Nitric Acid

Jai Ganesh · 2025-10-05 16:39:33

Nitric Acid

Gist

Nitric acid (HNO3) is a highly corrosive, colorless to yellow fuming liquid mineral acid used in manufacturing fertilizers, explosives, and nylon, and in various industrial processes and chemical laboratories. Known historically as aqua fortis and spirit of nitre, it is a strong, monoprotic acid that can cause severe burns on contact with skin and tissues and must be handled with extreme caution.

Nitric acid (HNO3) has a variety of industrial and laboratory uses, primarily in the production of fertilizers and explosives, but also in chemical manufacturing, metal processing (like etching and purification), and wastewater treatment. It is also used for making dyes, polymers, and in some medical and research applications.

Summary

Nitric acid is an inorganic compound with the formula HNO3. It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into oxides of nitrogen. Most commercially available nitric acid has a concentration of 68% in water. When the solution contains more than 86% HNO3, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as red fuming nitric acid at concentrations above 86%, or white fuming nitric acid at concentrations above 95%.

Nitric acid is the primary reagent used for nitration – the addition of a nitro group, typically to an organic molecule. While some resulting nitro compounds are shock- and thermally-sensitive explosives, a few are stable enough to be used in munitions and demolition, while others are still more stable and used as synthetic dyes and medicines (e.g. metronidazole). Nitric acid is also commonly used as a strong oxidizing agent.

Physical and chemical properties

Commercially available nitric acid is an azeotrope with water at a concentration of 68% HNO3. This solution has a boiling temperature of 120.5 °C (248.9 °F) at 1 atm (100 kPa; 15 psi). It is known as "concentrated nitric acid". The azeotrope of nitric acid and water is a colourless liquid at room temperature.

Commercial-grade fuming nitric acid contains 98% HNO3 and has a density of 1.50 g/{cm}^{3}. This grade is often used in the explosives industry. It is not as volatile nor as corrosive as the anhydrous acid and has the approximate concentration of 21.4 M.

Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO2) leaving the solution with a reddish-brown color. The presence of dissolved nitrogen dioxide increases the density of the aqueous and the anhydrous acid, anhydrous nitric acid containing 25% NO2 has a density of around 1.60 g/{cm}^{3}, the density is lower in water-containing mixtures. The maximum density of anhydrous nitric acid is attained at 40% (w/w) NO2.

An inhibited fuming nitric acid, either white inhibited fuming nitric acid (IWFNA), or red inhibited fuming nitric acid (IRFNA), can be made by the addition of 0.6 to 0.7% hydrogen fluoride (HF). This fluoride is added for corrosion resistance in metal tanks. The fluoride creates a metal fluoride layer that protects the metal.

Details

Nitric acid is a (HNO3), colourless, fuming, and highly corrosive liquid (freezing point −42 °C [−44 °F], boiling point 83 °C [181 °F]) that is a common laboratory reagent and an important industrial chemical for the manufacture of fertilizers and explosives. It is toxic and can cause severe burns.

The preparation and use of nitric acid were known to the early alchemists. A common laboratory process used for many years, ascribed to a German chemist, Johann Rudolf Glauber (1648), consisted of heating potassium nitrate with concentrated sulfuric acid. In 1776 Antoine-Laurent Lavoisier showed that it contained oxygen, and in 1816 Joseph-Louis Gay-Lussac and Claude-Louis Berthollet established its chemical composition.

The principal method of manufacture of nitric acid is the catalytic oxidation of ammonia. In the method developed by the German chemist Wilhelm Ostwald in 1901, ammonia gas is successively oxidized to nitric oxide and nitrogen dioxide by air or oxygen in the presence of a platinum gauze catalyst. The nitrogen dioxide is absorbed in water to form nitric acid. The resulting acid-in-water solution (about 50–70 percent by weight acid) can be dehydrated by distillation with sulfuric acid.

Nitric acid decomposes into water, nitrogen dioxide, and oxygen, forming a brownish yellow solution. It is a strong acid, completely ionized into hydronium (H3O+) and nitrate (NO3−) ions in aqueous solution, and a powerful oxidizing agent (one that acts as electron acceptor in oxidation-reduction reactions). Among the many important reactions of nitric acid are: neutralization with ammonia to form ammonium nitrate, a major component of fertilizers; nitration of glycerol and toluene, forming the explosives nitroglycerin and trinitrotoluene (TNT), respectively; preparation of nitrocellulose; and oxidation of metals to the corresponding oxides or nitrates.

Additional Information

Nitric acid (HNO3) is an extremely important chemical used in the manufacture of fertilisers and explosives. It is made from ammonia by the Ostwald Process which was developed in 1902 by the German chemist Wilhelm Ostwald (below, left), who got the Nobel prize for Chemistry in 1909. This process reacts together O2 and ammonia NH3 at 850°C and 5 atmospheres pressure, with the help of platinum and rhodium catalysts, to make NO gas. This is then oxidised to NO2, which is then dissolved in water to make HNO3.

The Ostwald process was discovered just in time for the First World War, and it contributed greatly to the extended length of that war. This is because previously Germany had no nitrate deposits of its own from which to make the nitric acid that was essential for the production of the explosives used in artillery shells, such as TNT and nitroglycerine. In fact, most of the nitrates were only available from guano, which is the droppings of fish-eating sea birds, and is found in large quantities on the islands off the coast of Peru. When hostilities began, the shipping routes to Germany across the Atlantic were blocked, and Germany needed a new method to make nitric acid.

Coincidentally, again just before WW1 started, another German chemist, Fritz Haber, had found a method to turn inert nitrogen gas from air into ammonia. By 1913, the German chemical giant BASF (Badische Aniline und Soda Fabrik) had a plant operating in Ludwigshaven-Oppau, Germany making ammonia at the rate of 30 metric tons per day. Without question, the Haber-Bosch process for turning N2 into NH3, combined with the Ostwald process for turning NH3 into nitric acid, permitted Germany to continue making explosives, and extended the war for many years. Some people say that without these two chemical processes, WW1 might never have happened at all.

Properties of nitric acid

At room temperature, 100% pure anhydrous nitric acid is a liquid, but becomes a colourless white solid below -41°C and boils at 83°C. What we call 'concentrated nitric acid' is actually a solution of 68% by weight HNO3 in water (a 16M solution), and is often pale yellow as a result of photochemical decomposition which releases NO2. Dissolving even more NO2 into the pure material produces red 'fuming' nitric acid, which is an extremely powerful acid and oxidising agent using in the semiconductor industry for cleaning silicon wafers. The acid aqua regia (approx 3 vols HCl to 1 vol HNO3) contains free Cl2 and nitrosyl chloride (NOCl).

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#1 2025-10-05 16:39:33

Nitric Acid

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