Triamidodiphosphoric Acid, (NH₂)₃P₂O₃(OH)

Triamidodiphosphoric Acid, (NH₂)₃P₂O₃(OH), was prepared by similar reactions. It formed a series of salts in which it was monobasic. But in addition to the white monosilver salt, (NH₂)₃P₂O₃(OAg), an orange-coloured trisilver salt was prepared from ammoniacal silver nitrate, to which the constitution NH₂(NHAg)₂P₂O₃(OAg) was assigned.

Amido- and imido- derivatives of condensed phosphoric acid of still higher molecular weight have been prepared in great variety by Gladstone, and by Stokes. For example, monoimidotetramido- tetraphosphoric acid, NH=P₄O₇(NH₂)₄, has been obtained by heating the product of general reaction (b) to about 200° C., while tetramidotetraphosphoric acid, (HO)₂P₄O₇(NH₂)₄, was obtained by hydrolysing ammonium diamidotetraphosphate either with acids or with alkalies, thus

(NH₂)₂P₄O₇(OH)(ONH₄)₃ + HCl = (NH₂)₄P₄O₇(OH)₂ + NH₄Cl + 2H₂O

The structures assigned were

and

Many other derivatives have been obtained. Their general properties have already been indicated.

The imidodiphosphoric acids are isomeric with amidodiphosphoric acids. Monimidodiphosphoric acid,

HN={PO(OH)₂}₂, or

was prepared by heating trimetaphosphimic acid, dissolving in aqueous ammonia, and adding a salt of magnesium, which precipitated magnesium ortho- and pyro-phosphates, leaving a soluble magnesium salt which on treatment with ammoniacal silver nitrate gave a crystalline precipitate of NH=P₂O₂(OAg)₃OH. This, when treated with sodium chloride, gave a soluble non-crystallisable trisodium salt. A tetrasilver salt has also been prepared in two modifications—as a voluminous white precipitate which on boiling passes into a yellow form. These may have the imido- and the amido-structures respectively.

A dibasic imidodiphosphoric acid, which can be regarded as derived from the last-mentioned compound by the loss of the elements of water, was prepared by warming to about 50° C. a solution of 4 grams ammonium carbamate in 10 grams of phosphoryl chloride. Thus

3NH₂COONH₄ + 4POCl₃ = 2NH(POCl₂)₂ + 3CO₂ + 4NH₄Cl
NH(POCl₂)₂ + 3H₂O = NH(POOH)₂O + 4HCl

The barium or ferric salt may be precipitated, and from the former the free acid may be regenerated. Other derivatives were prepared by similar reactions.

These compounds, in which trivalent nitrogen bridges phosphoryl radicals, are obtained by heating amido-compounds, with loss of ammonia.

The potassium salt of nitrilodiphosphoric acid,



was obtained by heating potassium triamidodiphosphate, the ammonium salt by heating triamidodiphosphoric acid, and the silver salt by interaction between silver nitrate and a suspension of the sparingly soluble potassium salt in water.