Meta- and Pyro-phosphorous Acids

The acids HPO₂ and H₄P₂O₅ may be regarded as partly dehydrated forms of orthophosphorous acid. In their modes of preparation and properties they recall somewhat the meta- and pyro-phosphoric acids.

Metaphosphorous Acid

Metaphosphorous Acid was produced during the spontaneous combustion of dry phosphine in dry oxygen at a low pressure, probably according to the equation

PH₃ + O₂ = H₂ + HPO₂

It was deposited on the walls of the vessel in feathery crystals, which were readily hydrated by water giving H₃PO₃.

Pyrophosphorous Acid

Pyrophosphorous Acid has been made by treating PCl₃ with a little water, or by shaking the syrupy acid with a slight excess of PCl₃ and evaporating in a desiccator over KOH and P₂O₅. The crystalline acid was very deliquescent. Secondary phosphites when heated gave pyrophosphites with loss of water, thus

2BaHPO₃ = Ba₂P₂O₅ + H₂O

By the addition of the theoretical quantity of sulphuric acid the unstable H₄P₂O₅ was obtained.

Sodium dihydropyrophosphite was prepared by heating sodium hydrogen phosphite in a vacuum at 160° C.:—

2NaH₂PO₃ = Na₂H₂P₂O₅ + H₂O

The free acid was hydrated to phosphorous acid in solution with evolution of 4.9 Cals. per mol of H₄P₂O₅. Hydration was accelerated by strong acids. The dihydrogen pyrophosphite of sodium, Na₂H₂P₂O₅, similarly gave NaH₂PO₃. The acid appears to be dibasic, the salt Na₂H₂P₂O₅ being formed by neutralisation, with evolution of 14.3 Cals. per mol of water formed in the equation

H₄P₂O₅ + 2NaOH = Na₂H₂P₂O₅ + 2H₂O

Other salts of this acid have been prepared. As in the case of phosphorous acid the structure may be symmetrical or unsymmetrical, thus

or