Unit 8 Notes Topic 10 AP CHEMISTRY Titration Curves of Polyprotic Acids

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🧪 Polyprotic Acid Titration Curves

Unit 8 | Topic 10 – Multistep pH Curves & pKa Points

🌟 What Are Polyprotic Acids?

Polyprotic acids are acids that contain more than one ionizable (donatable) hydrogen atom. Each hydrogen ion (H⁺) is released in a separate step, meaning these acids ionize in stages.

🔬 Examples: H₂SO₄ (sulfuric acid), H₃PO₄ (phosphoric acid)

🔁 Stepwise Ionization of H₃PO₄

Step	Reaction
1	H₃PO₄ ⇌ H₂PO₄⁻ + H⁺
2	H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺
3	HPO₄²⁻ ⇌ PO₄³⁻ + H⁺

Each step has its own Ka value and thus a different pKa and equivalence point on the pH curve!

📈 The Curve: What Makes Polyprotic Titrations Special?

When titrating a polyprotic acid like H₃PO₄ with a strong base (like NaOH), you’ll see a multi-bump titration curve with multiple equivalence points and buffer regions.

• 🟢 1st Equivalence Point: H₃PO₄ → H₂PO₄⁻
• 🟡 2nd Equivalence Point: H₂PO₄⁻ → HPO₄²⁻
• 🔴 3rd Equivalence Point: HPO₄²⁻ → PO₄³⁻

Between each equivalence point lies a buffer region where a weak acid and its conjugate base are in balance.

🧠 Understanding Half-Equivalence Points

• Half-equivalence = exactly halfway to the equivalence point.
• At this point: [Acid] = [Conjugate Base]
• This means:
  pH = pKa
• You’ll get 3 half-equivalence points for a triprotic acid like H₃PO₄.

✅ Example: At midpoint between H₂PO₄⁻ ⇌ HPO₄²⁻, [H₂PO₄⁻] = [HPO₄²⁻] → pH = pKa₂

📐 Curve Breakdown Summary

• Multiple bumps or jumps: One for each H⁺ ion released
• Multiple buffer zones: Occur between each pair of steps
• Each inflection: Is an equivalence point (neutralization step complete)
• Each midpoint: Is a half-equivalence → pH = pKa

⚠️ Common Mistake: Students often draw only 1 equivalence point for polyprotic acids. Don’t forget: H₃PO₄ has THREE! The curve should show 3 separate rises with flat buffer sections between.

🧪 Formula Recap: Ka Expression

Ka = [A⁻][H⁺] / [HA]

This helps calculate Ka at each step using concentrations at equilibrium.