Phosphorus Oxyacids and Salts

Pure orthophosphoric acid, H₃PO₄ (shown in the figure below), forms colorless, deliquescent crystals that melt at 42 °C. The common name of this compound is phosphoric acid, and is commercially available as a viscous 82% solution known as syrupy phosphoric acid. One use of phosphoric acid is as an additive to many soft drinks.

One commercial method of preparing orthophosphoric acid is to treat calcium phosphate rock with concentrated sulfuric acid:

\({\text{Ca}}_{3}{({\text{PO}}_{4})}_{2}(s)+3{\text{H}}_{2}{\text{SO}}_{4}(aq)\;⟶\;2{\text{H}}_{3}{\text{PO}}_{4}(aq)+3{\text{CaSO}}_{4}(s)\)

A space filling model shows an orange atom labeled, “P,” bonded on four sides to red atoms labeled, “O.” Three of the red atoms are bonded to white atoms labeled, “H.” A Lewis structure is also shown in which a phosphorus atom is single bonded to four oxygen atoms, three of which have two lone pairs of electrons, and one of which has three lone pairs of electrons. The oxygen atoms with two lone pairs of electrons are single bonded to hydrogen atoms.

Orthophosphoric acid, H₃PO₄, is colorless when pure and has this molecular (left) and Lewis structure (right).

Dilution of the products with water, followed by filtration to remove calcium sulfate, gives a dilute acid solution contaminated with calcium dihydrogen phosphate, Ca(H₂PO₄)₂, and other compounds associated with calcium phosphate rock. It is possible to prepare pure orthophosphoric acid by dissolving P₄O₁₀ in water.

The action of water on P₄O₆, PCl₃, PBr₃, or PI₃ forms phosphorous acid, H₃PO₃ (shown in the figure below). The best method for preparing pure phosphorous acid is by hydrolyzing phosphorus trichloride:

\({\text{PCl}}_{3}(l)+3{\text{H}}_{2}\text{O}(l)\;⟶\;{\text{H}}_{3}{\text{PO}}_{3}(aq)+\text{3HCl}(g)\)

Heating the resulting solution expels the hydrogen chloride and leads to the evaporation of water. When sufficient water evaporates, white crystals of phosphorous acid will appear upon cooling. The crystals are deliquescent, very soluble in water, and have an odor like that of garlic. The solid melts at 70.1 °C and decomposes at about 200 °C by disproportionation into phosphine and orthophosphoric acid:

\(4{\text{H}}_{3}{\text{PO}}_{3}(l)\;⟶\;{\text{PH}}_{3}(g)+3{\text{H}}_{3}{\text{PO}}_{4}(l)\)

A space filling model shows an orange atom labeled, “P,” bonded on three sides to red atoms labeled, “O,” and on the other side to a white atom labeled, “H.” Two of the red atoms are bonded to white atoms labeled, “H.” A Lewis structure is also shown in which a phosphorus atom is single bonded to a hydrogen atom and three oxygen atoms, two of which have two lone pairs of electrons and single bonds to hydrogen atoms, and one of which has three lone pairs of electrons.

In a molecule of phosphorous acid, H₃PO₃, only the two hydrogen atoms bonded to an oxygen atom are acidic.

Phosphorous acid forms only two series of salts, which contain the dihydrogen phosphite ion, \({\text{H}}_{2}{\text{PO}}_{3}{}^{\text{−}},\) or the hydrogen phosphate ion, \({\text{HPO}}_{3}{}^{2-},\) respectively. It is not possible to replace the third atom of hydrogen because it is not very acidic, as it is not easy to ionize the P-H bond.

This lesson is part of:

Metals, Metalloids, and Nonmetals

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