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Oxoacids of Phosphorus

Last Updated : 07 Feb, 2022
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Oxoacids are acids that contain oxygen. Phosphorus is one such element that can be used to create a variety of oxoacids. H3PO4, H3PO3, and other common oxyacids The phosphorus atom is tetrahedrally surrounded by other atoms in phosphorus oxoacids. In general, it is obvious that these acids have at least one P=O bond and one P–OH bond. In addition to P=O and P–OH links, phosphorus oxoacids have P–P or P–H bonds. In these circumstances, the phosphorus oxidation state is less than +5. These acids are known to bounce between higher and lower oxidation states. For example, when heated, phosphorous acid decomposes into phosphoric acid and phosphine. 

P-H bonds in oxoacids cannot be ionizable to produce H+ ions. The H atoms bonded to oxygen in P-OH form, on the other hand, are ionizable. As a result, we can state that basicity is a feature exhibited by H atoms coupled to oxygen. As a result of the two P-OH linkages, phosphorous acid, H3PO3, is dibasic. Similarly, phosphoric acid, H3PO4, is tribasic due to the presence of three P-OH bonds. Phosphorus oxoacids with P-H bonds have significant reducing capabilities. Hypophosphorous acid, which has two P-H bonds, is an excellent reducing agent.

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

Oxoacids of Phosphorus

  • Phosphorus acid, H3PO3

Diprotic acid is phosphoric acid. It ionizes two protons as a result of this. The structural formula HPO(OH)2 is a better way to represent it. Phosphorous acid can be made by hydrolyzing phosphorus trichloride with acid or steam.

PCl3 + 3H2O → HPO(OH)2 + 3HCl

Phosphorus acid

  • Phosphoric acid, H3PO4

Phosphoric acid is classified as a triprotic acid. This indicates it has the ability to ionize three protons. When pure, it is a non-toxic acid. At room temperature and pressure, it is solid. Phosphoric acid can be made 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)

Phosphoric acid

  • Meta phosphoric Acid (HPO3)n

We can get it by heating orthophosphoric acid at around 850 degrees Celsius. As a monomer, it does not exist. It comes in the form of a cyclic trimer, cyclic tetramer, or polymer.

H3PO4 HPO3 + H2O

  • Hypophosphoric Acid (H4P2O6)

By performing controlled oxidation of red phosphorus with sodium chlorite, hypophosphoric acid is formed. Once the disodium salt of the acid is generated, it passes through a cation exchanger to produce hypophosphoric acid. The acid is tetrabasic in nature.

2P + 2NaClO2 + 2H2O → Na2H2P2O6 + 2HCl

Na2H2P2O6 + 2H → H4P2O6 + 2Na

Hypophosphoric Acid

  • Pyrophosphoric Acid (H4P2O7)

When orthophosphoric acid is heated to near 250°C, it transforms into tetrabasic acid.

2H3PO4 → H4P2O7 + H2O

Pyrophosphoric Acid

  • Orthophosphoric Acid (H3PO4)

When P4O10 is treated with bubbled water, orthophosphoric acid is produced. This acid has a tribasic structure.

P4O10+6H2O → 4H3PO4

Orthophosphoric Acid

Acidic Strength

Acidic strength order of the Oxoacids of Phosphorus: H₃PO₂, H₃PO₃, and H₃PO₄

The order of phosphorus acidity strength of oxoacids: H3PO4 > H3PO3 > H3PO2.

In the first example, all hydrogen atoms are connected to an oxygen atom as an OH group, which allows hydrogen to be easily released as protons. As a result, the acidity of phosphorus oxyacids will be the highest. In the second situation, two hydrogen atoms are bonded to the oxygen atoms, while a third hydrogen atom is attached to the P atom, which cannot be released as a proton.

Finally, only one hydrogen atom is bonded to oxygen in the third scenario, while the other two hydrogen atoms are attached directly to P, which cannot come out as protons. Furthermore, only one hydrogen can be produced as a proton. As a result, the acidity of a protonic acid is determined by how easily the species releases the H+ ion. As a result, the above-mentioned order is the result.

Basic Strength

The basicity of phosphorus oxoacids is defined as the number of ionizable H+ ions or protons in that acid. Although the structure of Phosphorus Oxoacids contains three H atoms, only those directly linked to the Oxygen (O) atom are easily ionized. However, there are only two such H atoms. As a result, the basicity of phosphorus oxoacids is 3.

Uses of Oxoacids of Phosphorus

Among the many applications for phosphorus oxoacids are:

  1. Phosphorus is utilized as a sulfuric acid replacement in the production of HI and HBr.
  2. It is used in the manufacture of soft drinks as a souring agent.
  3. It is utilized in the production of sodium, ammonium, and potassium phosphate salts.
  4. It’s utilized in the production of phosphatic fertilizers.

Sample Problems

Question 1: How do we obtain Hypophosphoric Acid?


The controlled oxidation of red phosphorus with sodium chlorite can produce this acid. When we obtain the hypophosphoric acid disodium salt, we run it via a cation exchanger to obtain hypophosphoric acid. It has a tetrabasic structure.

Question 2: Is H3PO2 an acid?


If we look at the structure of Phosphorus Oxoacids, we can see that P is attached to an oxygen atom with a double bond, two hydrogens with every single bond, and finally to a hydroxyl group. Acids are organic substances that can produce free H+ ions. There are three hydrogens in the chemical described above. The basicity here is 2, but the hydrogen connected to the electronegative atom, to the oxygen atom, is transferred as a hydrogen ion. This could lead to confusion. Because P-H bonds are stronger than O-H bonds in this case, the weaker link is easily broken. This chemical is definitely an acid because it can produce an H+ ion.

Question 3: What are Oxoacids?


To put it simply, oxyacids are acids that contain oxygen. One of these elements is phosphorus, which can be found in a variety of oxoacids. Examples include H3PO4, H3PO3, and various oxyacids.

Question 4: Why is H3PO3 dibasic?


The phosphorus atom is connected by two -OH groups, and one hydrogen atom is bonded directly to the phosphorus atom. It is simple to break the O-H bond. As a result, the two -OH groups are responsible for this compound’s diprotic nature. Whereas another Hydrogen is directly connected to the phosphorus atom, the bond is difficult to break due to its high stability. As a result, it is a diprotic acid rather than a triprotic acid.

Question 5: Write a short note on the basic strength of phosphorus oxoacids?


The number of ionizable H+ ions or protons in phosphorus oxoacids is defined as the acid’s basicity. Although Phosphorus Oxoacids have three H atoms in their structure, only those immediately connected to the Oxygen (O) atom are easily ionised. There are, however, only two such H atoms. As a result, phosphorus oxoacids have a basicity of 3.

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