Polyprotic Acids

Introduction to Polyprotic Acids

Phosphoric acid is a polyprotic acid.
Phosphoric acid is a polyprotic acid. Laguna Design / Getty Images
by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated March 06, 2017

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?

+
+

Ka1 > Ka2 > Ka3

Example of a Polyprotic Acid

3
4
  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

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