Nitric acid

Lore & Background

The discovery of mineral acids such as nitric acid is generally presumed to go back to 13th-century European alchemy. The conventional view is that nitric acid was first described in pseudo-Geber's De inventione veritatis. However, according to Eric John Holmyard and Ahmad Y. al-Hassan, nitric acid was also referenced in various earlier Arabic works such as the 'Chest of Wisdom' attributed to Jabir ibn Hayyan (8th century) or the work attributed to the Fatimid caliph al-Hakim bi-Amr Allah (985–1021). Nitric acid is also found in post-1300 works falsely attributed to Albert the Great and Ramon Llull.

In the 17th century, Johann Rudolf Glauber devised a process to obtain nitric acid by distilling potassium nitrate with sulfuric acid. In 1776 Antoine Lavoisier cited Joseph Priestley's work to point out that it can be converted from nitric oxide. In 1785 Henry Cavendish determined its precise composition and showed that it could be synthesized by passing a stream of electric sparks through moist air. In 1806, Humphry Davy reported the results of extensive distilled water electrolysis experiments concluding that nitric acid was produced at the anode from dissolved atmospheric nitrogen gas.

The industrial production of nitric acid from atmospheric air began in 1905 with the Birkeland–Eyde process. Another early production method was invented by French engineer Albert Nodon around 1913. Once the Haber process for the efficient production of ammonia was introduced in 1913, nitric acid production from ammonia using the Ostwald process overtook production from the Birkeland–Eyde process. This method of production is still in use today.

Chemistry and the Source of Its Red Color

Red fuming nitric acid is a powerful oxidizer that remains stable in storage, making it far more practical than its less concentrated relatives. Its composition is a blend of nitric acid, dinitrogen tetroxide, and a modest quantity of water. The striking crimson hue that gives the compound its name comes from the dinitrogen tetroxide, which partially decomposes into nitrogen dioxide, a reddish-brown gas. As the liquid becomes saturated with this gas, it releases toxic fumes carrying a suffocating odor.

Beyond its appearance, RFNA is a dangerous substance: it heightens the flammability of nearby combustible materials and reacts with water in a violently exothermic manner. The deliberate inclusion of dinitrogen tetroxide is not merely cosmetic. Because nitrogen dioxide is itself a decomposition product of nitric acid, adding the tetroxide shifts the equilibrium in favor of the acid, stabilizing it through Le Chatelier's principle. This same addition simultaneously boosts the mixture's oxidizing strength and depresses its freezing point, broadening the range of temperatures in which it can be handled and deployed.

A Workhorse of Rocket Propulsion

In the world of rocketry, RFNA has earned a reputation as a dependable oxidizer, most frequently paired with a separate fuel in bipropellant systems. It can also function as a monopropellant when amine nitrates are dissolved directly into the acid, serving as both fuel and oxidizer in a single tank, though this approach is notably inefficient and rarely chosen in practice. The compound's military history stretches back to World War II, when the German armed forces incorporated it into rocket propellants. Two notable formulations emerged: S-Stoff, a 96 percent nitric acid mixture with 4 percent ferric chloride acting as an ignition catalyst, and SV-Stoff, blending 94 percent nitric acid with 6 percent dinitrogen tetroxide.

Both earned the informal nickname Salbei, meaning sage. In the postwar era, inhibited RFNA became the oxidizer of choice for the Kosmos-3M, which went on to become the most frequently launched light orbital rocket in history. Across former Soviet states, the inhibited variant is commonly referred to by the French-derived name Mélange.

Taming the Corrosion Problem

One of the most persistent engineering challenges surrounding RFNA is its aggressive attack on virtually every common container material. To make the acid storable and transportable, formulators routinely add inhibitors, and any RFNA blended with such an additive is designated inhibited RFNA, or IRFNA. Hydrogen fluoride is the most widely cited inhibitor, typically present at roughly 0.6 percent. Its mechanism is elegant: HF reacts with the inner surface of the storage vessel to deposit a thin, nearly impervious layer of metal fluoride, effectively sealing the acid off from the underlying metal.

Other inhibitor families include iodine-based, phosphorus-based, potassium-based, and fluorine-based compounds, each yielding a distinct IRFNA grade. Material testing has shown that many stainless steels, along with tin, gold, and tantalum, exhibit strong corrosion resistance. Aluminium alloys fare less well, particularly at elevated temperatures, though their degradation rates remain within usable limits. Interestingly, the presence of phosphoric acid accelerates corrosion at high temperatures, while sulfuric acid actually suppresses it.

Beyond the Rocket: Industrial and Laboratory Roles

Although its fame rests largely on aerospace applications, RFNA finds a wider footprint across industry and the laboratory bench. In chemical manufacturing, it serves as a reagent for producing fertilizers, dye intermediates, and explosives, and it functions as an acidifying agent in pharmaceutical synthesis. In smaller-scale settings, it is employed in photoengraving and metal etching, where its strong oxidizing character makes it well suited to selectively removing material from surfaces. The variety of commercial and military formulations underscores how adaptable the base chemistry is.

The AK series, for example, holds a constant 80 percent nitric acid and 20 percent dinitrogen tetroxide ratio but swaps the inhibitor: fluorine-based in AK20F, iodine-based in AK20I, potassium-based in AK20K. The AK27 line shifts to a 73-27 split with iodine or phosphorus inhibitors. The IRFNA IIIa and IV HDA grades represent yet another family, with the latter pushing dinitrogen tetroxide to 44 percent. Each variant balances oxidizing power, storage stability, and material compatibility for a specific operational need.

Reader's Guide

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 at 1 atm. It is known as 'concentrated nitric acid'. The azeotrope of nitric acid and water is a colourless liquid at room temperature.

Contamination with nitrogen dioxide

Two solid hydrates are known: the monohydrate (HNO3·H2O or H3O+NO3−) and the trihydrate (HNO3·3H2O). An older density scale is occasionally seen, with concentrated nitric acid specified as 42 Baumé. Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles. The nitrogen dioxide (NO2) and/or dinitrogen tetroxide (N2O4) remains dissolved in the nitric acid coloring it yellow or even red at higher temperatures.

Fuming nitric acid

Commercial-grade fuming nitric acid contains 98% HNO3 and has a density of 1.50 g/cm3. Red fuming nitric acid (RFNA) contains substantial quantities of dissolved nitrogen dioxide. 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).

Anhydrous nitric acid

White fuming nitric acid (WFNA) is very close to anhydrous nitric acid. It is available as 99.9% nitric acid by assay, or about 24 molar. Anhydrous nitric acid is a colorless, low-viscosity liquid with a density of 1.512–3 g/cm3 that solidifies at -42 °C to form white crystals. Its dynamic viscosity under standard conditions is 0.76 mPa·s.

Structure and bonding

It boils at 83 °C. The two terminal N–O bonds are nearly equivalent and relatively short, at 1.20 and 1.21 Å. The third N–O bond is elongated because its O atom is bonded to H atom, with a bond length of 1.41 Å in the gas phase. The molecule is slightly aplanar (the O2N and NOH planes are tilted away from each other by 2°) and there is restricted rotation about the N–OH single bond. Nitric acid is normally considered to be a strong acid at ambient temperatures.