Ammonia

• 1. Introduction

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  In 2021, factories around the world produced 235 million tonnes of ammonia. That is an enormous weight for a gas best known for its sharp, pungent smell. Ammonia is a compound of nitrogen and hydrogen, yet its reach extends far beyond the chemistry laboratory. Animals work to expel it from their bodies, farmers spread it across fields, and astronomers have found it drifting between the stars. Where did its name come from, and who first captured the gas? How did a compound pulled from the air change farming? Why does the shape of its molecule matter so much? Can it fuel the ships of the future? And why is a substance that living things need so dangerous to fish?

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• 2. Name and Discovery

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  The Temple of Jupiter Amun, in the Roman province of Cyrenaica, gave ammonia its name. Pliny, in Book XXXI of his Natural History, describes a salt called hammoniacum, named for its source near that temple. Amun was the Egyptian deity whom the Greeks called Ammon. Priests and travellers at such temples were said to burn soils rich in ammonium chloride, which came from animal dung and urine.

  Pliny's description, however, does not match the properties of ammonium chloride. Herbert Hoover, in his commentary on an English translation of Georgius Agricola's De re metallica, judged it likely to have been common sea salt. Whatever the salt really was, it passed its name to ammonia and every ammonium compound. Anything containing ammonia, or resembling it, is called ammoniacal.

  Herodotus wrote of salt outcrops in a part of Libya inhabited by a people called the Ammonians. That area is now the Siwa oasis in northwestern Egypt, where salt lakes still exist. Strabo also mentioned the salt of this region. Yet Dioscorides, Apicius, Arrian, Synesius and Aëtius of Amida all described clear crystals used in cooking, which sounds like plain rock salt.

  Fermented urine, rich in ammonia thanks to bacteria, was a household tool in Classical Antiquity. It washed cloth and clothing, stripped hair from hides before tanning, and worked as a mordant in dyeing cloth. It also removed rust from iron, and ancient dentists used it to wash teeth.

  Sal ammoniac, called nushadir in Arabic, mattered to Muslim alchemists. The Book of Stones, likely written in the 9th century and attributed to Jābir ibn Hayyān, mentions it. European alchemists of the 13th century knew it too, and Albertus Magnus wrote about it. Dyers in the Middle Ages used fermented urine to shift the colour of vegetable dyes.

  In the 15th century, Basilius Valentinus showed that treating sal ammoniac with alkalis releases ammonia. Later, sal ammoniac was made by distilling the hooves and horns of oxen and neutralising the resulting carbonate with hydrochloric acid. That route earned ammonia the name spirit of hartshorn.

  Joseph Black first isolated the gas in 1756, by reacting ammonium chloride with calcined magnesia, a form of magnesium oxide. Peter Woulfe isolated it again in 1767, Carl Wilhelm Scheele in 1770, and Joseph Priestley in 1773. Priestley called it alkaline air. In 1785, Claude Louis Berthollet worked out its composition. The next challenge was to make ammonia from the nitrogen in the air, and a patent for that was sent in 1909.

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• 3. Fertiliser From Air

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  Fritz Haber and Robert LeRossignol invented a way to make ammonia from nitrogen in the air together with hydrogen. Their patent, number 1,202,995 at the United States patent office, was sent in 1909 and awarded in 1916. Carl Bosch later developed the industrial method, known today as the Haber-Bosch process.

  Germany first used it on an industrial scale during World War I. An allied blockade had cut off the supply of nitrates from Chile. The ammonia was turned into explosives that helped sustain the war effort. In 1918, the Nobel Prize in Chemistry went to Haber "for the synthesis of ammonia from its elements".

  Before the war began, most ammonia came from the dry distillation of nitrogenous vegetable and animal waste, camel dung among it. Distilling coal also yielded some, as did breaking down ammonium salts with alkaline hydroxides such as quicklime. The hydrogen needed for synthesis came from the electrolysis of water or the chloralkali process, until natural gas became available. In the 20th century, the steel industry made ammonia a byproduct of coking coal.

  Around 70% of industrial ammonia now becomes fertiliser, in forms such as urea and diammonium phosphate. In the United States the share is about 88%, and 30% of the agricultural nitrogen applied there is anhydrous ammonia. Worldwide, 110 million tonnes are applied to land every year, lifting yields of crops such as maize and wheat. The cheap fertiliser from the Haber process transformed agriculture, and African countries now hope to make renewable ammonia to depend less on imported fertiliser.

  A rival route using lithium was first reported in 1930, with lithium solutions in ethanol at pressures up to 1000 bar. In 1994, Tsuneto and colleagues used lithium electrodeposition in tetrahydrofuran at more moderate pressures. In 2020, a gas diffusion electrode reached a Faradaic efficiency of up to 47.5%, a measure of how much electric current goes into making ammonia. That ran at ambient temperature and 1 bar.

  In 2021, a tetraalkyl phosphonium salt replaced ethanol and pushed the figure to 69%. In 2022, a continuous-flow electrolyzer that used hydrogen gas as its proton source reached 61% at room temperature. Technology built on this chemistry is being developed for commercial fertiliser and for fuel.

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• 4. Shape and Behaviour

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  A bond angle of 106.7 degrees defines the ammonia molecule. A regular tetrahedron would give 109.5 degrees. The central nitrogen atom ends up with four electron pairs, three shared with hydrogen atoms and one lone pair. The lone pair repels more strongly than the bonding pairs and squeezes the angle smaller. The molecule becomes a trigonal pyramid, which is polar. Along with its talent for hydrogen bonding, that polarity makes ammonia highly miscible with water.

  At room temperature, the pyramid constantly flips through nitrogen inversion, like an umbrella turning itself inside out in a strong wind. The energy barrier is 24.7 kJ/mol, and the resonance frequency is 23.79 GHz, corresponding to microwaves with a wavelength of 1.260 cm. That absorption was the first microwave spectrum ever observed, and it was used in the first maser.

  The gas is lighter than air, with a density 0.589 times that of air. Strong hydrogen bonding between molecules makes it easy to liquefy. The liquid boils at minus 33.1 degrees Celsius and freezes into crystals at minus 77.7 degrees Celsius. Its critical point sits at 405 K and 11.35 MPa.

  Liquid ammonia is a widely studied solvent, and its most conspicuous trick is dissolving alkali metals. At concentrations below 0.06 mol/L the solutions are deep blue, holding metal cations and solvated electrons. Those are free electrons surrounded by a cage of ammonia molecules. Such solutions are strong reducing agents. At higher concentrations they look metallic and conduct like metals, and at low temperatures the two kinds can coexist as separate phases.

  A 1.0 M water solution of ammonia has a pH of 11.6, making ammonia a weak base. The saturated solution has a specific gravity of 0.880 and is often called .880 ammonia. In a classic demonstration, open bottles of concentrated ammonia and hydrochloric acid solutions send out vapours that meet between them. A cloud of ammonium chloride seems to appear out of nothing.

  Combustion is harder than it sounds. Ammonia burns only in narrow mixtures of 15-28% ammonia by volume in air, and in oxygen it gives a pale yellowish-green flame. Recent studies found that swirl combustors can burn it efficiently and stably, which rekindled interest in ammonia as a fuel for power.

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• 5. The Everyday Workhorse

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  Hockey rinks owe their ice to ammonia. Anhydrous ammonia is widely used in industrial refrigeration because of its high energy efficiency and low cost. Toxicity and the need for corrosion resistant parts keep it out of homes and small systems. It was commonly used before chlorofluorocarbons, known as Freons, became popular. Interest has grown again since vented CFCs and HFCs proved to be potent and stable greenhouse gases.

  On the International Space Station, ammonia circulates through radiator loops that regulate internal temperature. The pressure keeps it liquid throughout. Single-phase ammonia cooling systems also serve the power electronics in each pair of solar arrays.

  Goldsworthy Gurney in the UK tried ammonia engines experimentally in the 19th century. The St. Charles Streetcar Line in New Orleans used them in the 1870s and 1880s. During World War II, ammonia powered buses in Belgium. The Reaction Motors XLR99 rocket engine, which drove the X-15 hypersonic research aircraft, ran on liquid ammonia. It was less powerful than other fuels but left no soot in the reusable engine. Its density also roughly matches that of liquid oxygen, which simplified the aircraft's design.

  By 1895, ammonia was known to be strongly antiseptic. It takes 1.4 grams per litre to preserve beef tea, a broth. Anhydrous ammonia is used commercially today to reduce microbial contamination of beef. Lean finely textured beef, popularly called pink slime, starts as fatty trimmings of about 50-70% fat. Heat and centrifugation remove the fat, and ammonia then kills E. coli. The US Department of Agriculture judged the process effective and safe, but consumers complained about taste and smell.

  Smelling salts release ammonia vapour, which acts as a respiratory stimulant. Woodworkers have used its fumes to darken quartersawn white oak in Arts & Crafts and Mission-style furniture, because the fumes react with the natural tannins in the wood. Windex contained an ammonia solution until 2006. Ammonia has even filled balloons, since it offers about 45-48% of the lifting power of hydrogen or helium.

  Selective catalytic reduction, or SCR, uses ammonia with a vanadia-based catalyst to neutralise nitrogen oxide pollutants from diesel engines. Ammonia also scrubs emissions from burning fossil fuels, and the product becomes ammonium sulfate fertiliser.

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• 6. Fuel for Tomorrow

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  Making ammonia currently creates 1.8% of global emissions. The label green ammonia means it was made from green hydrogen, produced by electrolysis with electricity from renewable sources. Blue ammonia comes from blue hydrogen, made by steam methane reforming with the carbon dioxide captured and stored.

  In 2020, Saudi Arabia shipped 40 metric tons of liquid blue ammonia to Japan for use as a fuel, a world first. It was a by-product of petrochemical industries. Its energy density by volume is nearly double that of liquid hydrogen. Ammonia stays liquid at ambient temperature under its own vapour pressure, so it is easier to store than hydrogen. Its raw energy density of 11.5 MJ/L is about a third that of diesel.

  The International Energy Agency, or IEA, forecasts that ammonia will meet approximately 45% of shipping fuel demand by 2050. In 2020, DSME and MAN Energy Solutions announced the construction of an ammonia-based ship, and DSME planned to commercialise it by 2025. Researchers are also exploring ammonia for aircraft jet engines. Japan has a plan for ammonia co-firing in power generation, and in October 2021 the first International Conference on Fuel Ammonia, ICFA2021, was held.

  In June 2022, IHI Corporation burned liquid ammonia in a 2,000-kilowatt-class gas turbine and cut greenhouse gases by over 99% during combustion. In July 2022, the Quad nations of Japan, the US, Australia and India agreed at their first energy meeting to promote clean-burning hydrogen and ammonia fuels. ACWA Power and the city of Neom announced a green hydrogen and ammonia plant in 2020.

  Burning ammonia carries a catch, though. Nitrous oxide may also be a problem, since this greenhouse gas is known to possess up to 300 times the global warming potential of carbon dioxide.

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• 7. Toxic and Essential

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  Fish and amphibians have no internal system for detoxifying ammonia. They normally release it straight into the surrounding water. At around 0.05 mg/L, un-ionised ammonia harms fish, causing poor growth, reduced fertility and greater vulnerability to bacterial disease. Above 2.0 mg/L it causes gill and tissue damage, convulsions, coma and death. Lethal concentrations for various species range from 0.2 to 2.0 mg/L.

  Ammonia toxicity is believed to explain otherwise mysterious losses in fish hatcheries. In winter, reduced feeding and cooler water slow algal photosynthesis, so algae remove less ammonia. At scale there is no fast-acting remedy, so prevention is recommended over correction.

  Mammals avoid the problem. In humans, the liver converts ammonia to urea through the urea cycle, starting with the enzyme carbamoyl phosphate synthetase. Cirrhosis or defects in urea cycle enzymes can raise blood ammonia, a condition called hyperammonemia. It contributes to the confusion and coma of hepatic encephalopathy. Birds, reptiles and terrestrial snails convert it to uric acid, a solid that can be excreted with minimal water loss.

  Ammonia is nonetheless required for building amino acids and nucleic acids. Nitrogen makes up more than 75% of the atmosphere, but few creatures can use it directly. Enzymes called nitrogenases make ammonia from atmospheric nitrogen in certain organisms. Legumes team up with rhizobia bacteria that do this for them. Inside cells, glutamate carries ammonia safely, and in the bloodstream glutamine serves as its source.

  The US Occupational Safety and Health Administration sets a 15-minute limit of 35 ppm for gaseous ammonia and an 8-hour limit of 25 ppm. The immediately dangerous level was cut from 500 to 300 ppm. The average odour threshold is 5 ppm, so the smell warns well before harm. Repeated exposure dulls the nose, however, and desensitised people may miss it even at 100 ppm.

  The Whyalla Steelworks in South Australia makes coke from coal and discharges ammonia from its wastewater into marine waters, where it acts as a pollutant.

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• 8. Ammonia Beyond Earth

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  Agriculture supplies around 80% of the ammonia entering Earth's atmosphere, through fertilisers, livestock urea and manure, in that order. Forest fires and burning of agricultural waste add roughly equal amounts. Rain is the dominant sink, and in soil, ammonia-oxidizing archaea and bacteria convert it into nitrite. Those processes limit its atmospheric lifetime to 12 to 48 hours.

  With sulfuric and nitric acid, ammonia forms sulfate-nitrate aerosols, with ammonium sulfate prominent. These aerosols affect climate and human health. The hydroxyl radical can also oxidise ammonia, on a timescale as short as 30 days, producing greenhouse gases such as nitrous oxide. The oxidation can also generate nitric oxide, raising NOx levels in rural areas.

  Pluto shows what ammonia can do in the cold. Ammonia exists across the Solar System, on Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. On small icy bodies, a water and ammonia mixture acts as an antifreeze. It lets them keep internal oceans and active geology at temperatures far below what water alone would allow. The interior of Saturn may include frozen ammonia crystals.

  In 1968, ammonia became the first polyatomic molecule detected in interstellar space. The signal was microwave emission from the direction of the galactic core. Astronomers soon used it as a thermometer, because the relative intensity of its spectral lines reveals the temperature of the emitting gas. Triply deuterated ammonia was a surprise, since deuterium is scarce. Low temperatures are thought to let that molecule survive and accumulate.

  A survey of 149 dark cloud positions found that cores containing stars have broader lines than those without. Dark clouds sit near 10 K, with typical cores about 0.1 pc across and masses near 1 solar mass. These cold, dense cores are the sites of future star formation. In the galaxy IC 342, ammonia line ratios revealed hot gas above 70 K, closely tied to the innermost part of the nuclear bar.

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