Ammonium dihydrogen phosphate

Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP)^[5] is a chemical compound with the chemical formula (NH₄)(H₂PO₄). ADP is a major ingredient of agricultural fertilizers^[6] and dry chemical fire extinguishers. It also has significant uses in optics^[7] and electronics.^[8]

Ammonium dihydrogen phosphate^[1]

Names
IUPAC name Ammonium dihydrogen phosphate
Other names Monoammonium phosphate
Identifiers
CAS Number	7722-76-1 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:62982
ChemSpider	22812 Y
ECHA InfoCard	100.028.877
EC Number	231-764-5
E number	E342(i) (antioxidants, ...)
PubChem CID	24402
UNII	09254QB17T
CompTox Dashboard (EPA)	DTXSID5029689
InChI InChI=1S/H3N.H3O4P/c;1-5(2,3)4/h1H3;(H3,1,2,3,4) Y Key: LFVGISIMTYGQHF-UHFFFAOYSA-N Y InChI=1/H3N.H3O4P/c;1-5(2,3)4/h1H3;(H3,1,2,3,4) Key: LFVGISIMTYGQHF-UHFFFAOYAX
SMILES [O-]P(=O)(O)O.[NH4+]
Properties
Chemical formula	H6NO4P
Molar mass	115.025 g·mol−1
Appearance	white crystals
Odor	none
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) [2][3]
Solubility	insoluble in ethanol[2] insoluble in acetone
Refractive index (nD)	1.525
Structure
Crystal structure	tetragonal
Thermochemistry
Std enthalpy of formation (ΔfH⦵298)	−1445.07 kJ/mol[4]
Hazards
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H319
Precautionary statements	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)	1 0 0
Lethal dose or concentration (LD, LC):
LD50 (median dose)	5750 mg/kg (rat, oral)
Related compounds
Other anions	Ammonium phosphate Diammonium phosphate
Other cations	Monosodium phosphate Potassium dihydrogen phosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Chemical properties

Monoammonium phosphate is soluble in water and crystallizes from it as the anhydrous salt in the tetragonal system, as elongated prisms or needles.^[7] It is practically insoluble in ethanol.^[2]

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
₃PO
₄.^[9] At 125 °C the partial pressure of ammonia is 0.05 mm Hg.^[10]

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

Preparation

Monoammonium phosphate is industrially prepared by the exothermic reaction of phosphoric acid and ammonia in the correct proportions:^[12]

NH
₃ + H
₃PO
₄ → NH
₄H
₂PO
₄

Crystalline MAP then precipitates.

Uses

Agriculture

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
₂O
₅, or 27% of elemental phosphorus.

Fire extinguishers

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

Optics

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 n_o = 1.522 and n_e = 1.478 at optical wavelengths.^[7]

Electronics

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.^[8]

Toys

Being relatively non-toxic^{[citation needed]}, MAP is also a popular substance for recreational crystal growing, being sold as toy kits mixed with dyes of various colors.

Natural occurrence

The compound appears in nature as the rare mineral biphosphammite. It is formed in guano deposits.^[13][14] A related compound, that is the monohydrogen counterpart, is the even more scarce phosphammite.^[15][16]