Polyprotic Acids

Introduction to Polyprotic Acids

Phosphoric acid is a polyprotic acid.
Phosphoric acid is a polyprotic acid. Laguna Design / Getty Images

There are many different types of acids. This is an introduction to polyprotic acids, with an example of the ionization steps of a polyprotic acid.

What Is a Polyprotic Acid?

A polyprotic acid is an acid which contains more than one ionizable hydrogen (H+) per acid molecule. The acid ionizes one step at a time in an aqueous solution, with a separate ionization constant for each step. The initial dissociation is the primary source of H+, so it is the main factor in determining the pH of the solution.
The ionization constant is lower for subsequent steps.

Ka1 > Ka2 > Ka3

Example of a Polyprotic Acid

Phosphoric acid (H3PO4) is an example of a triprotic acid. Phosphoric acid ionizes in three steps:
  1. H3PO4(aq) ⇔ H+(aq) + H2PO4-(aq)

    Ka1 = [H+][H2PO4-]/[H3PO4] = 7.5 x 10-3

  2. H2PO4-(aq) ⇔ H+(aq) + HPO42-(aq)

    Ka2 = [H+][HPO42-]/[H2PO4-] = 6.2 x 10-8

  3. HPO42-(aq) ⇔ H+(aq) + PO43-(aq)

    Ka3 = [H+][PO43-]/[HPO42-] = 4.8 x 10-13

Learn More

Polyprotic Acid and Strong Base Titration Curve
Titration Basics
Introduction to Acids and Bases