Is Phosphoric Acid (H₃PO₄) a Strong or Weak Acid?
Is Phosphoric Acid (H₃PO₄) a strong or weak acid?
H3PO4 is a triprotic weak acid. Each successive dissociation is weaker: pKa1=2.15, pKa2=7.2, pKa3=12.4. It is harder to remove a proton from an increasingly negative species.
| Formula | H₃PO₄ |
| Name | Phosphoric Acid |
| Category | Weak acid |
| pKa | 2.15 |
| Conjugate | Dihydrogen phosphate (H₂PO₄⁻) |
| Key Concept | Polyprotic weak acid |
Definition
Phosphoric acid can donate three protons sequentially. None of its dissociations are complete (all weak), but the first is the strongest.
Acidic Proton / Active Site
All three -OH hydrogens can be donated, but they come off one at a time. The first is easiest (pKa 2.15), and each subsequent one requires more energy.
Conjugate Pair
H3PO4 -> H2PO4- -> HPO4^2- -> PO4^3-. Each step produces a more negatively charged conjugate base, which holds onto its remaining protons more tightly.
Strength Classification
pKa1 = 2.15 (weak acid). Phosphoric acid is used in soft drinks and biological buffers. Its three pKa values allow it to buffer at multiple pH ranges.
See acidic protons, conjugate base overlays, and pKa labels on interactive 3D molecules.
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