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Phosphorous Acid (H3PO3) - Reactions, Structure, Properties, Uses, FAQs

Phosphorous Acid (H3PO3) - Reactions, Structure, Properties, Uses, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:50 PM IST

Phosphorus acid (H3PO3) or phosphoric acid is an oxyacid of phosphorus. Phosphorous acid is also a conjugate acid of dihydrogen phosphite. This colorless compound is also given the name trihydroxy phosphorous acid since it contains three hydroxyl groups. Phosphorus acid chemical formula is H3PO3 or H3PO3 The name is phosphorus acid. Phosphorous acid, H3PO3 is a tribasic acid with one phosphorus, four oxygen, and three hydrogen atoms. And the basicity of phosphoric acid is 3 since it contains three acidic hydrogen atoms.

This Story also Contains
  1. Preparation of Phosphorous Acid, H3PO3
  2. Toxicity of Phosphorous Acid, H3PO3
  3. Structure of H3PO3 or H3PO3 Structure
  4. Phosphorous Acid
  5. Reactions of Phosphorous Acid, H3PO3
  6. Redox Properties
  7. Basicity of Phosphoric Acid
  8. Uses of Phosphorous Acid, H3PO3.
  9. Phosphinic acid and Phosphonic Acid

Preparation of Phosphorous Acid, H3PO3

Phosphorous acid or H3PO3 is produced by the slow combustion reaction of phosphorus in the form of a white volatile powder. The salt of phosphorus acid is called phosphites. Phosphoric acid is prepared by the reaction of phosphorus trichloride that is PCl3 with water. It is also prepared by the reaction of tetra phosphorus hexoxide P4O6. Industrially it is prepared by spraying PCl3 into steam at 100 degrees Celsius, the reaction heat is then used for the removal of HCl and excess water vapor from it.

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Toxicity of Phosphorous Acid, H3PO3

When phosphorous acid, H3PO3 contact with the eyes skin, and mucous membranes severely causes irritation and eye damage. It is also toxic when inhaled or by absorption by the skin. When a person inhales a particular amount of phosphorus acid it will cause irritation to the nose, throat, and lungs which will then lead to coughing, wheezing, and breathing problems. It severely damages the Respiratory system.

Phosphorus acid, H3PO3 is a white solid with sour order and is soluble in water. The taste of phosphorous acid eases garlic-like. From the chemical formula of phosphorus acid, we may think it is a tri-protic in nature that it ionizes two protons but actually it is diprotic. The molar mass of phosphorus acid is 81.9 g/mol with a density of 1.651g/cm3. The melting point of phosphorus acid eases 73.6 degrees Celsius and the boiling point of phosphorus acid is 200 degrees Celsius. It is soluble in water and also it is soluble in ethanol.

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Structure of H3PO3 or H3PO3 Structure

The structure of H3PO3 can be easily explained in this portion. Phosphorus acid, H3PO3 contains one covalent bond and unity has the name trihydroxy ortho phosphorus acid suggesting that it contains three OH groups that are 3 hydrogen bond acceptors. The geometry of phosphorus acid is pseudo-tetrahedral. One oxygen forms a double bond with a Phosphorus atom, three OH groups are bonded directly with the phosphorus atom. The structure of phosphorus acid is shown below.

Phosphorous acid structure

Phosphorous Acid

The structure of H3PO3 can be more accurately explained using the chemical formula HPO(OH)2. In the solid state phosphorous acid is tetrahedrally coordinated with one P-H and PO and two longer POH bonds. While it exists in equilibrium with the tautomer form containing three POH bonds. The one containing two hydroxyl groups is phosphonic acid while the other with three hydroxyl groups is phosphorous acid. The name ending with ‘ous’ means it is in the reduced form. The following structure shows this fact.

Tautomerism in phosphorous acid

Tautomerism in phosphorous acid

Other important oxyacids are phosphoric acid and hypophosphorous acid. These compounds also show tautomerism similar to phosphorous acid by the shift in the position of H. This is all about the structure of H3PO3.

Reactions of Phosphorous Acid, H3PO3

Acid property of H3PO3

Phosphorous acid, H3PO3 has a pKa value of 1.26-1.3. And it donates two hydrogen atoms only. The H atom which is bonded directly with the phosphorus is not readily ionized. The reaction can be represented as,

HPO(OH)2→HPO2(OH)-+H+

Thus one hydrogen ion is formed. The obtained product hydrogen phosphite ion is a weak acid with a pKa value of 6.7. It will also donate one hydrogen ion to form a phosphite ion. The reaction taking place is shown below.

HPO2(OH)-→HPO32-+H+

HPO32- is the second conjugate base of phosphorous acid with the name phosphite ion. This is the acidic property of phosphorous acid, the three hydrogens are not acidic

Redox Properties

Phosphorous acid, H3PO3 undergoes a disproportionation reaction at a temperature of 200 degrees Celsius to form phosphoric acid and phosphine.

4H3PO3→3H3PO4+PH3

This reaction is used for the preparation of phosphines that is PH3. Phosphorous acid slowly oxidizes in air to form phosphoric acid. The reaction of mercuric chloride with phosphorous acid forms mercurous chloride. While mercurous chloride reacts with phosphoric acid, H3PO3 to form mercury. Thus it shows redox properties.

Basicity of Phosphoric Acid

For phosphoric acid, H3PO3 three OH groups are bonded to the phosphorus atom so it is a tribasic group with a basicity of three. Basicity is the number of hydrogen atoms present in a chemical compound. Here the three hydrogen atoms are strongly bonded with an Electronegative atom of oxygen so it is acidic. While the basicity of phosphorus acid is only two. As all the hydrogen atoms are acidic it is dissociated completely in water except at low pH. When all three hydrogen ions have been removed the compound obtained is called orthophosphates. And the basicity of orthophosphoric acid is 2.

Uses of Phosphorous Acid, H3PO3.

  • Phosphorous acid, H3PO3 is widely used for the production of PVC stabilizers, amino methylene phosphonic acid, etc.

  • Because of its strong reducing nature, it can be used as a reducing agent.

  • It is used in the production of synthetic fibers and organophosphorus pesticides.

  • It is used for the production of amino trimethylene phosphonic acid which is used as a water treatment agent.

  • Phosphorous acid, H3PO3 is used to produce the fertilizer phosphate salt.

  • Phosphorous acid, H3PO3 also has many applications in the medical field mostly the dental field.

  • It is used for protecting ferrous materials including steel from rusting.

  • Basic lead phosphite is prepared by using phosphorous acid.

Phosphinic acid and Phosphonic Acid

The hypophosphorous acid known as phosphinic acid is also a phosphorous acid just like the phosphonic acid formula is H3PO3. The chemical formula of phosphinic acid is H3PO2 and it contains one phosphorus, three hydrogens, and only two oxygen atoms. It is a monoprotic compound in which only one hydrogen is acidic while for phosphonic acid it is diprotic with two acidic hydrogen atoms. The salt of hypophosphorous acid is hypophosphite. Like phosphonic acid, it also exists in equilibrium with the tautomer, HP(OH)2.

When phosphinic acid is heated it undergoes a disproportionation reaction to form phosphine and phosphoric acid. So it can be used for the synthesis of phosphoric acid. It is also used for reducing nickel. Its main application is the reduction of metal salts back to the bulk metal. Organo phosphinic acids can be easily synthesized using phosphinic acids and they have many important applications in various chemical reactions.

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Frequently Asked Questions (FAQs)

1. What is the formula of phosphorous acid?

 Formula of Phosphorous acid is H3PO3.

2. What is H3PO3?

H3PO3 is an acid with the name phosphorous acid. It is a di protonic acid with many applications in many fields. Readily undergo ionization to form two hydrogen ions.

3. What is the basicity of Phosphoric acid, H3PO3?

Phosphoric acid, H3PO3 is a colorless weak acid. For phosphoric acid the chemical formula is H3PO4. It contains one phosphorus atom, four oxygen atoms, and three hydrogen atoms. It is different from phosphorous acid, H3PO3 since it contains four oxygen atoms. For phosphoric acid, three OH groups are bonded to the phosphorus atom so it is a tribasic group with basicity of three. Basicity is the number of hydrogen atoms present in a chemical compound. And here the three hydrogen atoms are strongly bonded with an electronegative atom of oxygen so it is acidic. While the basicity of phosphorus acid is only two. As all the hydrogen atoms are acidic it is dissociated completely in water except at low pH. When all the three hydrogen ions have been removed the compound obtained is called orthophosphates.

4. Why is phosphorous acid considered a diprotic acid despite having three hydrogen atoms?
Phosphorous acid is considered a diprotic acid because only two of its three hydrogen atoms are ionizable. The third hydrogen atom is directly bonded to the phosphorus atom and does not readily dissociate in aqueous solutions.
5. How does the acidity of phosphorous acid compare to phosphoric acid?
Phosphorous acid is generally less acidic than phosphoric acid. This is because phosphorous acid is a diprotic acid (can donate two protons), while phosphoric acid is a triprotic acid (can donate three protons). The additional proton in phosphoric acid contributes to its higher acidity.
6. How does the presence of phosphorous acid affect the pH of aqueous solutions?
Phosphorous acid affects the pH of aqueous solutions by:
7. How does the reactivity of phosphorous acid compare to other oxoacids of phosphorus?
The reactivity of phosphorous acid compared to other oxoacids of phosphorus:
8. How does the presence of phosphorous acid affect the solubility of metal ions?
The presence of phosphorous acid affects the solubility of metal ions in several ways:
9. What is the IUPAC name for phosphorous acid?
The IUPAC name for phosphorous acid is phosphonic acid. This name reflects its structure, where one hydrogen atom is directly bonded to the phosphorus atom.
10. What is the significance of phosphorous acid in biochemistry?
In biochemistry, phosphorous acid itself is not particularly significant. However, its derivatives and related compounds play important roles:
11. What is the difference between phosphorous acid and phosphinic acid?
The main differences between phosphorous acid (H3PO3) and phosphinic acid (H3PO2) are:
12. What is the role of phosphorous acid in the synthesis of phosphonates?
Phosphorous acid plays a crucial role in the synthesis of phosphonates:
13. What is the relationship between phosphorous acid and phosphites?
The relationship between phosphorous acid and phosphites is as follows:
14. What is the molecular formula of phosphorous acid?
The molecular formula of phosphorous acid is H3PO3. This formula represents three hydrogen atoms, one phosphorus atom, and three oxygen atoms in a single molecule of phosphorous acid.
15. What is the difference between phosphorous acid and phosphorus acid?
There is no difference between phosphorous acid and phosphorus acid. Both terms refer to the same compound, H3PO3. "Phosphorous acid" is the more commonly used name, while "phosphorus acid" is an alternative name sometimes encountered in literature.
16. What are the spectroscopic characteristics of phosphorous acid?
The spectroscopic characteristics of phosphorous acid include:
17. What are the environmental fate and degradation pathways of phosphorous acid?
The environmental fate and degradation pathways of phosphorous acid include:
18. What are the challenges in the industrial production of phosphorous acid?
Challenges in the industrial production of phosphorous acid include:
19. How does phosphorous acid react with bases?
Phosphorous acid reacts with bases to form phosphite salts. As a diprotic acid, it can form both monohydrogen phosphites and dihydrogen phosphites, depending on the amount of base added and the reaction conditions.
20. How does phosphorous acid participate in esterification reactions?
Phosphorous acid can undergo esterification reactions with alcohols to form phosphite esters. These reactions typically involve the replacement of the -OH groups with -OR groups, where R represents an alkyl or aryl group from the alcohol.
21. How does phosphorous acid behave in aqueous solutions?
In aqueous solutions, phosphorous acid partially dissociates into ions. As a diprotic acid, it undergoes a two-step dissociation:
22. How does phosphorous acid react with metals?
Phosphorous acid can react with certain metals, particularly more reactive ones, to produce hydrogen gas and metal phosphites. The reaction is generally less vigorous compared to stronger acids. For example:
23. How does phosphorous acid participate in condensation reactions?
Phosphorous acid can participate in condensation reactions, particularly with aldehydes and ketones. These reactions, known as Pudovik reactions or phospha-aldol reactions, result in the formation of α-hydroxyphosphonates. The phosphorous acid acts as a nucleophile, attacking the carbonyl group of the aldehyde or ketone.
24. How does phosphorous acid behave as a reducing agent?
Phosphorous acid acts as a reducing agent due to the presence of the P-H bond. It can donate electrons to other species, reducing them while being oxidized itself. This property makes it useful in various chemical reactions and industrial processes.
25. What is the oxidation state of phosphorus in phosphorous acid?
The oxidation state of phosphorus in phosphorous acid is +3. This is lower than the oxidation state of phosphorus in phosphoric acid (+5), which contributes to its reducing properties.
26. How does phosphorous acid interact with transition metal complexes?
Phosphorous acid can interact with transition metal complexes in several ways:
27. How does temperature affect the stability of phosphorous acid?
Temperature affects the stability of phosphorous acid in the following ways:
28. How does phosphorous acid contribute to corrosion inhibition?
Phosphorous acid contributes to corrosion inhibition through several mechanisms:
29. How does the structure of phosphorous acid differ from phosphoric acid (H3PO4)?
Phosphorous acid (H3PO3) has one hydrogen atom directly bonded to the phosphorus atom, while phosphoric acid (H3PO4) has all hydrogen atoms bonded to oxygen atoms. This structural difference results in different chemical properties and reactivity.
30. What is the geometry around the phosphorus atom in phosphorous acid?
The geometry around the phosphorus atom in phosphorous acid is pyramidal. The phosphorus atom is at the center, with three bonds (two P-O bonds and one P-H bond) arranged in a tetrahedral-like structure, with a lone pair of electrons occupying the fourth position.
31. How does the presence of a lone pair on phosphorus affect the properties of phosphorous acid?
The lone pair on the phosphorus atom in phosphorous acid affects its properties in several ways:
32. What is the significance of phosphorous acid in geochemistry?
The significance of phosphorous acid in geochemistry includes:
33. What are the safety considerations when handling phosphorous acid?
When handling phosphorous acid, important safety considerations include:
34. How does phosphorous acid interact with biological membranes?
Phosphorous acid interacts with biological membranes in the following ways:
35. What is the significance of phosphorous acid in agriculture?
Phosphorous acid and its derivatives have several applications in agriculture:
36. What are the main industrial uses of phosphorous acid?
Phosphorous acid is used in various industrial applications, including:
37. What is the role of phosphorous acid in organic synthesis?
In organic synthesis, phosphorous acid is used as:
38. How does phosphorous acid contribute to flame retardancy in materials?
Phosphorous acid and its derivatives can act as flame retardants by:
39. What are the environmental impacts of phosphorous acid?
Phosphorous acid can have both positive and negative environmental impacts:
40. What is the role of phosphorous acid in the phosphorus cycle?
Phosphorous acid plays a minor role in the phosphorus cycle. It can be formed through the hydrolysis of certain phosphorus compounds and can be oxidized to phosphoric acid in natural environments. However, its presence is generally less significant compared to other phosphorus-containing compounds in the cycle.
41. How does phosphorous acid participate in polymerization reactions?
Phosphorous acid can participate in polymerization reactions in several ways:
42. What is the role of phosphorous acid in analytical chemistry?
Phosphorous acid plays several roles in analytical chemistry:
43. How does phosphorous acid contribute to the phosphorylation of organic compounds?
Phosphorous acid contributes to the phosphorylation of organic compounds through:

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