Orthophosphoric Acid - H3PO4 (Phosphoric Acid)

What is Phosphoric Acid?

Orthophosphoric acid refers to phosphoric acid. Phosphoric Acid is a weak acid with the 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 bones and helps in metabolic processes. In its liquid form, it appears as a clear, colourless solution and in its solid form, it appears as a transparent, crystalline solid.

Table of Content

• Structure of orthophosphoric Acid H₃PO₄
• Physical Properties of Phosphoric Acid H₃PO₄
• Preparation of orthophosphoric Acid H₃PO₄
• Synthesis of Phosphoric Acid – H₃PO₄
• Chemical Properties of Phosphoric Acid H₃PO₄
• Basicity of Orthophosphoric Acid H₃PO₄
• Uses of Phosphoric Acid H₃PO₄
• Health Hazard of Phosphoric acid
• FAQs

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 a tribasic acid.  All three hydrogens are acidic, the pKa value of first, second and third hydrogen are respectively pKa₁ = 2.14, pKa₂ = 7.20, and pKa₃ = 12.37

Read more: Uses of Phosphoric Acid

Structure of Phosphoric Acid (H₃PO₄)

Structure of phosphoric acid

Physical Properties of Phosphoric Acid (H₃PO₄)

1. Phosphoric acid is an odourless, colourless, 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 para phosphoric 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 para phosphoric acid a white one. Albumen is coagulated by the latter but not by the former.

Preparation of Phosphoric acid

It can be conveniently prepared by dissolving P₂O₅ in water followed by boiling the solution to form a thick syrup.

P₂O₅ + 3H₂O ⇌ 2H₃PO₄

Red phosphorus when heated with conc. HNO₃ yields orthophosphoric acid.

P + 5HNO₃ → H₃PO₄ + H₂O + 5NO₂

On a large scale, it is prepared by treating phosphorite rock with dil.H2SO4.

Ca₃(PO₄)₂ + 3H₂SO₄ → 3CaSO₄ + 2H₃PO₄

Synthesis of Phosphoric Acid – H₃PO₄ 

• Phosphoric acid may be obtained by adding sulphuric acid to the phosphate of baryta suspended in water.
• The sulphuric 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.

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 undergoes dehydration in three steps and is referred to as tribasic.
• As a result mono and di sodium 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 di phosphate 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.

Basicity of Orthophosphoric Acid (H₃PO₄)

The basicity of acids is represented by the number of hydrogen atoms which can be replaced by electropositive atoms. Basicity of an acid is the number of hydrogen ions that can be produced by the ionization of one molecule of the acid in its aqueous solution.

An acid which produces three hydrogen or hydronium ions by the ionization of one molecule of the acid, is called a tribasic acid or tri protic acid. For example, H₃PO₄ and H₃PO₃ are tribasic acids.

Uses of Phosphoric Acid (H₃PO₄)

Phosphoric acid (H₃PO₄) has many essential applications, in particular in the manufacture of fertilizers. Many acids are derived from phosphate rocks by a wet process based on the reaction between phosphate rocks and acid solutions. This acid (H₃PO₄) is a medium-strong acid, but is also highly corrosive to ferrous or ferrous alloys.

• Around 90% of phosphoric acid produced is used as fertilizers
• It is used 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 sulphuric or nitric acid. Its concentrated solutions are irritants to the skin and mucous membranes. The vapours 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, and hydrogen chloride.

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