The p-Block Elements

Oxoacids of Phosphorus

Oxoacids of Phosphorus are Hypophosphoric acid(H3PO4), Metaphosphoric acid(HPO2), Pyrophosphoric acid (H4P2O7), Hypophosphorous acid(H3PO2), Phosphorous acid (H3PO3), Peroxophosphoric acid (H3PO5), Orthophosphoric acid (H3PO5). Oxoacids are acids containing oxygen. Let us Learn more about Oxoacids and Oxoacids of Phosphorus.

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What are Oxoacids?

In simple terms, oxoacids are the acids containing oxygen. Phosphorus is one such element that forms a number of oxoacids. A few common oxyacids include H3PO4, H3PO3, etc.

In oxoacids of phosphorus, we see that the phosphorus is tetrahedrally surrounded by other atoms. Generally, it is clear that there are at least one P=O bond and one P–OH bond in these acids. P–P or P–H bonds are also present besides the P=O bonds and P–OH bonds in oxoacids of phosphorus. In these cases, the oxidation state of phosphorus is less than +5.

These acids are generally seen to jump to higher and lower oxidation states. For example, upon heating, phosphorous acid disproportionates to result in phosphoric acid and phosphine.

Browse more Topics under The P Block Elements

Oxoacids of Phosphorus

The P-H bonds in oxoacids are not ionisable to give H+ ions. On the other hand, the H atoms attached to oxygen in P-OH form are ionisable. Hence, we can say that basicity is the property exhibited by the H atoms that are attached to oxygen.

As a result, phosphorous acid, H3PO3 is dibasic as it has two P-OH bonds. Similarly, phosphoric acid, H3PO4 is tribasic as it has three P-OH bonds. The oxoacids of phosphorus that have P-H bonds exhibit strong reducing properties. For example, hypophosphorous acid containing two P-H bonds is a very good reducing agent.

4AgNO3 + 2H2O + H3PO2  4Ag + 4HNO3 + H3PO4

Few Popular Oxoacids of Phosphorus

In this section, we will look at some of the most important and popular oxoacids of phosphorus.

Name Formula
Hypophosphoric acid H4P2O6
Metaphosphoric acid HPO2
Pyrophosphoric acid H4P2O7
Hypophosphorous acid H3PO2
Phosphorous acid H3PO3
Peroxophosphoric acid H3PO5
Orthophosphoric acid H3PO4

1) Phosphorus acid, H3PO3

Phosphorous acid is a diprotic acid. This means that it ionizes two protons. We can describe it in a better manner by the structural formula HPO(OH)2. We can prepare phosphorous acid by the hydrolysis of phosphorus trichloride with acid or steam.

PCl3    +   3H2O        HPO(OH)2    +    3HCl


2) Phosphoric acid, H3PO4

Phosphoric acid is a triprotic acid. This means that it can ionise three protons. It is a non-toxic acid, when pure. It is solid at the room temperature and pressure. We can prepare phosphoric acid by adding sulfuric acid to tricalcium phosphate rock:

Ca5(PO4)3X    +    5H2SO4    +    10H2O       3H3PO4    +    5CaSO4.2H2O   +     HX

(X can be F, Cl, Br, and OH).

Learn more about P Block Elements here in detail

3) Polymetaphosphoric Acid (HPO3)n

We can obtain it by warming orthophosphoric acid to around 850 K. It does not exist as a monomer. It exists as a cyclic trimer, cyclic tetramer or polymer.

Solved Example for You

Q: How do we obtain Hypophosphoric Acid (H4P2O6)?

Ans: This acid can be set up by the controlled oxidation of red phosphorus with sodium chlorite. When we get the disodium salt of the hypophosphoric acid, we pass it through a cation exchanger to yield hypophosphoric acid. It is tetrabasic in nature.

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