Ammonium dihydrogen phosphate
3D model (JSmol)
|E number||E342(i) (antioxidants, ...)|
CompTox Dashboard (EPA)
|Molar mass||115.025 g·mol−1|
|Melting point||190 °C (374 °F; 463 K)|
|(g/dL) 28 (10 °C)|
36 (20 °C)
44 (30 °C)
56 (40 °C)
66 (50 °C)
81 (60 °C)
99 (70 °C)
118 (80 °C)
173 (100 °C) 
|Solubility||insoluble in ethanol |
insoluble in acetone
Refractive index (nD)
Std enthalpy of
|P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|5750 mg/kg (rat, oral)|
Potassium dihydrogen phosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP) is a chemical compound with the chemical formula (NH4)(H2PO4). ADP is a major ingredient of agricultural fertilizers and some fire extinguishers. It also has significant uses in optics and electronics.
Solid monoammonium phosphate can be considered stable in practice for temperatures up to 200 °C, when it decomposes into gaseous ammonia NH
3 and molten phosphoric acid H
4. At 125 °C the partial pressure of ammonia is 0.05 mm Hg.
3 + H
4 → NH
Crystalline MAP then precipitates.
The largest use of monoammonium phosphate by weight is in agriculture, as an ingredient of fertilizers. It supplies soil with the elements nitrogen and phosphorus in a form usable by plants. Its NPK label is 12-61-0 (12-27-0), meaning that it contains 12% by weight of elemental nitrogen and (nominally) 61% of phosphorus pentoxide P
5, or 27% of elemental phosphorus.
Monoammonium phosphate is a widely used crystal in the field of optics due to its birefringence properties. As a result of its tetragonal crystal structure, this material has negative uniaxial optical symmetry with typical refractive indices no = 1.522 and ne = 1.478 at optical wavelengths.
Monoammonium phosphate crystals are piezoelectric, a property required in some active sonar transducers (the alternative being transducers that use magnetostriction). In the 1950s ADP crystals largely replaced the quartz and Rochelle salt crystals in transducers because they are easier to work than quartz and, unlike Rochelle salt, are not deliquescent.
Being relatively non-toxic, MAP is also a popular substance for recreational crystal growing, being sold as toy kits mixed with dyes of various colors.
The compound appears in nature as the rare mineral biphosphammite. It is formed in guano deposits. A related compound, that is the monohydrogen counterpart, is the even more scarce phosphammite
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