Ammonium dihydrogen phosphate: a multifunctional inorganic compound and its chemical properties

Ammonium dihydrogen phosphate is a colourless or white crystals, used as buffering agent, dough conditioner, leavening agent.

Chemical properties

Ammonium dihydrogen phosphate is soluble in water and crystallizes from it as the anhydrous salt in the tetragonal system, as elongated prisms or needles. It is practically insoluble in ethanol.

Solid ammonium dihydrogen phosphate can be considered stable in practice for temperatures up to 200 °C, when it decomposes into gaseous ammonia NH3 and molten phosphoric acid H3PO4. At 125 °C the partial pressure of ammonia is 0.05 mm Hg.¹

Applications

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 P2O5, or 27% of elemental phosphorus.

Fire extinguishers

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

Method of assay

Dissolve about 500 mg of the sample, accurately weighed, in 50 ml of water, and titrate to a pH of 8.0 with 0.1 N sodium hydroxide. Each ml of 0.1 N sodium hydroxide is equivalent to 11.50 mg of Ammonium dihydrogen phosphate.

1.Dejun Xu, Xing Xiong, Lin Yang, Zhiye Zhang, and Xinlong Wang (2016): "Determination of the Solubility of Ammonium Dihydrogen Phosphate in Water-Ethanol System at Different Temperatures from 283.2 to 343.2 K".Journal of Chemincal Engineering Data, volume 61, issue 1, pages 78–82.