Di Ammonium hydrogen phosphate; chemical formula (NH4)2(HPO4) is one of a series of water-soluble ammonium phosphate salts that can be produced when ammonia reacts with phosphoric acid.Solid diammonium phosphate shows a dissociation pressure of ammonia as given by the following expression and equation: (NH4)2HPO4(s) NH3(g) + (NH4) H2PO4(s) At 100 �°C, the dissociation pressure of diammonium phosphate is approximately 5 mmHg.