Nitric Acid (HNO3): Formula, Strengths, Uses and Handling

The short answer

Nitric acid is HNO₃, CAS 7697-37-2, molecular weight 63.01 g/mol. Commercial acid is normally supplied at 60 to 68 %, with a density of about 1.41 g/cm³ at the 68 % azeotrope.

It behaves differently from hydrochloric and sulphuric acid because it is not only a strong acid but a strong oxidising agent. That is why it dissolves copper and silver, which hydrochloric acid will not touch; why it passivates stainless steel rather than corroding it; why it reacts dangerously with organic material; and why its supply is regulated in most of the world.

Formula, molecular weight and identifiers

Weak, concentrated and fuming acid

The break at 68 % is a physical one. Nitric acid and water form an azeotrope at approximately 68 % that boils unchanged at 120.5 °C, so no amount of distillation will concentrate the acid past that point. Higher strengths are made by dehydrating the azeotrope with concentrated sulphuric acid or magnesium nitrate, which is why fuming grades are a different product commercially, not just a stronger one.

Physical properties

The brown fume is the practical hazard signature. Nitrogen dioxide has a delayed effect on the lungs — a worker can inhale a damaging dose and feel little at the time, with pulmonary oedema developing hours later. Any nitric acid exposure involving visible brown fume is a medical matter regardless of how the person feels immediately.

Acid and oxidiser: why that matters

Most acids attack a metal by the hydrogen ion, releasing hydrogen gas. Nitric acid mostly does not work that way. The nitrate ion is itself reduced — to nitrogen dioxide with concentrated acid, to nitric oxide with dilute — which means nitric acid dissolves metals that sit below hydrogen in the reactivity series and that hydrochloric or dilute sulphuric acid cannot touch, copper and silver being the standard examples.

The same oxidising power is the reason for the handling rules that look excessive next to other acids:

• Organic materials. Contact with wood, paper, rags, sawdust, alcohols, ketones, glycols or oils can ignite or detonate. Spill absorbent must be inorganic — vermiculite, sand or a proprietary acid absorbent, never sawdust or cellulose.
• Reducing agents and metal powders. Violent reaction. Segregate completely.
• Bunding and drainage. A nitric acid bund should contain nothing combustible, and the store should hold no combustible goods at all.
• Aqua regia. Three parts hydrochloric to one part nitric dissolves gold and platinum. It is unstable, evolves chlorine and nitrosyl chloride, and must be made fresh and never stored in a closed container.

Why it protects stainless steel instead of eating it

Nitric acid is one of the few strong acids that stainless steel handles well, and the reason is the oxidising power again. Nitric acid oxidises the chromium at the steel surface into a thin, tight, self-repairing chromium oxide film, and that film then protects the metal underneath. The acid is effectively building the steel's own armour.

This is the basis of an entire industrial process. Passivation of stainless steel — typically 20 to 50 % nitric acid, sometimes with sodium dichromate, at controlled temperature and time — strips free iron and contamination from a machined or welded surface and restores full corrosion resistance. Citric acid passivation has displaced it in many plants on safety and effluent grounds, but nitric remains the reference process in the standards.

The same effect passivates aluminium, which is why concentrated nitric acid can be shipped in aluminium tanks that would be destroyed by hydrochloric acid. It does not extend to dilute acid: below roughly 50 % the passivating film no longer forms reliably on aluminium, so a strength that is safe in one vessel is not safe in the same vessel diluted.

The Ostwald process

Essentially all nitric acid is made by the Ostwald process in three steps.

• Oxidation of ammonia. Ammonia and air pass over a platinum–rhodium gauze at around 900 °C and are converted to nitric oxide and water.
• Oxidation of NO. The nitric oxide is cooled and oxidised further by air to nitrogen dioxide.
• Absorption. Nitrogen dioxide is absorbed in water in a tall absorption column to give nitric acid, typically at 55 to 68 % depending on the pressure the plant runs.

Because the feedstock is ammonia, nitric acid economics track ammonia and therefore natural gas. And because the same plants feed ammonium nitrate production, weak acid supply is usually tied to fertiliser demand rather than to chemical demand.

What nitric acid is used for

• Fertiliser. By a wide margin the largest use: reaction with ammonia to make ammonium nitrate and calcium ammonium nitrate, and with phosphate rock in the nitrophosphate route to NPK fertilisers.
• Nylon intermediates. Oxidation of cyclohexanol and cyclohexanone to adipic acid, the diacid half of nylon 6,6.
• Metal treatment. Pickling and passivation of stainless steel, bright dipping of copper and brass, and etching in printing and electronics.
• Analytical laboratories. Sample digestion for ICP-OES, ICP-MS and atomic absorption; trace-metal grade acid is a distinct, much purer product.
• Electronics. Silicon wafer etching and cleaning, on semiconductor-grade acid with parts-per-billion metal limits.
• Metal refining and assay. Parting of gold and silver, and aqua regia dissolution of precious metals.
• Intermediates. Nitration in the manufacture of dyes, pigments, pharmaceutical intermediates and polyurethane precursors.

Scapex Exports supplies nitric acid 68 % technical, concentrated nitric acid and nitric acid AR grade.

Safety, regulation and export documents

• Classification. UN 2031, Class 8, with subsidiary risk 5.1 at higher strengths; red fuming acid is UN 2032. Packing group depends on concentration — confirm with your forwarder for the strength you are moving.
• Health. Causes severe burns; yellow staining of skin (the xanthoproteic reaction with protein) is a sign of a real chemical burn. Nitrogen dioxide inhalation can cause delayed pulmonary oedema; treat any fume exposure as a medical emergency even if the person seems well.
• Materials. Stainless steel 304L and 316L, aluminium at high concentration, glass, PTFE, PVDF and some FRP systems. Not carbon steel, not copper or brass, not natural rubber, not most plastics that are fine with hydrochloric acid.
• Storage. Cool, dark, ventilated, bunded, with nothing combustible in the store. Vent through a scrubber. Keep completely separate from organics, reducing agents, alkalis and cyanides.
• Regulatory diligence. Nitric acid above set concentration thresholds is a regulated explosives precursor in the European Union, India, the United States and elsewhere. Expect end-use and consignee questions before a quotation, and check whether your destination requires an import permit, an end-use certificate or licensed-consignee status. This is standard compliance, not an obstacle.
• Packing. 35 to 50 kg HDPE carboys, 250 kg HDPE drums, stainless or aluminium road tankers and ISO tanks depending on strength.

An export consignment should carry a certificate of analysis stating HNO₃ assay, specific gravity, chloride, sulphate, iron, heavy metals and residue on ignition; a 16-section GHS safety data sheet; a dangerous goods declaration with the correct UN number, class, subsidiary risk and packing group; commercial invoice and packing list; certificate of origin; and any end-use declaration or import permit the destination requires.

Enquire with a specification

Nitric acid is supplied by Scapex Exports as a manufacturer and supplier from qualified Indian producers, in technical, concentrated and analytical grades, with certificate of analysis, safety data sheet and full export and dangerous goods documentation.

Send the strength, grade, impurity limits, packing, quantity, destination port and end use, and you will get a quotation written against that specification together with a clear answer on what the destination's precursor rules require.

Frequently asked questions