Phosphoric Acid - Formula, Uses, Properties, Preparation, FAQs

Q: 1. What is orthophosphoric acid used for?

Phosphoric acid (H 3 PO 4 ), also known as orthophosphoric acid, is the most important oxygen acid of phosphorus and is used to generate fertiliser phosphate salts. This is also utilised in the production of dental cements, albumin derivatives, and the sugar and textile industries.

Q: 6.What is the metaphosphoric acid formula ?

The chemical formula for metaphosphoric acid is HPO 3

Phosphoric Acid - Formula, Uses, Properties, Preparation, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:38 PM IST

Orthophosphoric Acid - H₃HO₄ (Phosphoric Acid)

Phosphoric acid is referred to as orthophosphoric acid. Phosphoric acid has the chemical formula H₃PO₄ and is a weak acid. Phosphoric acid is an acid with four oxygen atoms, one phosphorus atom, and three hydrogen atoms. Phosphoric(V) acid or orthophosphoric acid are other names for it. It can be found in teeth and bones, and it aids in metabolic activities. It appears as a clear, colourless solution in liquid form and as a transparent, crystalline solid in solid form.

This Story also Contains

Orthophosphoric Acid - H3HO4 (Phosphoric Acid)
Preparation of Phosphoric Acid
Physical Properties of Phosphoric Acid (H3PO4)
Chemical Properties of Phosphoric Acid (H3PO4)
Basicity of Orthophosphoric Acid (H3PO4)
Uses of Phosphoric Acid (H3PO4)

Also Phosphoric acid formula or phosphoric acid chemical formula is PO₄ and Orthophosphoric acid formula is H₃PO₄ similarly phosphorous acid formula is H₃PO₄; Phosphorous acid is also known Orthophosphorous acid. It aids in the binding of divalent cations as a sequestering agent. In orthodontics and dentistry, it is commonly employed. In biogeochemistry and biochemistry, phosphoric acid plays a significant role. It's called tribasic acid. The three zones are the one where pH equals pK, the one where pH equals 4.7, and the one where pH is equal to 9.8.

IUPAC Name	Ortho Phosphoric Acid
Chemical Formula	H₃PO₄
Molar Mass	97.99 g/mol
Density	2.030 g.cm^-3
Melting Point	42.4 °C (108.3 °F; 315.5 K)
Boiling Point	407 °C (765 °F; 680 K)

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Preparation of Phosphoric Acid

It's easy to make by dissolving P₂O₅ in water and then heating the solution to make a thick syrup.

P₂O₅ + 3H₂O → 2H₂PO₄

When red phosphorus is cooked with conc.HNO₃, orthophosphoric acid is formed.

P + 5HNO₃ → H₃PO₄ + H₂O + 5NO₂

It is produced on a big scale by processing phosphorite rock with dilute H₂SO₄.

Ca₃(PO₄)₂ + 3H₂SO₄ → 3CaSO₄ + 2H₃PO₄

Synthesis of Phosphoric Acid – H₃PO₄

Sulphuric acid is added to the phosphate of baryta suspended in water, and the sulphuric acid reacts with the baryta to generate an insoluble salt that precipitates while the phosphoric acid stays in solution. When phosphorus is slowly added to nitric acid, phosphoric acid is formed, which is mixed with the remaining nitric acid.

Physical Properties of Phosphoric Acid (H₃PO₄)

1. Phosphoric acid is an odourless, colourless, viscous liquid with a high degree of reddening litmus property. It cannot be obtained without the use of water.

2. When a transparent fragile glass is subjected to red heat and then cooled, it develops.

3. Because phosphoric acid damages glass and porcelain when heated to redness, this fusion should be done in a platinum crucible.

4. When the acid is studied after being exposed to heat, it is shown to have acquired new qualities, despite the fact that its composition remains unchanged.

5. It has been given the label para phosphoric as a result of this, while the term phosphoric is used to refer to the acid in the first stated condition.

6. Silver nitrate produces a yellow precipitate with phosphoric acid and a white precipitate with para phosphoric acid. The latter, but not the former, coagulates albumen.

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Chemical Properties of Phosphoric Acid (H₃PO₄)

Phosphoric acid is a solid that is commonly found in a viscous aqueous solution.

It is slightly acidic, with three probable deprotonation stages leading to the formation of phosphates.
Phosphoric acid, like carboxylic acids, can dimerize to generate phospho anhydrides by a dehydration reaction.
Phosphoric acid is classified as tribasic because it can dissociate into three different forms.
As a result, phosphoric acid mono, disodium, and potassium salts are commonly utilised as pH buffers.
Multimerization is one of the most important reactions involving phosphoric acid and its derivatives.
Two phosphoric acid molecules may interact with the loss of water to generate a diphosphate ester, also known as pyrophosphate, similar to carboxylic acids.
However, because phosphoric acid has more -OH functions, triphosphates can form.
Phosphoric acid salts are solid and, unless a strong mineral acid is present, many are generally water insoluble.

Basicity of Orthophosphoric Acid (H₃PO₄)

The quantity of hydrogen atoms that can be substituted by electro – positive atoms determines an acid's basicity. The quantity of hydrogen ions that can be created by ionising one molecule of an acid in its aqueous solution is known as basicity. A tribasic acid, also known as a triprotic acid, is an acid that creates three hydrogen or hydronium ions when one molecule of the acid is ionised. H₃PO₄ and H₃PO₃ are tribasic acids, for example.

Acid	Basicity
Hypophosphorous acid (H₃PO₂)	Oxidation number of Phosphorus = +1 Basicity = 1
Phosphorus acid (H₃PO₃)	Oxidation number of Phosphorus = +3 Basicity = 2
Orthophosphoric acid (H₃PO₄)	Oxidation number of Phosphorus = +5 Basicity = 3
Pyrophosphoric acid (H₄P₂O₇)	Oxidation number of Phosphorus = +5 Basicity = 4
Metaphosphoric acid (HPO₃)	Oxidation number of Phosphorus = +5 Basicity = 1

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Uses of Phosphoric Acid (H₃PO₄)

Phosphoric acid (H₃PO₄) is used in a variety of ways, including the production of fertilisers. Many acids are made from phosphate rocks via a wet process that involves phosphate rocks reacting with acid solutions. H₃PO₄ is a medium-strong acid that is very corrosive to ferrous and ferrous alloys.

It is used in fertilisers - approximately 90% of the phosphoric acid generated is utilised in fertilisers.
It's a supplement feed for pigs, cattle, and poultry.
It is used as a pH adjuster in skincare and cosmetics.
It is used as a sanitising agent in the brewing, food, and dairy sectors.
It is used to acidify beverages and foods such as jam and cola.
It is used to eliminate rust from metals surfaces.

Health Hazard

Concentrated sulphuric or nitric acid are more corrosive and dangerous than phosphoric acid. The skin and mucous membranes are irritated by its concentrated solutions. P₂O₅ fumes in the vapours can irritate the throat and induce coughing, however at 10mg/m³ they might be tolerated. Phosphoric acid, in both solid and liquid form, is noncombustible. If inhaled, it can be lethal. Fire smoke contains phosphine, phosphoric acid, and hydrogen chloride, which can irritate respiratory passages, eyes, and skin.

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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. 1. What is orthophosphoric acid used for?

Phosphoric acid (H₃PO₄), also known as orthophosphoric acid, is the most important oxygen acid of phosphorus and is used to generate fertiliser phosphate salts. This is also utilised in the production of dental cements, albumin derivatives, and the sugar and textile industries.

2. 2. What happens when orthophosphoric acid is heated?

The orthophosphoric anhydrous acid is a crystalline white solid with a melting point of 42.35 degrees Celsius. There will be a loss of component oxygen when phosphoric acid is heated to temperatures of roughly 200 ° C. Dehydration produces a series of acids, from pyrophosphoric acid (H₄P₂O₇) through metaphosphoric acid (H₄P₂O₈).

3. 3. Why is phosphoric acid used in coke?

Phosphoric acid is a colourless and odourless crystalline liquid. It delivers soft drinks a tart flavour while also preventing the growth of mould and germs, which may easily flourish in a sugar solution. The phosphoric acid in soda contributes a significant amount of its acidity. It is then treated again to become phosphoric acid.

4. 4.What are the side effects of orthophosphoric acid?

Ingestion causes burning of the lips, tongue, throat, and stomach. Nausea, vomiting, stomach pains, and diarrhoea are all possible symptoms. It's possible that irreparable damage will occur. Long-term planning ( Chronic) Exposure's effects include: Skin contact at low concentrations might result in raw, red, peeling skin (dermatitis).

5. 5. Does phosphoric acid dissolve rust?

Few stronger acids will completely eradicate rust, but they will etch and pit the surface of any remaining ferrous metal, weakening it. One of the few acids that may eliminate rust without oxidising or damaging the iron beneath it is phosphoric acid.

6. 6.What is the metaphosphoric acid formula ?

The chemical formula for metaphosphoric acid is HPO₃

7. Why is phosphoric acid considered a triprotic acid?

Phosphoric acid is considered a triprotic acid because it can donate three protons (H+ ions) in aqueous solutions. It undergoes three-step dissociation, releasing one proton at each step, resulting in the formation of dihydrogen phosphate, hydrogen phosphate, and phosphate ions.

8. How does the structure of phosphoric acid contribute to its acidity?

The structure of phosphoric acid contributes to its acidity through the presence of three hydroxyl (-OH) groups attached to the central phosphorus atom. These OH groups can release protons (H+ ions) in solution, making it a triprotic acid. The electron-withdrawing effect of the oxygen atoms also increases the acid's ability to donate protons.

9. How does phosphoric acid behave as a weak acid in solution?

As a weak acid, phosphoric acid only partially dissociates in aqueous solutions. It releases protons (H+ ions) in a stepwise manner, with each step having a different dissociation constant (Ka). This partial dissociation results in the coexistence of the acid and its conjugate bases in solution, maintaining a dynamic equilibrium.

10. How does phosphoric acid compare to other common acids in terms of strength?

Phosphoric acid is a moderately strong acid compared to other common acids. It is stronger than acetic acid (found in vinegar) but weaker than hydrochloric acid or sulfuric acid. Its first dissociation step has a relatively high Ka value, but subsequent dissociations are much weaker, making it less corrosive than strong mineral acids.

11. What is the significance of phosphoric acid in biochemistry?

In biochemistry, phosphoric acid plays crucial roles:

12. How does phosphoric acid interact with metals?

Phosphoric acid interacts with metals in several ways:

13. How does temperature affect the properties of phosphoric acid?

Temperature affects phosphoric acid properties in several ways:

14. How does the concentration of phosphoric acid affect its properties?

The concentration of phosphoric acid significantly affects its properties:

15. What is the difference between ortho-phosphoric acid and meta-phosphoric acid?

Ortho-phosphoric acid (H3PO4) is the common form of phosphoric acid, while meta-phosphoric acid (HPO3) is a dehydrated form:

16. How does phosphoric acid participate in buffer solutions?

Phosphoric acid participates in buffer solutions due to its multiple dissociation steps:

17. What is the chemical formula of phosphoric acid?

The chemical formula of phosphoric acid is H3PO4. This formula represents one phosphorus atom bonded to four oxygen atoms, with three hydrogen atoms attached to three of the oxygen atoms.

18. What is the significance of phosphoric acid in the phosphorus cycle?

Phosphoric acid plays a crucial role in the phosphorus cycle:

19. How does phosphoric acid influence soil chemistry?

Phosphoric acid influences soil chemistry in several ways:

20. What are the isotopic variations of phosphorus in phosphoric acid, and their significance?

Phosphorus in phosphoric acid primarily exists as the stable isotope ^31P. However, trace amounts of radioactive ^32P and ^33P can occur:

21. How does phosphoric acid participate in organophosphorus chemistry?

Phosphoric acid is crucial in organophosphorus chemistry:

22. What are the spectroscopic characteristics of phosphoric acid?

Spectroscopic characteristics of phosphoric acid include:

23. How does phosphoric acid compare to other phosphorus oxoacids?

Phosphoric acid (H3PO4) compares to other phosphorus oxoacids as follows:

24. What are the thermodynamic properties of phosphoric acid reactions?

Thermodynamic properties of phosphoric acid reactions include:

25. How does phosphoric acid interact with biological membranes?

Phosphoric acid interacts with biological membranes in several ways:

26. How does phosphoric acid behave in non-aqueous solvents?

In non-aqueous solvents, phosphoric acid behaves differently:

27. How is phosphoric acid prepared on an industrial scale?

Phosphoric acid is primarily prepared on an industrial scale through two methods:

28. What are the challenges in the purification of phosphoric acid?

Purification of phosphoric acid faces several challenges:

29. What is the environmental impact of phosphoric acid production and use?

The environmental impact of phosphoric acid includes:

30. What is the role of phosphoric acid in the production of phosphate fertilizers?

Phosphoric acid is a key component in the production of phosphate fertilizers. It reacts with various compounds to form different types of phosphate fertilizers:

31. How does phosphoric acid behave as a dehydrating agent?

Phosphoric acid acts as a dehydrating agent due to its ability to form condensed phosphates:

32. What are the main industrial uses of phosphoric acid?

The main industrial uses of phosphoric acid include:

33. What is the role of phosphoric acid in cola drinks?

In cola drinks, phosphoric acid serves multiple purposes:

34. What is the difference between food-grade and industrial-grade phosphoric acid?

Food-grade phosphoric acid is highly purified and meets strict quality standards for use in food and beverages. It contains minimal impurities and is safe for human consumption. Industrial-grade phosphoric acid may contain higher levels of impurities and is used for non-food applications such as metal treatment or fertilizer production.

35. What safety precautions should be taken when handling phosphoric acid?

When handling phosphoric acid, important safety precautions include:

36. How does phosphoric acid contribute to eutrophication?

Phosphoric acid can contribute to eutrophication when phosphate-containing fertilizers or detergents enter water bodies. The excess phosphorus promotes algal blooms, which can deplete oxygen levels in the water, harming aquatic life. This highlights the importance of proper use and disposal of phosphoric acid and its derivatives in agricultural and industrial applications.

37. What is the role of phosphoric acid in tooth enamel interactions?

Phosphoric acid interacts with tooth enamel in several ways:

38. How does phosphoric acid affect protein structure and function?

Phosphoric acid affects protein structure and function through several mechanisms:

39. How does phosphoric acid contribute to corrosion inhibition?

Phosphoric acid contributes to corrosion inhibition in several ways:

40. What is the role of phosphoric acid in the production of pharmaceuticals?

In pharmaceutical production, phosphoric acid serves several purposes:

41. What is the role of phosphoric acid in flame retardants?

Phosphoric acid plays a role in flame retardants through several mechanisms:

42. What is the role of phosphoric acid in the production of polyphosphates?

Phosphoric acid is key in polyphosphate production: