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.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
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.
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Anne Marie Helmenstine, Ph.D.
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?

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+

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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