Red fuming nitric acid

Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant. It consists of 84% nitric acid (), 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. 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 pharmaceutical acidifiers. 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.