3.2 Oxoacids

Many p-block elements form oxoacids but here the focus will be on those of phosphorus with a few illustrative examples of other elements.The chemistry of phosphorous compounds in natural water and the body is not quite as simple as it has been treated so far, so the topic will be expanded upon here.

For instance, there are several oxoacids of phosphorus. The three most important are phosphoric acid, H₃PO₄, phosphorous acid, H₃PO₃, and hypophosphorous acid, H₃PO₂. Table 3 lists other names for these acids.

Therefore H₃PO₄, H₃PO₃, and H₃PO₂ are also called phosphoric(V), phosphoric(III) and phosphoric(I) acid respectively.

Table 3 Common phosphorus oxoacids and their anions.

Formula and common name	Common anion name	Structure
H3PO4 phosphoric acid or orthophosphoric acid	phosphate or orthophosphate
H3PO3 phosphorous acid or phosphonic acid	phosphite or phosphonate
H3PO2 hydrophosphorous acid or phosphinic acid	hypophosphite or phosphinate
H4P2O7 diphosphoric acid	diphosphate
(HPO3)n (in the limit where n = ∞) metaphosphoric acid	metaphosphate

Some acids are polyprotic (or taking the alternative viewpoint polybasic) meaning they ionise stepwise, unlike hydrochloric acid, HCl, which is a monoprotic acid. Note only the protons attached to oxygen are ionisable. Phosphoric acid is triprotic undergoing three successive ionisations:

As phosphoric acid is triprotic, it can form three series of salts with a metal such as sodium: the dihydrogen phosphate, the hydrogen phosphate and the normal phosphate.

Note that Equations 21-23 have the form:

The pairs acid(1)/base(1) and acid(2)/base(2) are called a conjugate acid and base. Thus, H₃O⁺ and H₂O are a conjugate acid and base pair; so are H₃PO₄(aq) and H₂PO₄−(aq). An acid is transformed to its conjugate base by losing a proton and vice versa.