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Oxoacids of Phosphorus– Definition, Types of Oxoacids, Uses, Practice Problems & FAQs

Oxoacids of Phosphorus– Definition, Types of Oxoacids, Uses, Practice Problems & FAQs

Oxygen, the ‘omnipresent soul’ has a superpower to react with almost every element on earth! This natural instinct of Mr. Oxygen gave birth to a wide number of compounds called oxoacids. An oxoacid or oxyacid is a type of acid that has an oxygen atom linked to a hydrogen atom as well as at least one other element. Phosphorus is one such element who wanted to partner with oxygen in creating a number of oxoacids and the result is what we are going to study in detail now!


  • What are the Oxoacids of Phosphorus?
  • Phosphorus Acid
  • Phosphoric Acid
  • Hypophosphoric Acid
  • Pyrophosphoric Acid
  • Metaphosphoric Acid
  • Acidic Strength of Oxoacids of phosphorus
  • Properties of Oxoacids of Phosphorus
  • Uses of Oxoacids of Phosphorus
  • Practice Problems
  • Frequently Asked Questions-FAQs

What are the Oxoacids of Phosphorus?

Phosphorus is capable of forming many oxoacids with oxygen. In terms of loss or gain of H2O molecules of oxygen atoms, the oxoacid compositions are linked. Oxoacids generally dissociate in water to produce H+ ions. The oxidation states of Phosphorus in the oxoacids varies from +1 to +5.


Hypophosphorous Acid (Phosphinic Acid) - H3PO2

Orthophosphorus Acid (Phosphonic Acid) - H3PO3

Pyrophosphorus Acid - H4P2O5

Hypophosphoric Acid- H4P2O6

Orthophosphoric Acid- H3PO4

Pyrophosphoric Acid- H4P2O7

Metaphosphoric Acid- HPO3n[Exist in polymeric form]

Orthophosphorus Acid

  • The formula of orthophosphorus acid/phosphorus acid is H3PO3.
  • Orpthophosphorus acid is also called phosphonic acid. 
  • Phosphorus acid is a diprotic acid in nature with two donatable protons.
  • It ionizes to give two H+ ions. Hence its basicity is 2.
  • The structural formula is HPO-(OH)2 or H3PO3 is used to characterize it and the hybridisation of phosphorus in orthophosphorus acid is sp3. Geometry of orthophosphorus acid is tetrahedral. 

  • One way to produce orthophosphorus acid is hydrolysis of phosphorus trichloride or phosphorus trioxide with steam or acid.

PCl3+3H2O → H3PO3+3HCl

P2O3 + 3H2O → 2H3PO3

  • Oxidation state of central P atom in H3PO3 is +3.
  • Orthophosphorus acid, or phosphorus acid, on heating disproportionates to give orthophosphoric acid and phosphine.

4H3PO3 3H3PO4 + PH3

Orthophosphoric Acid 

Orthophosphoric acid (H3PO4) is one of the most popular acids that is used in many industries, especially in the manufacturing of fertilizers. The salts of this acid which are known as phosphates. Orthophosphoric acid is also known as phosphoric acid. 

  • Phosphoric acid is a triprotic acid in nature with three donatable protons.
  • It ionizes to give three H+ ions. Hence its basicity is 3.
  • The structural formula is PO-(OH)3 or H3PO4 is used to characterize it and the hybridisation of phosphorus in orthophosphoric acid is sp3. Geometry of orthophosphoric acid is tetrahedral.
  • This acid is a non-toxic acid and in its pure form, it is solid at room temperature.
  • It has a molar mass of 97.99 g mol-1.
  • The acid is mostly available in the form of an aqueous solution (almost 85%) and is an odourless, colourless, and non-volatile liquid. 
  • Orthophosphoric acid is a sticky liquid, this acid is treated as a weak acid but it still can cause irritation on the skin and can damage the eyes if the eyes are exposed to phosphoric acid and can damage the membranes in the nose.

Preparation of orthophosphoric acid: It is prepared by adding sulfuric acid to calcium phosphate rock:

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

Where X = F, Cl, Br, -OH

Another method of preparing orthophosphoric acid is by bubbling phosphorus pentoxide with water.

P4O10 + 6H2O 4H3PO4

Chemical properties of orthophosphoric acid:

  • Oxidation state of P in H3PO4 is +5. 
  • Phosphoric acid or H3PO4 can release up to three H+ ions. Due to this property, it can react differently in comparison to other mineral acids. It is a tribasic acid. 
  • Reaction with bases usually results in the formation of three classes of salts.
  • When the molecules of phosphoric acid are exposed to high temperatures it forms dimers, trimers and even long polymeric chains as seen in polyphosphoric acids and metaphosphoric acids.

Metaphosphoric Acid 

  • Metaphosphoric acid is prepared by heating orthophosphoric acid at around 850 K. Metaphosphoric acid can exist as a polymer, a cyclic trimer or a cyclic tetramer but never as a monomer.
  • Cyclo triphosphate acid, trimetaphosphoric acid, and trimetaphosphate are all names for cyclic meta-phosphoric acid (H3O9P3). It belongs to the non-metal phosphates class of inorganic chemicals.

Hypophosphoric Acid 

  • Hypophosphoric acid is made by oxidizing red phosphorus with sodium chlorite in a regulated manner. 

P4+ 4NaClO2 + 4H2O → 2Na2H2P2O6 + 4HCl

When acid's disodium salt is created, it passes via a cation exchanger to become hypophosphoric acid. 

Na2H2P2O6 + 2H-Resin → H4P2O6 + 2Na-Resin

  • It is tetrabasic in nature. The oxidation state of P here, is +4.
  • It has four P-OH, One P-P, Two P=O

Pyrophosphoric Acid 

  • When orthophosphoric acid (H3PO4) is heated to near 250°C, it transforms into tetrabasic pyrophosphoric acid.

  • Oxidation state of P in H4P2O7 is +5
  • It has four P-OH, two P=O, one P-O-P.
  • Pyrophosphoric acid is a radiopharmaceutical that is used to examine the bone and cardiovascular abnormalities. It is also found in some medications that help prevent iron deficiency anaemia.

Pyrophosphorus Acid 

  • Oxidation state of P in pyrophosphorus acid is +3.
  • It contains two P-OH, Two P-H, Two P=O, One P-O-P
  • It can be obtained by reacting phosphorus trichloride with H3PO3.

PCl3+H3PO3+2H2O H4P2O5 + 3HCl

Hypophosphorous Acid (Phosphinic Acid)

  • Phosphinic acid is a strongly reducing acid in nature due to the presence of two P-H bonds. It has one P-OH, two P-H, and one P=O bond. 

  • Oxidation state of P in hypophosphorous acid is +1.
  • It can be prepared from white phosphorus (P4) and alkali.

P4+ 3NaOH +3H2O PH3+3NaH2PO2

NaH2PO2+ HCl H3PO2+ NaCl

Acidic Strength of Oxoacids of Phosphorus

The acidic strength of these oxoacids follow the order: H3PO4>H3PO3>H3PO2

All hydrogen atoms are connected to an oxygen atom as an -OH group in the first scenario

(H3PO4), allowing hydrogen to be easily released as protons. As a result, the acidic strength of phosphorus oxyacids will be the highest.

Two hydrogen atoms are bonded to the oxygen atoms in H3PO3, whereas the third hydrogen is attached to the P atom (as P-H bond), which cannot come out as a proton.

Finally, in the third case (H3PO2), only one hydrogen atom is attached to oxygen, and the other two hydrogen atoms are attached directly to P, which cannot come out as protons. Hence, only one hydrogen atom can be given out as an H+ion. 

Thereby, the acidity of a protonic acid is decided by the ease with which the species releases the H+ions. Thus, the resultant order of acidic strength can be given as: H3PO4>H3PO3>H3PO2.

Properties of Oxoacids of Phosphorus

  • In oxoacids of phosphorus, P are tetrahedrally surrounded by other atoms. Generally, all these acids are known to form at least one P=O bond and one P–OH bond.
  • P–P or P–H bonds are also found in addition to P=O bonds and P–OH bonds in oxoacids of phosphorus where the oxidation state of phosphorus is less than +5.
  • These acids are generally seen to be disproportionate to either lower or higher oxidation states. For instance, when phosphorous acid is heated, it gives phosphine and phosphoric acid (orthophosphoric acid).

4H3PO3→ 3H3PO4+PH3

  • Oxoacids of phosphorus containing P–H bonds have strong reducing properties, for example, Hypophosphorous or Phosphinic acid (H3PO2) is a strong reducing agent and can reduce silver nitrate to metallic silver.

4AgNO3+2H2O+H3PO2 → 4Ag+4HNO3+H3PO4

  • Presence of P–H bonds only determines the reducing nature of an oxoacid and not their basicity. This is because P–H bonds cannot ionize to give H+.
  • The basicity is caused by only those H-atoms that are linked to oxygen in the P–OH configuration. 
  • As a result, H3PO3 and H3PO4 are dibasic and tribasic, respectively, because H3PO3 has two and H3PO4 has three P–OH linkages in their structure.

Uses of Oxoacids of Phosphorus

  • Phosphoric acid is used for the production of fertilizers, especially phosphate based ones. As a flavouring agent in animal or poultry feed.
  • Nylon fibres, polyamides, polyester fibre, polyacrylonitrile, alkyd resins, epoxies, fatty acid esters, and glycerols are among the chemicals and polymers that use hypophosphorous acid as a decolorizing agent and for colour stabilization.
  • Hypophosphoric acid is a kind of phosphoric acid that can be used as a bleaching agent as well as a good reducer. It is mostly used as a wetting agent in the industry. It is used as a stimulant in the majority of pharmaceuticals.
  • Pyrophosphoric acid is a radiopharmaceutical that is used to examine bone and cardiovascular abnormalities. It is also found in some medications that help prevent iron deficiency anaemia.
  • Metaphosphoric acid is used to protect vitamin C in solution against any kind of atmospheric oxidation.
  • The synthesis of basic lead phosphite, a stabilizer in PVC and related chlorinated polymers, is the most important usage of phosphorus acid (phosphonic acid).
  • Peroxy monophosphoric acid (H3PO5) is an electrophilic reagent used in organic synthesis to oxidize alkenes, alkynes, aromatic compounds, and amines. Only relatively acid-stable epoxides can be made from alkenes due to their strongly acidic character, such as trans-stilbene oxide from trans-stilbene.

Practice Problems

Q 1. What is the basicity of H3PO3?

a. 2
b. 3
c. 1
d. 0

Answer: (A)

The basicity corresponds to the number of hydrogen atoms attached to the oxygen atom as -OH group. H3PO3 has two -OH groups and hence its basicity is 2 as it has two replaceable hydrogens. 

So, option A is the correct answer. 

Q 2. Oxoacids must definitely contain which of the following elements?

a. Be
b. P
c. O
d. S

Answer: (C)

Oxoacids are generally acids having an element attached to -OH group. So it must have the element Oxygen. So, option C is the correct answer.

Q 3. Silver nitrate is reduced to _____in the presence of hypophosphorous acid. 

a. AgO
b. Ag
c. HNO3
d. None of these

Answer: (B)

Hypophosphorous or Phosphinic acid (H3PO2) is a strong reducing agent and can reduce silver nitrate to metallic silver. So option B is the correct answer. 

4AgNO3+2H2O+H3PO2 → 4Ag+4HNO3+H3PO4

Q 4. Which oxoacid of phosphorus when heated undergoes polymerisation to form metaphosphoric acid?

a. Phosphinic acid
b. Phosphoric acid
c. Phosphorus Acid
d. Metaphosphoric acid

Answer: (B)

When phosphoric acid is heated, it forms metaphosphoric acid which exists in the polymeric forms.

So option B is the correct answer. 

Frequently Asked Questions-FAQs

Q 1. Which oxoacids of phosphorus has P-P bond?

Answer: Hypophosphoric acid has one P-P bond. This acid is a tetrabasic acid as it has four donatable H+ ions. Hence its basicity is 4.

Q 2. What is the major difference between phosphate and phosphite ions?

Answer: If phosphate and phosphite ions are compared, the phosphate ion contains one extra oxygen atom than the phosphite ion, making the properties, chemistry and behaviour of compounds of phosphate ion different from the compounds of phosphite ion. Compounds of phosphate ion in general are less soluble than the compounds of phosphite ion. 

Q 3. What is the relation between reducing property and the presence of P-H bonds in oxoacids of phosphorus?

Answer: Reducing nature of oxoacids is directly proportional to the number of P-H bonds present in an oxoacid of phosphorus. Hence greater the number of P-H bonds, the stronger will be its reducing power. For example, H3PO2 has two P-H bonds, so it is a very strong reducing agent.

Q 4. What is the oxidation state of P in H3PO3?

Answer: Let oxidation state of P be x.

So, 3(+1) +x +3(-2)=0

⇒ x-3= 0

∴ x=+3

So oxidation state of P in H3PO3is +3

Related Topics

Allotropes of Phosphorus


Phosphorus halides



Alkali Metals

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