Monamidophosphoric Acid, NH₂PO(OH)₂

Monamidophosphoric Acid, NH₂PO(OH)₂, has been obtained by several reactions, among which are the hydrolysis of diphenyl amidophosphate by means of alkali, thus

NH₂PO(OC₆H₅)₂ + 2NaOH = NH₂PO(OH)₂ + 2C₆H₅ONa

and the action of nitrous acid on diamidophosphoric acid. The sodium salt (which results in the former method) may be converted into the lead salt, which may then be decomposed by hydrogen sulphide at 0° C. The acid is precipitated from the filtrate by alcohol in the form of tabular or cubic crystals. It is very soluble in water, has a sweetish taste, and is hydrolysed after long standing at room temperature or rapidly in hot solution, into ammonium dihydrogen phosphate, thus

NH₂PO(OH)₂ + H₂O = NH₄OPO(OH)₂

Ammonium and hydroxylamine salts of this acid furnish interesting examples of isomerism. Thus, ammonium hydrogen amidophosphate, NH₂PO(OH)ONH₄ (prepared by double decomposition between the silver salt and ammonium sulphide, with subsequent precipitation with alcohol as needle-shaped crystals), is isomeric with hydrazine phosphite, N₂H₄.H₃PO₃, while hydroxylamine amidophosphate, NH₂PO(OH)(ONH₃OH), is isomeric with hydrazine phosphate, N₂H₄.H₃PO₄.

The mono- and di-sodium salts and the corresponding potassium salts have also been prepared. The salt NH₂PO(OH)(OK) was obtained by hydrolysing diphenylamidophosphate with a boiling solution of KOH, acidifying the cold solution with acetic acid and washing the precipitate with alcohol. It forms rhombohedral crystals which are very soluble in water. The solution is neutral and is gradually hydrolysed on standing. Lithium, silver and lead amidophosphates were precipitated by adding salts of these metals to alkali amidophosphates.