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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 ). In other words, an oxi ...
used as a
rocket propellant Rocket propellant is the reaction mass of a rocket. This reaction mass is ejected at the highest achievable velocity from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemic ...
. It consists of 84%
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
(), 13%
dinitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russia rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium ...
and 1–2%
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
. The color of red fuming nitric acid is due to the dinitrogen tetroxide, which breaks down partially to form nitrogen dioxide. 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. It is usually used with an
inhibitor Inhibitor or inhibition may refer to: In 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 neurotra ...
(with various, sometimes secret, substances, including
hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock ...
; 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; 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, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, 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 is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
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. Other uses for RFNA include fertilizers, dye intermediates, explosives, and pharmaceutic aid as acidifier. 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, fluorine-based inhibitor * AK27I: 73% HNO3, 27% N2O4, iodine-based inhibitor * AK27P: 73% HNO3, 27% N2O4, fluorine-based inhibitor


Experiments

; 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 purpose of the HF is to act as a corrosion inhibitor. ;
Water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
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 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.


See also

* White fuming nitric acid


References

{{Reflist , refs= {{Cite book , title=Ignition! An Informal History of Liquid Rocket Propellants , last=Clark , first=John D. , chapter=9: What Ivan Was Doing , publisher=Rutgers University Press , year=1972 , isbn=0813507251 , url=https://orgprepdaily.files.wordpress.com/2010/03/9.pdf , page=116


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