Red fuming nitric acid (RFNA) is a storable

oxidizer An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...

used as a

rocket propellant Rocket propellant is used as reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines. Overvi ...

. It consists of

nitric acid Nitric acid is an inorganic compound with the formula . 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 nitrogen oxide, oxides of nitrogen. Most com ...

(),

dinitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russian rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium ...

() and a small amount of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

. The color of red fuming nitric acid is due to the dinitrogen tetroxide, which breaks down partially to form

nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, is an intermediate in the s ...

. The nitrogen dioxide dissolves until the liquid is saturated, and produces toxic fumes with a suffocating odor. RFNA increases the flammability of combustible materials and is highly exothermic when reacting with water. Since nitrogen dioxide is a product of decomposition of nitric acid, its addition stabilizes nitric acid in accordance with Le Chatelier's principle. Addition of dinitrogen tetroxide also increases oxidizing power and lowers the freezing point. It is usually used with an

inhibitor Inhibitor or inhibition may refer to: Biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotransmi ...

(with various, sometimes secret, substances, including

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

; any such combination is called ''inhibited RFNA'', ''IRFNA'') because nitric acid attacks most container materials. Hydrogen fluoride for instance will passivate the container metal with a thin layer of metal fluoride, making it nearly impervious to the nitric acid. It can also be a component of a

monopropellant Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with biprop ...

; with substances like amine nitrates dissolved in it, it can be used as the sole fuel in a rocket. This is inefficient and it is not normally used this way. During

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, the German military used RFNA in some rockets. The mixtures used were called ''S- Stoff'' (96% nitric acid with 4%

ferric chloride Iron(III) chloride describes the inorganic compounds with the formula (H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated f ...

as an ignition catalyst) and ''SV-Stoff'' (94% nitric acid with 6% dinitrogen tetroxide) and nicknamed ''Salbei'' ( sage). Inhibited RFNA was the oxidizer of the world's most-launched light orbital rocket, the

Kosmos-3M The Kosmos-3M ( meaning "''Cosmos''", GRAU index 11K65M) was a Russian space launch vehicle, member of the Kosmos (rocket family), Kosmos rocket family. It was a liquid-fueled two-stage launch vehicle, first launched in 1967 and with over 420 s ...

. In former-Soviet countries inhibited RFNA is known as

Mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically con ...

. Other uses for RFNA include fertilizers, dye intermediates, explosives, and pharmaceutical acidifiers. It can also be used as a laboratory reagent in photoengraving and metal etching.

Compositions

* ''IRFNA IIIa'': 83.4% HNO₃, 14% NO₂, 2% H₂O, 0.6% HF * ''IRFNA IV HDA'': 54.3% HNO₃, 44% NO₂, 1% H₂O, 0.7% HF * ''S-Stoff'': 96% HNO₃, 4% FeCl₃ * ''SV-Stoff'': 94% HNO₃, 6% N₂O₄ * ''AK20'': 80% HNO₃, 20% N₂O₄ * ''AK20F'': 80% HNO₃, 20% N₂O₄, fluorine-based inhibitor * ''AK20I'': 80% HNO₃, 20% N₂O₄, iodine-based inhibitor * ''AK20K'': 80% HNO₃, 20% N₂O₄, potassium-based inhibitor * ''AK27I'': 73% HNO₃, 27% N₂O₄, iodine-based inhibitor * ''AK27P'': 73% HNO₃, 27% N₂O₄, phosphorus-based inhibitor

Corrosion

; Hydrofluoric acid content of IRFNA: When RFNA is used as an oxidizer for rocket fuels, it usually has a HF content of about 0.6%. The HF forms a metal fluoride layer on the surface of the storage vessels, inhibiting corrosion. ; Water content of RFNA: To test the water content, a sample of 80% HNO₃, 8–20% NO₂, and the rest H₂O depending on the varied amount of NO₂ in the sample. When the RFNA contained HF, there was an average H₂O% between 2.4% and 4.2%. When the RFNA did not contain HF, there was an average H₂O% between 0.1% and 5.0%. When the metal impurities from corrosion were taken into account, the H₂O% increased, and the H₂O% was between 2.2% and 8.8% ; Corrosion of metals in RFNA: Stainless steel, aluminium alloys, iron alloys, chrome plates, tin, gold and tantalum were tested to see how RFNA affected the corrosion rates of each. Experiments were performed using 16% and 6.5% RFNA samples and the different substances listed above. Many different stainless steels showed resistance to corrosion. Aluminium alloys did not endure as well as stainless steels especially in high temperature, but the corrosion rates were not high enough to prohibit the use of this with RFNA. Tin, gold and

tantalum Tantalum is a chemical element; it has Symbol (chemistry), symbol Ta and atomic number 73. It is named after Tantalus, a figure in Greek mythology. Tantalum is a very hard, ductility, ductile, lustre (mineralogy), lustrous, blue-gray transition ...

showed high corrosion resistance similar to that of stainless steel. These materials are better though because at high temperatures the corrosion rates did not increase much. Corrosion rates at elevated temperatures increase in the presence of phosphoric acid. Sulfuric acid decreased corrosion rates.

References

External links

National Pollutant Inventory – Nitric Acid Fact Sheet
* https://web.archive.org/web/20030429160808/http://www.astronautix.com/props/nitidjpx.htm Rocket oxidizers Oxidizing acids

Compositions

Corrosion

See also

References

Further reading

External links