Ammonium dihydrogen phosphate ( ADP ), also known as monoammonium phosphate ( MAP ), is a chemical compound with the chemical formula (NH4 ) ( H2PO4 ) . ADP is a major component of agricultural fertilizers and some fire extinguishers . It also has important uses in optics  and electronics .
|chemical formula||H 6 N O 4 P|
|molar mass||115.025 g·mol −1|
|density||1.80 g / cm3|
|Melting point||190 °C (374 °F; 463 K)|
|Solubility in water||(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 )||1.525|
Monoammonium phosphate is soluble in water and crystallizes as an anhydrous salt in a tetragonal system in the form of elongated prisms or needles.  It is practically insoluble in ethanol.
Solid monoammonium phosphate can be considered stable in practice for temperatures up to 200 °C, when it decomposes into the gaseous ammonia NH3 and molten phosphoric acid H3PO4 The partial pressure of ammonia at 125 °C is 0.05 mm Hg. 
A solution of stoichiometric monoammonium phosphate is acidic (pH 4.7 at 0.1% concentration, 4.2 at 5%). [11 1]
Monoammonium phosphate is prepared industrially by the exothermic reaction of phosphoric acid and ammonia in the correct ratio:
NH3 + H3PO4 → NH6PO4
The crystalline MAP then precipitates.
The largest use of monoammonium phosphate by weight is in agriculture, as an ingredient in fertilizers. It supplies the soil with nitrogen and phosphorus elements 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 61% of (nominal) phosphorus pentoxide P2O5, or 27% elemental phosphorus.
The compound is also a component of ABC powder in some dry chemical fire extinguishers.
Monoammonium phosphate is a widely used crystal in the field of optics due to its divalent properties. As a result of its quaternary crystal structure, this material has negative axial optical symmetry with specific refractive index n o = 1.522 and n e = 1.478 at optical wavelengths .
Monoammonium phosphate crystals are piezoelectric, a property essential in some active sonar transducers (alternative transducers that use magnetostriction). In the 1950s ADP crystals replaced quartz and Rochelle salt crystals in transducers largely because they are easier to work with than quartz and, unlike Rochelle salt, are not fragile. 
Being relatively non-toxic [ citation needed ] , MAP is also a popular substance for growing recreational crystals, being sold as a mixed toy kit with a variety of paint dyes.
The compound appears in nature as the rare mineral biphosphite. It is formed in guano deposits. A related compound, the monohydrogen equivalent, is the even rarer phosphamite  https://www.ima-mineraology.org/Minlist.htm