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Introduction
Polyprotic acids can release multiple protons (H⁺) per molecule.
Diprotic Acids: Acids that can release two protons per molecule.
Triprotic Acids: Acids that can release three protons per molecule.
These acids dissociate in steps. Each step has its own acid dissociation constant:
K₁ - Usually the strongest dissociation step.
K₂
K₃ - For triprotic acids only, usually the weakest step.
For example, here is the dissociation of phosphoric acid, a triprotic acid:
H₃PO₄ (aq) ⇌ H⁺ (aq) + H₂PO₄⁻ (aq) K₁ = 7.5 × 10⁻³
H₂PO₄⁻ (aq) ⇌ H⁺ (aq) + HPO₄²⁻ (aq) K₂ = 6.2 × 10⁻⁸
HPO₄²⁻ (aq) ⇌ H⁺ (aq) + PO₄³⁻ (aq) K₃ = 4.8 × 10⁻¹⁴
Solving Problems with Polyprotic Acids
It gets progressively harder to donate the next proton. The values of K₂ and K₃ are usually so miniscule (<10⁻³) that they are negligible in determining pH. Therefore, you can solve for the pH primarily based on the value of K₁.
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