Ammonium nitrate forms colourless hygroscopic crystals that melt at 169.6 °C. The solid can be in five different polymorphic crystal forms, with transformation temperatures at -16.9 °C, 32.3 °C, 84.2 °C and 125.2 °C. The tendency of ammonium nitrate crystals to cake together is mainly due to the first two phase transformations near room temperature.
The phase transition between the polymorphs IV and III at 32.2 °C is relevant for the handling and storage of the substance. In formulations for fertilizers or explosives, this behavior can lead to undesirable changes in morphology and thus in properties. This phase transition can be suppressed by doping with various salts to obtain so-called phase-stabilized ammonium nitrate. Suitable salts can be various potassium salts, such as potassium fluoride, potassium chloride, potassium nitrate, potassium carbonate, potassium sulphate, potassium rhodanide and potassium dichromate, which must, however, be added in a proportion of 1 to 2 Ma% (mass percent). The effect can also be achieved with a much lower amount of 0.1 Ma% potassium hexacyanidoferrate(II) K4[Fe(CN)6]-3H2O.
Industrial production is carried out by introducing ammonia gas into 40% nitric acid or by combining ammonia solution with nitric acid:
HNO3 + NH3 → NH4NO3
Ammonium nitrate is the main component of many fertilizers (ammonium nitrate-urea solution, compound fertilizers (“blue grain”), calcium ammonium nitrate (Nitramoncal, brand name of Chemie Linz, internally: NAC)).
It is also used for explosives. Ammonium nitrate is contained, for example, in ANC and Donarite explosives, but also in illegally produced explosives such as ANNM.
Ammonium nitrate was also used temporarily as a propellant for airbags in motor vehicles. However, under the influence of high ambient temperature and humidity, it proved to be insufficiently stable over the long term and was replaced by other propellants. In the USA alone, over 40 million vehicles had to be recalled because at least one airbag contained a gas generator containing ammonium nitrate.
Although it is considered to be oxidizing and can explode when heated, ammonium nitrate is not actually an explosive substance within the meaning of the Explosives Act. Nevertheless, the handling of it is regulated in the Federal Republic of Germany by the Explosives Act, and thus ammonium nitrate may now only be used in fertilizers in a mixture with harmless substances such as lime because of its potential danger (KAS27). Typical concentrations used to be 26 and 28 % N (nitrogen content) by Chemie Linz. Today it is 27 % N, which means about 70 % NH4NO3 and the rest lime and some oil against caking of the balls. Higher nitrogen contents of up to 46 % can be achieved with urea, its amide nitrogen is more slowly available.
Ammonium nitrate is the cause of many disasters with serious explosions:
- One of the most devastating of these catastrophes was the explosion of the Oppau ammonia plant at BASF in Ludwigshafen on the Rhine on 21 September 1921, where ammonium sulphate nitrate fertiliser that had solidified there was usually loosened by means of dynamite before being discharged. A change in the production process probably resulted in a local accumulation of ammonium nitrate in the product. The blasts triggered two explosions in quick succession, in which about 400 of a total of 4500 tonnes of fertiliser in a silo detonated, causing one of the largest explosion damages in history: 559 people were killed, injured in 1977 and a large part of the factory and surrounding buildings were destroyed. The explosion could be heard as far away as Munich, 300 kilometres away.
- In the Texas City explosion on April 16, 1947, the two freighters Grandcamp (France) and Highflyer (USA) loaded with ammonium nitrate exploded in the port of Texas City (Texas, USA). There were 500 to 600 dead, over 100 missing, 8000 injured, hundreds homeless and 65 million US dollars in damages.
- On 28 July 1947, the freighter Ocean Liberty (Norway), loaded with ammonium nitrate, exploded in the port of Brest (France). There were 26 dead and over 100 injured.
- Attack in Oklahoma City, USA, by the assassins Nichols/McVeigh on 19 April 1995
- Exactly 80 years after the explosion at the Oppau ammonia plant (see above), 31 people died on 21 September 2001 in an ammonium nitrate explosion at the AZF fertiliser factory in Toulouse, France, and there were also thousands injured and huge material damage.
- In the railway accident at Ryongchŏn on 22 April 2004, an ammonium nitrate-loaded train wagon exploded in Ryongchŏn in North Korea, killing at least 161 people, injuring an estimated 1300 and destroying or damaging 8000 homes.
- Attacks in Norway 2011 by the assassin Anders Behring Breivik on July 22, 2011
- At least 14 people died and 180 others were injured in a fire and a violent explosion at the West Fertilizer Company on April 17, 2013
- On August 12, 2015, hundreds of people were killed or injured in an explosion in Tianjin, China. According to the British newspaper The Guardian, 800 tons of ammonium nitrate were stored on site, among many other substances. This amount is a good explanation for the crater, about 100 m wide, left by the explosion. It is not yet known which substance was the main cause of the destruction.