42 AMMONIUM PHOSPHATE, DIBASIC

vapor and oxygen—two mols solid producing seven mols of gaseous products. This can cause a dangerous explosion. In the presence of readily oxidizable substances, such as fuel oil soaked into the pores of the solid or finely divided metal, the ignition is self-sustained—occurring at lower temperatures, and the explosivity is enhanced. Also, it can explode dangerously in a fire. At ordinary temperatures, the compound is stable and safe to handle. Calcium carbonate, phosphate or other substances are mixed with fertilizer grade ammonium nitrate to reduce its explosivity. There are many cases of loss of human lives from ammonium nitrate fire or explosion.

AMMONIUM PHOSPHATE, DIBASIC

[7783–28–0]

Formula: (NH4)2HPO4; MW 132.07;

Synonyms: diammonium hydrogen phosphate; secondary ammonium phosphate

Uses

Dibasic ammonium phosphate is used as a fertilizer; as a fire extinguisher; a flame retardant for plywood, papers, and fabrics; to prevent afterglow in matches; in purifying sugar; as a flux for soldering tin, copper, zinc and brass; and to control precipitation of alkali-soluble or acid-insoluble colloidal dyes on wool.

Physical Properties

Colorless monoclinic crystal; saline taste; refractive index 1.52; density 1.619 g/cm3; melts at 155°C (decomposes); very soluble in water (57 g/100 g at 10°C and 106.7g/100g at 70°C, respectively); insoluble in alcohol, acetone, and liquid ammonia.

Preparation

(NH4)2HPO4 is made by reacting ammonia with phosphoric acid:

2NH3 + H3PO4 → (NH4)2HPO4

Reactions

Heating at 70°C results in decomposition to monoammonium phosphate and ammonia:

(NH4)2 HPO4 → (NH4)H2PO4 + NH3

A boiling, dilute solution of diammonium phosphate evolves ammonia (the pH of the solution decreases), which also occurs slowly at ambient temperatures. The solid and its solution create an ammonia vapor pressure. Reactions with mineral acids produce the corresponding ammonium salts.