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Red fuming nitric acid
Names
	IUPAC name Nitric acid
	Other names Red fuming nitric acid
Identifiers
CAS Number	52583-42-3 ;
ChemSpider	None;
CompTox Dashboard (EPA)	DTXSID5029685 ;
Properties
Chemical formula	HNO3 + NO2
Appearance	Liquid, red fumes
Density	Increases as free NO2 content increases
Boiling point	83 °C (181 °F; 356 K)
Solubility in water	Miscible in water
Hazards
	Occupational safety and health (OHS/OSH):
Main hazards	Skin and metal corrosion; serious eye damage; toxic (oral, dermal, pulmonary); severe burns
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant. It consists of nitric acid (H N O₃), dinitrogen tetroxide (N₂O₄) and a small amount of water. 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.^[1]

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.^[2]

It is usually used with an inhibitor (with various, sometimes secret, substances, including hydrogen fluoride;^[3]^: 62 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, the German military used RFNA in some rockets. The mixtures used were called S-Stoff (96% nitric acid with 4% ferric chloride as an ignition catalyst^[3]^: 115–9) 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. In former-Soviet countries inhibited RFNA is known as Mélange.

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.^[4]

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^[5]^[6]: 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 by forming a metal fluoride layer on the surface of the storage vessels.
Water content of RFNA^[7]: 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^[5]: 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.

References

^ Sugur, V. S.; Manwani, G. L. (October 1983). "Problems in Storage and Handling of Red Fuming Nitric Acid". Defence Science Journal. 33 (4): 331–337. doi:10.14429/dsj.33.6188.
^ Schmidt, Eckart W. (2022). "Red Fuming Nitric Acid". Encyclopedia of Oxidizers. De Gruyter. pp. 3881–3962. doi:10.1515/9783110750294-029. ISBN 978-3-11-075029-4.
^ ^a ^b Clark, John Drury (23 May 2018). Ignition!: An Informal History of Liquid Rocket Propellants. Rutgers University Press. p. 302. ISBN 978-0-8135-9918-2. OCLC 281664.
^ O'Neil, Maryadele J. (2006). The Merck index: an encyclopedia of chemicals, drugs, and biologicals. Merck. p. 6576. ISBN 978-0-911910-00-1.
^ ^a ^b Karplan, Nathan; Andrus, Rodney J. (October 1948). "Corrosion of Metals in Red Fuming Nitric Acid and in Mixed Acid". Industrial and Engineering Chemistry. 40 (10): 1946–1947. doi:10.1021/ie50466a021.
^ Phelps, Edson H.; Lee, Fredrick S.; Robinson, Raymond B. (October 1955). Corrosion Studies in Fuming Nitric Acid (PDF) (Technical report). Wright Air Development Center. 55-109. Archived (PDF) from the original on July 27, 2018. Retrieved 2024-01-02.
^ Burns, E. A.; Muraca, R. F. (1963). "Determination of Water in Red Fuming Nitric Acid by Karl Fischer Titration". Analytical Chemistry. 35 (12): 1967–1970. doi:10.1021/ac60205a055.