Red fuming nitric acid
Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant. It consists of 84% nitric acid (HNO3), 13% dinitrogen tetroxide and 1–2% 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.
Red fuming nitric acid
|HNO3 + NO2|
|Appearance||Liquid, red fumes|
|Density||Increases as free NO2 content increases|
|Boiling point||120.5 °C (248.9 °F; 393.6 K)|
|Miscible in water|
|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).
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It is usually used with an inhibitor (with various, sometimes secret, substances, including hydrogen fluoride; 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) 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.
- 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
- 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 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 hold up as well as stainless steels especially in high temperatures 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.
- "Problems in Storage and Handling of Red Fuming Nitric Acid" (PDF). Archived from the original on September 27, 2013. Retrieved 2013-09-26.CS1 maint: bot: original URL status unknown (link)
- Clark, John D. (1972). Ignition! An Informal History of Liquid Rocket Propellants. Rutgers University Press. p. 62. ISBN 0-8135-0725-1.
- Clark, John D. (1972). "9: What Ivan Was Doing". Ignition! An Informal History of Liquid Rocket Propellants (PDF). Rutgers University Press. p. 116. ISBN 0813507251.
- O'Neil, Maryadele J. (2006). The Merck index: an encyclopedia of chemicals, drugs, and biologicals. Merck. p. 6576. ISBN 978-0-911910-00-1.
- 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.
- "Corrosion Studies in Fuming Nitric Acid" (PDF). Retrieved 23 May 2017.
- 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.