noble metals. Whether you need a dilute 5% solution for equipment descaling or a 70% ACS grade for semiconductor etching, selecting the correct concentration of nitric is critical for operational success and safety.

In its pure form, nitric acid is a clear, colorless liquid. However, it often appears pale yellow due to the decomposition of the acid into nitrogen dioxide (NO2) over time or when exposed to light. It is highly soluble in water and miscible with most organic solvents, though mixing it with organic compounds requires extreme caution due to its oxidizing properties.

Nitric Acid vs. Nitrous Acid: Key Differences

While their names are similar, nitric acid and nitrous acid behave very differently in chemical processes.

• Nitric Acid (HNO3): A strong, highly stable mineral acid and a powerful oxidizing agent. It fully dissociates in water and is manufactured on a massive industrial scale for fertilizers, explosives, and metal treatment.
• Nitrous Acid (HNO2): A weak, unstable acid that exists only in solution or as nitrite salts. It is typically generated in situ (on-site) for specific laboratory reactions, such as the preparation of diazonium salts from amines.

For industrial cleaning, etching, and passivation, nitric acid is the required chemical. Nitrous acid lacks the strength and stability for these applications.

Nitric Acid vs. Hydrochloric Acid

Both nitric acid and hydrochloric acid are strong mineral acids, but they serve different primary functions due to their chemical behavior.

Hydrochloric acid is a non-oxidizing acid. It is highly effective at removing rust and scale from steel without heavily attacking the base metal, making it ideal for pickling. Nitric acid is a strong oxidizing acid. Instead of simply dissolving metal oxides, it actively oxidizes surfaces, which is why it is used for passivation and etching.

When nitric acid and hydrochloric acid are mixed together, they form aqua regia. This highly corrosive, fuming yellow mixture is one of the few reagents capable of dissolving noble metals like gold and platinum, a property utilized in precious metal recovery and analytical chemistry.

Industrial Applications by Concentration

The utility of nitric acid depends entirely on its concentration. Using the wrong grade can lead to incomplete reactions or excessive material degradation.

• 5% Technical Grade: A dilute, fuming liquid used primarily for light descaling, pH adjustment, and specialized cleaning protocols where stronger oxidizers would damage equipment.
• 40% Technical Grade: A mid-strength clear, colorless liquid miscible with water and alcohol. It is heavily utilized in metal passivation, intermediate chemical synthesis, and industrial cleaning.
• 65% ACS Grade: A high-purity reagent with a boiling point of 121°C (249.8°F). It is the standard concentration for laboratory analytical work, wet chemistry, and precious metal refining.
• 70% ACS Grade - Low Particle: The highest standard concentration, boiling at 83°C (181.4°F). This grade is strictly controlled for trace impurities and is essential for semiconductor manufacturing, critical etching, and trace metal analysis.

Metal Passivation and Etching

One of the most common industrial uses for nitric acid is the passivation of stainless steel. During machining and fabrication, free iron is deposited on the surface of stainless steel components. If left untreated, this iron will rust, compromising the integrity of the part.

Submerging the component in a nitric acid bath (typically using a 40% solution, diluted as per manufacturer specifications) dissolves the free iron. More importantly, the oxidizing nature of the acid promotes the formation of a thin, dense chromium oxide layer on the surface. This passive layer protects the steel from future corrosion.

In higher concentrations (65% to 70%), nitric acid is used as an etchant. It rapidly oxidizes and dissolves metals like copper, silver, and zinc, making it a staple in printed circuit board (PCB) manufacturing and jewelry making.

Safety, Storage, and Handling

Nitric acid is a highly hazardous chemical. The 70% concentration is classified under Hazard Class 8 (Corrosive). It causes severe skin burns and eye damage upon contact. The fumes generated by concentrated nitric acid contain nitrogen oxides, which are highly toxic if inhaled.

Always store nitric acid in a cool, dry, well-ventilated area away from direct sunlight. Because it is a strong oxidizer, it must be strictly segregated from organic materials, reducing agents, and flammable solvents like acetone or methanol. Mixing nitric acid with organics can result in violent, explosive reactions.

Operators must wear appropriate personal protective equipment (PPE), including acid-resistant gloves, chemical splash goggles, and face shields. Always consult the specific product Safety Data Sheet (SDS) for exact UN numbers, packing groups, and emergency response procedures before handling.

Frequently Asked Questions