Polyprotic Acids

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.

K_a1 > K_a2 > K_a3

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)

   K_a1 = [H⁺][H₂PO₄^-]/[H₃PO₄] = 7.5 x 10^-3

2. H₂PO₄^-(aq) ⇔ H⁺(aq) + HPO₄^2-(aq)

   K_a2 = [H⁺][HPO₄^2-]/[H₂PO₄^-] = 6.2 x 10^-8

3. HPO₄^2-(aq) ⇔ H⁺(aq) + PO₄^3-(aq)

   K_a3 = [H⁺][PO₄^3-]/[HPO₄^2-] = 4.8 x 10^-13

Learn More

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