Orthophosphoric Acid - H3PO4

What is Phosphoric Acid?

Orthophosphoric acid refers to phosphoric acid. Phosphoric Acid is a weak acid with chemical formula H3PO4.

Phosphoric Acid is an acid-containing four atoms of oxygen, one atom of phosphorus, and three atoms of hydrogen. It is also known as phosphoric(V) acid or orthophosphoric acid. It is present in teeth and bone and helps in metabolic processes. In its liquid form, it appears as a clear, colorless solution and in its solid form, it appears as transparent, crystalline solid.

As a sequestering agent, it helps in the binding of divalent cations. It is widely used in orthodontics and dentistry. Phosphoric acid is important in biogeochemistry and biochemistry. It is tribasic acid. The three regions are the region where pH is equal to pK, the region where pH is equal to 4.7 and the region where pH is equal to 9.8.

Structure of Phosphoric Acid (H3PO4)

Phosphoric Acid structure

Structure of phosphoric acid

Synthesis of Phosphoric Acid – H3PO

  • Phosphoric acid may be obtained by adding sulfuric acid to the phosphate of baryta suspended in water.
  • The sulfuric acid unites with the baryta forming an insoluble salt which precipitates while the phosphoric acid remains in solution.
  • When phosphorus is gradually added to nitric acid, phosphoric acid is generated and remains mingled with the residual nitric acid.

Physical Properties of Phosphoric Acid (H3PO4)

  1. Phosphoric acid is an odorless, colorless, viscous liquid, possessing in a high degree the property of reddening litmus. It cannot be obtained free from water.
  2. When exposed to a red heat and afterward cooled it forms a transparent brittle glass.
  3. This fusion should be effected in a platinum crucible since phosphoric acid when heated to redness attacks either glass or porcelain.
  4. The acid if examined after this exposure to heat is found although its composition remains the same to have acquired new properties.
  5. On this account the name of paraphosphoric has been given to it; while the term phosphoric is applied to designate the acid in the state first described.
  6. Nitrate of silver yields with phosphoric acid a yellow precipitate with paraphosphoric acid a white one. Albumen is coagulated by the latter but not by the former.
H3PO4 Phosphoric Acid
Molecular Weight/ Molar Mass 97.994 g/mol
Density 1.88 g/cm³
Boiling Point 158 °C
Melting Point 42.35 °C

Chemical Properties of Phosphoric Acid (H3PO4)

  • Phosphoric acid is a deliquescent solid, generally encountered as a viscous aqueous solution.
  • It is weakly acidic, with three possible sequential deprotonation steps, forming phosphates.
  • Like carboxylic acids, phosphoric acid can dimerize via a dehydration reaction to form phospho anhydrides.
  • Phosphoric acid is referred to as being tribasic, in that it has three possible dissociation steps.
  • As a result mono and disodium and potassium salts of phosphoric acid are routinely used as pH buffers.
  • One of the most important reactions of phosphoric acid and its derivatives is multimerization.
  • As with carboxylic acids, two phosphoric acid molecules may combine with the loss of water to form a diphosphate ester also referred to as pyrophosphate.
  • However, as phosphoric acid has further -OH functionalities triphosphates may also be formed.
  • Salts of phosphoric acid are solid and many are relatively water-insoluble unless a strong mineral acid is present.

Uses of Phosphoric Acid (H3PO4)

  • It is used as fertilizers – around 90% of phosphoric acid produced is used as fertilizers
  • It is used in as a supplement feed for pigs, cattle, poultry
  • It is used in skincare products, cosmetics as a pH adjuster
  • It is used in brewing, food and dairy industries as a sanitizing agent
  • It is used in beverages and food like jam and cola to acidify them
  • It is used to remove rust from the surface to metals

Health Hazard

Phosphoric acid is less corrosive and hazardous than is concentrated sulfuric or nitric acid. Its concentrated solutions are irritants to the skin and mucous membranes. The vapors contain P2O5 fumes can cause irritation to the throat and coughing but could be tolerated at <10mg/m3.

Phosphoric acid is a noncombustible substance in both solid and liquid forms. It may be fatal if inhaled. Fumes from fires are irritating to respiratory passages, eyes, skin, and may contain phosphine, phosphoric acid, hydrogen chloride.

Frequently Asked Questions

1. Phosphoric acid is a strong acid. True or false?

Ans: False. Phosphoric acid is a weak acid.

2. What is the amount of phosphoric acid present in coke?

Ans: The average amount per can of coke has 17 mg per 100 ml of Coca cola.

3. List two uses of phosphoric acid.

Ans: It is used in fertilizers and in dental cements.

4. Which is stranger? Hydrochloric acid or phosphoric acid.

Ans: Hydrochloric acid is stranger than phosphoric acid.

5. Phosphoric acid is combustible. True or false?

Ans: False. It is non combustible.

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Learn more about the chemical behavior and importance of H3PO4 with the expert faculties at BYJU’S – India’s largest education company.

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