Polyprotic Acid

What is an Acid?

According to Bronsted and Lowry, acid is a substance capable of donating a proton.

A substance capable of donating one proton is known as Monoprotic Acid.

In contrast, a substance capable of donating two protons is known as Diprotic Acid.

A substance capable of donating more than one proton is known as Polyprotic Acid.

Table of Content

• What is a Polyprotic Acid?
• Examples of Polyprotic Acid
• Demonstration of Polyprotic Acid
• Dissociation Constant value of different Polyprotic Acid
• What is a Dissociation Constant
• FAQ

What is a Polyprotic Acid?

A substance capable of donating more than one proton is known as Polyprotic Acid. Diprotic and Triprotic are specific types of polyprotic acid capable of donating two and three protons, respectively.

Polyprotic acids have multiple dissociation constants, such as Ka₁, Ka₂, Ka₃ and equivalence points, depending on the number of times dissociation occurs.

Examples of Polyprotic Acid

• Sulfuric Acid (H₂SO₄) is an example of a diprotic polyprotic acid. Consecutive deprotonation of sulfuric acid (H₂SO₄) yields HSO₄^– and SO₄^2- respectively.
• Sulfurous Acid (H₂SO₃) is an example of a diprotic polyprotic acid. Consecutive deprotonation of sulfurous acid (H₂SO₃) yields HSO₃^– and SO₃^2- respectively.
• Orthophosphoric Acid (H₃PO₄) is an example of a triprotic polyprotic acid. Consecutive deprotonation of orthophosphoric acid (H₃PO₄) yields H₂PO₄^–, HPO₄^2-, and PO₄^3- respectively.
• Carbonic Acid (H₂CO₃) is an example of a diprotic polyprotic acid. Consecutive deprotonation of carbonic acid (H₂CO₃) yields HCO₃^– and CO₃^2- respectively.

Demonstration of Polyprotic Acid

We can demonstrate polyprotic acid using the Titration Curve. A titration curve has multiple dissociations constants as Ka₁, Ka₂, Ka₃ etc. and equivalence points.

Dissociation Constant value of different Polyprotic Acid

The dissociation constant tells us about the solution’s acidity. The greater the dissociation constant, the more elevated would be its acidity.

Here is the value of the dissociation constant of a few acids.

Acid with the highest dissociation constant Ka value will dissociate readily. Sulfuric acid (1) will dissociate readily from the nine listed in the table.

• The value of the first dissociation constant Ka is highest, followed by the second and so on.
• After every successive proton loss, the proton becomes less acidic.

What is a Dissociation Constant (Ka)?

Ka or Acid dissociation constant measures the strength of an acid. It also tells us how much an acid dissociates in an aqueous solution.

Consider dissociation of acid HA:

HA ⥦ H⁺ + A^–

For the above reaction, the dissociation constant Ka would be equivalent to