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

• Introduction to p Block Elements
• Some Important Compounds of Carbon and Silicon
• Trend and Anomalous Properties of Carbon
• Trends and Properties of Boron and Aluminium
• Ammonia
• Chlorine
• Dinitrogen
• Dioxygen
• Group 13 Elements: Boron Family
• Group 14 Elements: Carbon Family
• Group 15 Elements
• Group 16 Elements
• Group 17 Elements
• Group 18 Elements
• Hydrogen Chloride
• Interhalogen Compounds
• Nitric Acid and Oxides of Nitrogen
• Oxoacids of Halogens
• Oxoacids of Sulphur
• Ozone
• Phosphine
• Phosphorus – Allotropic Forms
• Phosphorus Halides
• Simple Oxides
• Sulphur – Allotropic Forms
• Sulphuric Acid
• Sulphuric Dioxid

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.