Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydrazine hydrate ().
Hydrazine is mainly used as a foaming agent in preparing polymer foams, but applications also include its uses as a precursor to pharmaceuticals and agrochemicals, as well as a long-term storable propellant for in-space spacecraft propulsion. Additionally, hydrazine is used in various rocket fuels and to prepare the gas precursors used in airbags. Hydrazine is used within both nuclear and conventional electrical power plant steam cycles as an oxygen scavenger to control concentrations of dissolved oxygen in an effort to reduce corrosion. , approximately 120,000 tons of hydrazine hydrate (corresponding to a 64% solution of hydrazine in water by weight) were manufactured worldwide per year.
Hydrazines are a class of organic substances derived by replacing one or more hydrogen atoms in hydrazine by an organic group.
In 2019, the first examples of isomorphism between organic hydrazinosolvates and their corresponding hydrates were discovered, demonstrating that hydrazine molecules can undergo isomorphous substitution by water within crystalline frameworks such as 18-crown-6 complexes.
Etymology and history The name "hydrazine" was coined by Emil Fischer in 1875; he was trying to produce organic compounds that consisted of mono-substituted hydrazine. By 1887, Theodor Curtius had produced hydrazine sulfate by treating organic diazides with dilute sulfuric acid; however, he was unable to obtain pure hydrazine, despite repeated efforts. On p. 129, Curtius admits: "" [Free hydrazine has not been analyzed yet.] Pure anhydrous hydrazine was first prepared by the Dutch chemist Lobry de Bruyn in 1895.
The nomenclature is a bi-valent form, with prefix hydr- used to indicate the presence of hydrogen atoms and suffix beginning with -az-, from azote, the French word for nitrogen.
Applications Gas producers and propellants The largest use of hydrazine is as a precursor to blowing agents. Specific compounds include azodicarbonamide and azobisisobutyronitrile, which produce of gas per gram of precursor. In a related application, sodium azide, the gas-forming agent in airbags, is produced from hydrazine by reaction with sodium nitrite.
Hydrazine is also used as a long-term storable propellant on board space vehicles, such as the Dawn mission to Ceres and Vesta, and to both reduce the concentration of dissolved oxygen in and control pH of water used in large industrial boilers. The F-16 fighter jet, Space Shuttle, and U-2 spy plane use hydrazine to fuel their Emergency Start System in the event of an engine stall. The Space Shuttle and its solid rocket boosters used hydrazine in their auxiliary power units to produce hydraulic power for engine gimbaling and control of aerodynamic surfaces.
Precursor to pesticides and pharmaceuticals
Hydrazine is a precursor to several pharmaceuticals and pesticides. Often these applications involve conversion of hydrazine to heterocyclic rings such as pyrazoles and pyridazines. Examples of commercialized bioactive hydrazine derivatives include cefazolin, rizatriptan, anastrozole, fluconazole, metazachlor, metamitron, metribuzin, paclobutrazol, diclobutrazole, propiconazole, hydrazine sulfate, diimide, triadimefon, and the diacylhydrazine insecticides.
Hydrazine compounds can be effective as active ingredients in insecticides, miticides, nematicides, fungicides, antiviral agents, attractants, herbicides, or plant growth regulators.
Small-scale, niche, and research
The Italian catalyst manufacturer Acta (chemical company) has proposed using hydrazine as an alternative to hydrogen in fuel cells. The chief benefit of using hydrazine is that it can produce over 200 mW/cm2 more than a similar hydrogen cell without requiring (expensive) platinum catalysts. Because the fuel is liquid at room temperature, it can be handled and stored more easily than hydrogen. By storing the hydrazine in a tank full of a double-bonded carbon-oxygen carbonyl, the fuel reacts and forms a safe solid called hydrazone. By then flushing the tank with warm water, the liquid hydrazine hydrate is released. Hydrazine has a higher electromotive force of 1.56 V compared to 1.23 V for hydrogen. Hydrazine breaks down in the cell to form nitrogen and hydrogen which bonds with oxygen, releasing water. Hydrazine was used in fuel cells manufactured by Allis-Chalmers Corp., including some that provided electric power in space satellites in the 1960s.
A mixture of 63% hydrazine, 32% hydrazine nitrate and 5% water is a standard propellant for experimental bulk-loaded liquid propellant artillery. The propellant mixture above is one of the most predictable and stable, with a flat pressure profile during firing. Misfires are usually caused by inadequate ignition. The movement of the shell after a mis-ignition causes a large bubble with a larger ignition surface area, and the greater rate of gas production causes very high pressure, sometimes including catastrophic tube failures (i.e. explosions). From January–June 1991, the U.S. Army Research Laboratory conducted a review of early bulk-loaded liquid propellant gun programs for possible relevance to the electrothermal chemical propulsion program.
The United States Air Force (USAF) regularly uses H-70, a 70% hydrazine 30% water mixture, in operations employing the General Dynamics F-16 Fighting Falcon fighter aircraft and the Lockheed U-2 "Dragon Lady" reconnaissance aircraft. The single jet engine F-16 utilizes hydrazine to power its Emergency Power Unit (EPU), which provides emergency electrical and hydraulic power in the event of an engine flame out. The EPU activates automatically, or manually by pilot control, in the event of loss of hydraulic pressure or electrical power in order to provide emergency flight controls. The single jet engine U-2 utilizes hydrazine to power its Emergency Starting System (ESS), which provides a highly reliable method to restart the engine in flight in the event of a stall.
Rocket fuel
Hydrazine was first used as a component in rocket fuels during World War II. A 30% mix by weight with 57% methanol (named M-Stoff in the German Luftwaffe) and 13% water was called C-Stoff by the Germans. The mixture was used to power the Messerschmitt Me 163B rocket-powered fighter plane, in which the German high test peroxide T-Stoff was used as an oxidizer. Unmixed hydrazine was referred to as B-Stoff by the Germans, a designation also used later for the ethanol/water fuel for the V-2 missile.
Hydrazine is used as a low-power monopropellant for the maneuvering (RCS/Reaction control system) thrusters of spacecraft, and was used to power the Space Shuttle's auxiliary power units (APUs). In addition, mono-propellant hydrazine-fueled rocket engines are often used in terminal descent of spacecraft. Such engines were used on the Viking program landers in the 1970s as well as the Mars landers Phoenix (May 2008), Curiosity (August 2012), and Perseverance (February 2021).
During the Soviet space program, unsymmetrical dimethylhydrazine (also discovered by Fischer in 1875) was used instead of hydrazine. Together with nitric oxidizers it became known as "devil's venom" due to its highly dangerous nature.
In all hydrazine mono-propellant engines, the hydrazine is passed over a catalyst such as iridium metal supported by high-surface-area alumina (aluminium oxide), which causes it to decompose into ammonia (), nitrogen gas (), and hydrogen () gas according to the three following reactions:
Reaction 1: Reaction 2: Reaction 3:
The first two reactions are extremely exothermic (the catalyst chamber can reach in a matter of milliseconds,) and they produce large volumes of hot gas from a small volume of liquid, making hydrazine a fairly efficient thruster propellant with a vacuum specific impulse of about 220 seconds. Reaction 2 is the most exothermic, but produces a smaller number of molecules than that of reaction 1. Reaction 3 is endothermic and reverts the effect of reaction 2 back to the same effect as reaction 1 alone (lower temperature, greater number of molecules). The catalyst structure affects the proportion of the that is dissociated in reaction 3; a higher temperature is desirable for rocket thrusters, while more molecules are desirable when the reactions are intended to produce greater quantities of gas.
Since hydrazine is a solid below , it is not suitable as a general purpose rocket propellant for military applications. Other variants of hydrazine that are used as rocket fuel are monomethylhydrazine, , also known as MMH (melting point ), and unsymmetrical dimethylhydrazine, , also known as UDMH (melting point ). These derivatives are used in two-component rocket fuels, often together with dinitrogen tetroxide, . A 50:50 mixture by weight of hydrazine and UDMH was used in the engine of the service propulsion system of the Apollo command and service module, both the ascent and descent engines of the Apollo Lunar Module and Titan II ICBMs and is known as Aerozine 50. These reactions are extremely exothermic, and the burning is also hypergolic (it starts burning without any external ignition).
There are ongoing efforts in the aerospace industry to find a replacement for hydrazine, given its potential ban across the European Union. Promising alternatives include nitrous oxide-based propellant combinations, with development being led by commercial companies Dawn Aerospace, Impulse Space, and Launcher. The first nitrous oxide-based system ever flown in space was by D-Orbit onboard their ION Satellite Carrier in 2021, using six Dawn Aerospace B20 thrusters. Another alternative is more safe blends of hydrazine with much lower vapor pressure, hence reduced inhalation hazard. Aerojet Rocketdyne has developed the HPB-G28 blend that has 150 times lower vapor pressure, the same specific impulse, and a 35% higher density specific impulse than neat hydrazine. HPB-G28 can be used with same thrusters and catalysts as hydrazine, but has freezing point of , making propellant line heating unnecessary. It contains 65% (by mol) hydrazine, 27% hydroxyethylhydrazinium nitrate (HEHN) and 8% hydrazinium nitrate.
Occupational hazards Health effects
Potential routes of hydrazine exposure include dermal, ocular, inhalation and ingestion.
Hydrazine exposure can cause skin irritation/contact dermatitis and burning, irritation to the eyes/nose/throat, nausea/vomiting, shortness of breath, pulmonary edema, headache, dizziness, central nervous system depression, lethargy, temporary blindness, seizures and coma. Exposure can also cause organ damage to the liver, kidneys and central nervous system. Hydrazine is documented as a strong skin sensitizer with potential for cross-sensitization to hydrazine derivatives following initial exposure. In addition to occupational uses reviewed above, exposure to hydrazine is also possible in small amounts from tobacco smoke.
The official U.S. guidance on hydrazine as a carcinogen is mixed but generally there is recognition of potential cancer-causing effects. The National Institute for Occupational Safety and Health (NIOSH) lists it as a "potential occupational carcinogen". The National Toxicology Program (NTP) finds it is "reasonably anticipated to be a human carcinogen". The American Conference of Governmental Industrial Hygienists (ACGIH) grades hydrazine as "A3—confirmed animal carcinogen with unknown relevance to humans". The U.S. Environmental Protection Agency (EPA) grades it as "B2—a probable human carcinogen based on animal study evidence".