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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

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