Chemistry » Metals, Metalloids, and Nonmetals » Occurrence, Preparation, and Compounds of Oxygen

# Phosphorus Oxyacids and Salts

## 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:

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

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:

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

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.