Ammonium Dihydrogen Phosphate

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 [7] and electronics .

chemical properties

Monoammonium phosphate is soluble in water and crystallizes as an anhydrous salt in a tetragonal system in the form of elongated prisms or needles. ^[7] 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. ^[10]

A solution of stoichiometric monoammonium phosphate is acidic (pH 4.7 at 0.1% concentration, 4.2 at 5%). ^{[11 1]}

Preparation

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.

Use

agriculture

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.

Fire Extinguisher

The compound is also a component of ABC powder in some dry chemical fire extinguishers.

optics

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 .

Electronics

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. ^[8]

toys

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.

natural phenomena

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 ^[15] https://www.ima-mineraology.org/Minlist.htm