Polyprotic acids are acids that can donate more than one proton, H+, per molecule. Examples include carbonic acid, H2CO3, sulfuric acid, H2SO4, and phosphoric acid, H3PO4. They matter because each proton is released in a separate step, so their solutions can contain several related chemical species at once.
This stepwise behavior controls pH, buffering, titration curves, and many biological and environmental systems.
Each ionization step has its own acid dissociation constant, such as Ka1, Ka2, and Ka3. Usually Ka1 is larger than Ka2, and Ka2 is larger than Ka3, because it becomes harder to remove a positive proton from an increasingly negative ion. At different pH values, different forms of the acid dominate, such as H3A, H2A-, HA2-, or A3-.
During titration with a strong base, a polyprotic acid can produce multiple buffer regions and multiple equivalence points if the Ka values are far enough apart.
Key Facts
- A polyprotic acid can donate two or more protons in separate ionization steps.
- For a triprotic acid: H3A ⇌ H+ + H2A-, Ka1 = [H+][H2A-]/[H3A].
- Second ionization: H2A- ⇌ H+ + HA2-, Ka2 = [H+][HA2-]/[H2A-].
- Third ionization: HA2- ⇌ H+ + A3-, Ka3 = [H+][A3-]/[HA2-].
- For most polyprotic acids, Ka1 > Ka2 > Ka3, so pKa1 < pKa2 < pKa3.
- At pH = pKa for a step, the acid and conjugate base pair for that step have equal concentrations.
Vocabulary
- Polyprotic acid
- An acid that can donate more than one proton per molecule in separate steps.
- Stepwise ionization
- The process in which a polyprotic acid loses its protons one at a time, with a different equilibrium constant for each step.
- Ka
- The acid dissociation constant that measures how strongly an acid donates a proton in a specific equilibrium step.
- Dominant species
- The acid or conjugate base form present at the highest concentration at a particular pH.
- Equivalence point
- The point in a titration where the moles of added base match the moles of acidic protons neutralized for a given stage.
Common Mistakes to Avoid
- Treating all protons as if they dissociate at the same time is wrong because each proton has its own Ka value and equilibrium step.
- Assuming Ka1, Ka2, and Ka3 are equal is wrong because removing each later proton is usually harder as the remaining species becomes more negatively charged.
- Using the total acid concentration as the concentration of every species is wrong because the acid is distributed among forms such as H3A, H2A-, HA2-, and A3- depending on pH.
- Missing multiple equivalence points in a titration is wrong when the acid has well-separated Ka values, because each removable proton can create a separate neutralization stage.
Practice Questions
- 1 Phosphoric acid has pKa1 = 2.15, pKa2 = 7.20, and pKa3 = 12.35. At what pH are [H2PO4-] and [HPO4 2-] equal?
- 2 A 25.0 mL sample of 0.100 M H2A is titrated with 0.100 M NaOH. What volume of NaOH is needed to reach the first equivalence point, and what volume is needed to reach the second equivalence point?
- 3 A triprotic acid H3A has pKa values of 2.0, 6.5, and 11.0. Explain which species is dominant at pH 4.0 and why it is between two ionization steps.