Intermediate Topic 527 of 1102 Acids, Bases & Electrochemistry
Polyprotic Acids
Acids with more than one ionizable proton, each step having a smaller Ka.
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In-depth explanation of Polyprotic Acids
Polyprotic acids can release more than one proton, and each release has its own constant, with Ka1 much greater than Ka2, which is much greater than Ka3. Each step is weaker because removing a proton from an already negative ion is harder, since the remaining charge holds the next proton more tightly. In most cases the first step controls the pH, and the later steps contribute only small corrections. Intermediate species such as the bicarbonate ion are amphiprotic, able to act as both acid and base. Polyprotic acids are central to biology, carbonate chemistry and natural buffering.
Going deeper: Polyprotic Acids
Key ideas: Each proton comes off in stages with Ka1 much greater than Ka2 much greater than Ka3 (H3PO4: 7.5e-3, 6.2e-8, 4.2e-13). Usually pH is determined by Ka1 alone, and [A2-] equals Ka2 when only the first dissociation matters. Species distribution depends on pH relative to each pKa.
Common mistakes and connections: Mistakes: treating each dissociation as equal in [H+] contribution, or multiplying pH calculations three times. Sulfuric acid is the exception: its first step is strong, so second-step calculations need a true ICE table. Each successive proton is harder to remove because of increasing negative charge. Titration curves show multiple equivalence points.
Visualization of Polyprotic Acids
Titrating phosphoric acid shows three buffer plateaus at pKa 2.15, 7.20 and 12.35, with each proton harder to remove than the last. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Phosphoric acid has successive pKa values of 2.15, 7.20 and 12.35, so it provides useful buffering near pH 2, pH 7 and pH 12.
Frequently asked questions about Polyprotic Acids
What is Polyprotic Acids?
Acids with more than one ionizable proton, each step having a smaller Ka.
Is Polyprotic Acids a beginner, intermediate or advanced topic?
Polyprotic Acids is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Polyprotic Acids?
Study these earlier topics in Acids, Bases & Electrochemistry first: Ka and Kb Relationship, Percent Ionization, pH of Weak Acid Solutions.
What comes after Polyprotic Acids?
The next topics in Acids, Bases & Electrochemistry are Acid Strength and Structure and Hydrolysis of Salts.
Can I practise Polyprotic Acids with quizzes?
Yes. The interactive lesson includes a self-check question on Polyprotic Acids, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Polyprotic Acids fits in Acids, Bases & Electrochemistry
Polyprotic Acids is topic 17 of 105 in Acids, Bases & Electrochemistry.