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Phosphorous acid is also known as phosphonic acid, and orthophosphorous acid is a common chemical having the molecular formula H3PO3. This acid is diprotic. It is one of the several oxygen acids of phosphorous and is used as a reducing agent in chemical analysis. It is a highly unstable compound, it readily absorbs moisture, and is converted to phosphoric acid (H3PO4) in the presence of oxygen or when heated above 180 °C (360 °F).
| Table of Contents |
Keywords: Phosphorous acid, Tautomerism, Phosphine, Diprotic, Hydrogen bond acceptor, Tetrahedral
What is Phosphorous Acid?
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- Phosphorous acid, H3PO3, is also known as trihydroxy ortho phosphorous acid.
- The name suggests that it contains three OH groups, these three hydroxyl groups are 3 hydrogen bond acceptors.
- The molecular arrangement and geometry of phosphorous acid are pseudo tetrahedral.
- In the structure one oxygen forms a double bond with a phosphorous atom, and three hydroxyls (-OH) groups are bonded directly with the phosphorous atom.
- However, the structure exists in tautomer form, which is explained below. The structure of phosphorous acid is shown in the diagram below.
Phosphorous Acid
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Tautomerism in Phosphorous Acid
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- The molecular formula HPO(OH)₂ can be used to further explain the structure of H3PO3.
- Phosphorous acid is tetrahedrally coordinated in the solid-state, with one P-H and P-O bond and two longer P-OH bonds.
- It's in a state of equilibrium in the tautomer form, which has three P-OH bonds.
- The one with two hydroxyl groups is phosphonic acid, whereas the other has three hydroxyl groups and is known as phosphorous acid.
- The tautomers can be seen in the diagram below.
Tautomerism in Phosphorous Acid
Physical and Chemical Properties of Phosphorous Acid
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All the physical and chemical properties of phosphorous acid are mentioned in the table below:
| Property Name | Property Value |
|---|---|
| Molecular Weight | 81.996 |
| Hydrogen Bond Donor Count | 3 |
| Hydrogen Bond Acceptor Count | 3 |
| Rotatable Bond | 0 |
| Exact Molecular Mass | 81.98198095 |
| Monoisotopic Mass | 81.98198095 |
| Formal Charge | 0 |
| Density | 1.65 g/cm³ |
| Boiling Point | 200 °C |
| Melting Point | 73.6 °C |
| Chemical Formula | H3PO3 |
| Odor | Sour odor |
| Appearance | White solid, deliquescent |
| Nature of internal bonds | Covalent |
| Solubility | Soluble in Water and Alcohol |
| Uses | Used as:
|
Synthesis of Phosphorous Acid
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The acid can be synthesized both in the laboratory and on an industrial scale. Both the processes are mentioned below:
- Laboratory Scale Synthesis
In laboratories, the acid anhydride form of phosphorous acid undergoes hydrolysis with water and produces phosphorous acid.
P4O6 + 6H2O → 4 HPO(OH)2
- Industrial Scale Synthesis
In industrial-scale synthesis, phosphorous trichloride is hydrolyzed with water or steam to produce the acid.
PCl3 + 3 H2O → HPO(OH)2 + 3 HCl
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Application of Phosphorous Acid
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There are many applications of phosphorous acid
- phosphorous acid is used to make Phosphine (PH3) at a laboratory scale.
4 H3PO3 → 3 H3PO4 + PH3
- In the presence of oxygen, phosphoric acid slowly oxidizes to phosphoric acid.
- Both phosphorous acid and its deprotonated derivatives are effective reducing agents, albeit they are not always rapid.
- Phosphoric acid or its salts are formed when they are oxidized.
- It converts noble metal cation solutions to metals. When phosphorous acid is treated with a cold mercuric chloride solution, a white mercuric chloride precipitate forms:
H3PO3 + 2 HgCl2 + H2O → Hg2Cl2 + H3PO4 + 2HCl
- On heating or standing, phosphorous acid reduces mercury chloride further to mercury:
H3PO3 + Hg2Cl2 + H2O → 2 Hg + H3PO4 + 2HCl
- Phosphorous acid is used in the synthesis of basic lead phosphite, a stabilizer in PVC and related chlorinated polymers.
- Basic lead phosphonate PVC stabilizer, amino methylene phosphonic acid, and hydroxyethane diphosphonic acid are all made with phosphorous acid.
- It is also used to make phosphorous acid, synthetic fibers, organophosphorous insecticides and ATMP, a highly efficient water treatment agent.
Read More: Stearic Acid
Safety Hazards of Phosphorous Acid
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- When phosphorous acid, H3PO3, comes into contact with the eyes, it causes significant irritation and damage to the skin and mucous membranes.
- When inhaled or absorbed through the skin, it is also harmful.
- When a person inhales a certain amount of phosphorous acid, the nose, throat, and lungs get irritated, resulting in coughing, wheezing, and breathing difficulties.
- The respiratory system is seriously harmed when phosphorous acid is ingested.
Things to Remember
- Phosphorous acid also known as phosphonic acid and orthophosphorous acid is a chemical having the molecular formula H3PO3.
- Phosphorous acid is diprotic in nature.
- It is one of the several oxygen acids of phosphorous and is used as a reducing agent in chemical analysis.
- Phosphorous acid is also known as trihydroxy ortho phosphorous acid because it contains three OH groups, these three hydroxyl groups are 3 hydrogen bond acceptors.
- The molecular arrangement and geometry of phosphorous acid are pseudo tetrahedral.
- The pKa of phosphoric acid ranges from 1.26 to 1.3.
- HP(O)₂(OH) is a diprotic acid, and the hydrogen phosphite ion is a weak acid.
- Hydrogen phosphite is the conjugate base HP(O)₂(OH), and the phosphite ion is the second conjugate base, HPO.
- Phosphoric acid or its salts are formed when phosphorous acid is oxidised. It converts noble metal cation solutions to metals.
- When phosphorous acid is treated with a cold mercuric chloride solution, a white mercuric chloride precipitate forms.
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Sample Questions
Ques 1: Briefly explain the structure of phosphorous acid. (2 marks)
Ans: phosphorous acid, H3PO3, is also known as trihydroxy ortho phosphorous acid. The name suggests that it contains three OH groups, these three hydroxyl groups are 3 hydrogen bond acceptors. The molecular arrangement and geometry of phosphorous acid are pseudo tetrahedral. In the structure one oxygen forms a double bond with a phosphorous atom, and three hydroxyls (-OH) groups are bonded directly with the phosphorous atom.
Ques 2: Write a short note on the tautomer of phosphorous acid. (2 marks)
Ans: The molecular formula HPO(OH)2 can be used to explain the structure of H3PO3 further. Phosphorous acid is tetrahedrally coordinated in the solid-state, with one P-H and P-O bond and two longer P-OH bonds. It's in a state of equilibrium in the tautomer form, which has three P-OH bonds. The one with two hydroxyl groups is phosphonic acid, whereas the other has three hydroxyl groups and is known as phosphorous acid.
Ques 3: How is phosphorous acid synthesized in the laboratory? (2 marks)
Ans: In laboratories, the acid anhydride form of phosphorous acid undergoes hydrolysis with water and produces phosphorous acid.
P4O6 + 6 H2O → 4 HPO(OH)2
Ques 4: How is phosphorous acid synthesized in industry scale? (2 marks)
Ans: In industrial-scale synthesis, phosphorous trichloride is hydrolyzed with water or steam to produce the acid.
PCl3 + 3 H2O → HPO(OH)2 + 3 HCl
Ques 5: Write a short note on phosphorous acid used as a reducing agent. (2 marks)
Ans: Phosphorous acid disproportionates to phosphoric acid and phosphine when heated to 200°C:
4 H3PO3 → 3 H3PO4 + PH3
This procedure is used to make PH3 at a laboratory scale. In the presence of oxygen, phosphoric acid slowly oxidizes to phosphoric acid. Both phosphorous acid and its deprotonated derivatives are effective reducing agents, albeit they are not always rapid.
Ques 6: How is phosphorous acid used as a Ligand? (2 marks)
Ans: Phosphorous acid is known to coordinate as the otherwise unusual P(OH)3 tautomer when treated with metals of the d6 configuration. Mo(CO)5(P(OH)3) and [Ru(NH3)4(H2O)(P(OH)3) are two examples.
The luminous salt potassium diplatinum (II) tetrakispyrophosphite is made by heating a combination of potassium tetrachloroplatinate and phosphorous acid.
Ques 7: Write the reaction involved using phosphorous acid, and mercury. (2 marks)
Ans: Phosphoric acid or its salts are formed when they are oxidized. It converts noble metal cation solutions to metals. When phosphorous acid is treated with a cold mercuric chloride solution, a white mercuric chloride precipitate forms:
H3PO3 + 2 HgCl2 + H2O → Hg2Cl2 + H3PO4 + 2 HCl
On heating or standing, phosphorous acid reduces mercury chloride further to mercury:
H3PO3 + Hg2Cl2 + H2O → 2 Hg + H3PO4 + 2 HCl
Ques 8: What are the harmful effects of phosphorous acid? (2 marks)
Ans: When phosphorous acid, H3PO3, comes into contact with the eyes, it causes significant irritation and damage to the skin and mucous membranes. When inhaled or absorbed through the skin, it is also harmful. When a person inhales a certain amount of phosphorous acid, the nose, throat, and lungs get irritated, resulting in coughing, wheezing, and breathing difficulties. The respiratory system is seriously harmed.
Ques 9: What are the different compounds synthesized from phosphorous acid? (2 marks)
Ans: The synthesis of basic lead phosphite, a stabilizer in PVC and related chlorinated polymers, is the most important usage of phosphorous acid (phosphonic acid). Basic lead phosphonate PVC stabilizer, amino methylene phosphonic acid, and hydroxyethane diphosphonic acid are all made with it. It's also used to make phosphorous acid, synthetic fibers, organophosphorous insecticides, and ATMP, a highly efficient water treatment agent.
Ques 10: Explain why phosphorous acid is stronger than phosphoric acid? (2 marks)
Ans: Because hydrogen does not attract electrons as strongly as oxygen, a portion of the molecule is more positive, giving phosphoric acid a greater dipole moment. Phosphorous acid is more acidic than phosphoric acid because of its greater acidity constant, which corresponds to a lower pKa value.
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