Ammonium nitrate

Ammonium nitrate: Hi, Has anyone else had any trouble with ammonium nitrate absorbing water? Mine has become a clear mixture of liquid and crystals. The crystals are a solid lump that needs to be prodded to separate. It was suggested by one of my teachers to place it in the oven to dry it out. I'm not in favour of doing this procedure, and so I won't be taking his advice.

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Publication Date: 31 July 2015
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Ammonium nitrate

Before handling a chemical, the first port of call must be to consult the Safety Data Sheets (SDSs), which contain important safety information. The following is information about ammonium nitrate taken from a Safety Data Sheet.

  • Ammonium nitrate is hygroscopic and absorbs water from the atmosphere. If kept at high humidity, ammonium nitrate can dissolve in the water it has absorbed due to its deliquescent nature. Therefore it should be stored in a cool dry place. It could be stored in a desiccator to prevent it absorbing moisture from the surrounding air and coalescing into a large solid lump.
  • Ammonium nitrate is also a powerful oxidiser, so it should also be kept away from heat, hot surfaces, open flames, combustible materials and other sources of ignition.
  • Ammonium nitrate is not combustible, however, being a strong oxidant, it can intensify the development of a fire involving combustible materials such as rags, wooden articles, clothing and plastics. It can also cause skin corrosion.
  • When ammonium nitrate is heated, it decomposes to give off toxic gases. In an open, well-ventilated space, it decomposes completely to give nitrous oxide, ammonia and nitric acid with white fumes and vapours. If heated in confinement (e.g. in an oven), or in the presence of contaminants or both, ammonium nitrate will decompose to give brown toxic fumes of nitrogen dioxide.
  • In extraordinary circumstances, ammonium nitrate can detonate as an explosive. Factors such as contamination or confinement can increase the explosive sensitivity of ammonium nitrate. Thus, ammonium nitrate is widely used in the manufacture of explosives.
  • Ammonium nitrate is also known as Security Sensitive Ammonium nitrate (SSAN) and is therefore subject to state government legislation. Under this legislation, it is only permitted in schools in quantities up to 3kg and in some jurisdictions is subject to certain paperwork requirements, such as applying for an exemption and/or ordering using an End User Declaration (EUD).

When ammonium nitrate has absorbed moisture, it is very challenging to dry it out.

Science ASSIST strongly advises not to heat ammonium nitrate under any circumstances because of the hazards of decomposition or explosion, which could be triggered by contamination and other factors.

Recommendations

Science ASSIST recommends that you do one of the following with your deliquesced ammonium nitrate.

  • Remove the excess solution from the salt and greatly dilute the solution so that it can be spread on the garden as a fertiliser. Weigh the remaining mass and treat it as the dry salt in making up a solution. E.g. approximately 2M or 4M depending upon the amount. This can always then be diluted further if required.
  • Dissolve most of it in a limited amount of distilled water at 20° C to form a saturated solution. Given that ammonium nitrate has a solubility of 150 g/100 mL at 20o C, a saturated solution can be calculated to be approximately 18.7 M. Keep this as a saturated solution and use for the purposes of a salt bridge, or dilute as required.

Science ASSIST also recommends

  • Safe handling: Ammonium nitrate should only be used in a well-ventilated area and proper PPE such as protective clothing, nitrile rubber gloves and safety glasses should be worn.
  • Alternative chemical: In jurisdictions where ammonium nitrate is not permitted, or to avoid the difficulties of storage, consider alternatives such as ammonium salts and nitrate salts that can be used instead of ammonium nitrate.

References

Cagnina Stefania, Patricia Rotureau and Carlo Adamoa. (2013). ‘Study of Incompatibility of Ammonium Nitrate and its Mechanism of Decomposition by Theoretical Approach’ Chemical Engineering Transactions. Vol 31. http://www.aidic.it/cet/13/31/138.pdf (accessed November 2016)

Department of Mines and Petroleum, (2013), Safe storage of solid ammonium nitrate – code of practice (3rd edition): Resources Safety, Department of Mines and Petroleum, Western Australia. http://www.dmp.wa.gov.au/documents/Code_of_Practice/DGS_COP_StorageSolidAmmoniumNitrate.pdf

‘Material Safety Data Sheet: ammonium nitrate’, (30 April 2014) Merck Millipore website. http://www.merckmillipore.com/INTERSHOP/web/WFS/Merck-AU-Site/en_US/-/USD/ProcessMSDS-Start?PlainSKU=MDA_CHEM-101188&Origin=PDP (Link updated, July 2019).

‘Material Safety Data Sheet: ammonium nitrate’, (2 December 2014) Fisher Scientific Website https://www.fishersci.com/msds?productName=A676212

‘Material Safety Data Sheet: ammonium nitrate solution’, (1 February 2013) Calamco Website http://calamco.com/pdf/ammonium_nitrate_msds.pdf

Ammonium nitrate

There was a NSW directive to dispose of ammonium nitrate in excess of 3kg in 2006 by using it as a fertiliser on school grounds. You should not subject it to any heat. 

You can easily substitute for the cation and anion, however, if using for endothermic reactions, use sodium nitrate instead.

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