Nitric acid

Lore & Background

The discovery of nitric acid is generally presumed to go back to 13th-century European alchemy, first described in pseudo-Geber's De inventione veritatis (after c. The recipe in the Ṣundūq al-ḥikma, once attributed to Jabir ibn Hayyan, is now recognized as a known forgery; the earliest clear descriptions of nitric acid are from 13th-century European alchemy. In the 17th century, Johann Rudolf Glauber devised a process to obtain nitric acid by distilling potassium nitrate with sulfuric acid.

Reader's Guide

Nitric acid's significance lies in its dual role as a fundamental industrial chemical and a historical milestone in alchemy and chemistry. As the primary reagent for nitration, it enables the production of nitro compounds used in explosives, munitions, synthetic dyes, and medicines such as metronidazole. Its strong oxidizing properties make it indispensable in various chemical processes. The historical development of nitric acid production—from medieval alchemical recipes to the Birkeland–Eyde and Ostwald processes—illustrates the evolution of chemical manufacturing. The compound's physical properties, including its azeotropic concentration and tendency to decompose into nitrogen dioxide, have practical implications for storage and handling. The development of inhibited fuming nitric acid with hydrogen fluoride improved corrosion resistance for metal tanks, enabling its use in rocketry and other demanding applications. Nitric acid remains a cornerstone of the chemical industry, with the Ostwald process still in use today.

Did You Know?

• Red fuming nitric acid contains substantial dissolved nitrogen dioxide, giving it a reddish-brown color and a density up to 1.60 g/cm³.
• Anhydrous nitric acid solidifies at −42 °C to form white crystals and boils at 83 °C.

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.