Ammonium dihydrogen phosphate
Ammonium dihydrogen phosphate (NH4H2PO4) is an inorganic salt formed from ammonia and phosphoric acid. It is a white, water-soluble crystalline solid that gives mildly acidic solutions. It is widely used as a fertiliser, in dry-powder fire extinguishers and flame retardants, as a buffer component in laboratory work, and as a source of nonlinear optical crystals. It is supplied in technical, fertiliser, food and analytical reagent grades.
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Ammonium dihydrogen phosphate
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What is ammonium dihydrogen phosphate?
Ammonium dihydrogen phosphate, also called monoammonium phosphate (MAP), has the formula NH4H2PO4 and a molar mass of about 115.03 g/mol (CAS 7722-76-1, EC 231-764-5). It forms colourless to white crystals or powder that dissolve readily in water. Its aqueous solutions are mildly acidic. When heated, it decomposes and releases ammonia.
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Where is it used?
- Agriculture: It is a major fertiliser that supplies both nitrogen and phosphorus.
- Fire safety: It is the active ingredient in many ABC dry-powder extinguishers and is used in flame-retardant treatments.
- Laboratory: It is a component of phosphate buffers and HPLC mobile phases. Ammonium phosphate buffers suit some LC-MS methods better than sodium or potassium phosphate.
- Food and fermentation: It is permitted as a food additive in some jurisdictions (E 342) and is used as a yeast nutrient.
- Optics: Single crystals, known as ADP crystals, have piezoelectric and nonlinear optical properties.
What should buyers check when procuring it?
Specify the grade you need: technical, fertiliser, FCC/food or ACS reagent. Check the CoA for assay, pH of a solution, insoluble matter and heavy-metal limits. GHS classification varies between suppliers, so always review the supplier's SDS. Check REACH or TSCA status for your region. Store the material dry in closed containers, because it is mildly hygroscopic and can release ammonia on decomposition.
Related concepts
Related salts include diammonium hydrogen phosphate ((NH4)2HPO4), which is more alkaline and is also used as a fertiliser, and sodium and potassium dihydrogen phosphate, which are common alternative buffer salts. Phosphoric acid is the parent acid. Phosphate buffer systems and their pKa values explain how this salt behaves in buffers.
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