Skip to main content
WAChemistrySyllabus dot point

How do acids that can donate more than one proton ionise, and why does each successive ionisation get weaker?

Describe the stepwise ionisation of polyprotic acids and explain why successive ionisation constants decrease

A focused answer to the WACE Year 12 Chemistry dot point on polyprotic acids, their stepwise ionisation, why each successive Ka is smaller, and how this affects pH and titrations, with a worked example and common exam mistakes.

Generated by Claude Opus 4.76 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

What this dot point is asking

A monoprotic acid such as hydrochloric or ethanoic acid donates only one proton per molecule. A polyprotic acid can donate more than one: diprotic acids (two protons, such as sulfuric acid H2SO4\text{H}_2\text{SO}_4 and carbonic acid H2CO3\text{H}_2\text{CO}_3) and triprotic acids (three protons, such as phosphoric acid H3PO4\text{H}_3\text{PO}_4).

Stepwise ionisation

A polyprotic acid does not lose all its protons at once. Phosphoric acid ionises in three distinct steps:

H3PO4⇌H++H2PO4−Ka1\text{H}_3\text{PO}_4 \rightleftharpoons \text{H}^+ + \text{H}_2\text{PO}_4^- \quad K_{a1}

H2PO4−⇌H++HPO42−Ka2\text{H}_2\text{PO}_4^- \rightleftharpoons \text{H}^+ + \text{HPO}_4^{2-} \quad K_{a2}

HPO42−⇌H++PO43−Ka3\text{HPO}_4^{2-} \rightleftharpoons \text{H}^+ + \text{PO}_4^{3-} \quad K_{a3}

Each step has its own equilibrium constant, and the values fall steeply, typically by a factor of around 10510^5 each time.

Why successive constants decrease

After the first proton leaves, the remaining species carries a negative charge. Removing a second positive proton (H+\text{H}^+) from an already negatively charged ion requires overcoming a stronger electrostatic attraction, so it is harder and less favourable. Removing a third proton from a doubly negative ion is harder still. This is why

Ka1≫Ka2≫Ka3K_{a1} \gg K_{a2} \gg K_{a3}

Sulfuric acid is a special case

Sulfuric acid is strong in its first ionisation (H2SO4→H++HSO4−\text{H}_2\text{SO}_4 \rightarrow \text{H}^+ + \text{HSO}_4^-, essentially complete) but weak in its second (HSO4−⇌H++SO42−\text{HSO}_4^- \rightleftharpoons \text{H}^+ + \text{SO}_4^{2-}, an equilibrium). This mixed behaviour is worth remembering because it differs from a fully weak acid like phosphoric acid.

Effect on titrations

Because the protons come off in distinct steps, a titration of a polyprotic acid against a strong base can show more than one equivalence point, one for each proton being neutralised. A diprotic acid can show two end points and a triprotic acid up to three, provided the successive constants are far enough apart to resolve them.

Why this matters

Polyprotic acids appear throughout chemistry and biology: carbonic acid buffers blood and oceans, phosphoric acid buffers cells, and sulfuric acid is the most produced industrial chemical. Understanding stepwise ionisation explains their buffering, their titration curves and their pH.

Discuss this in the community ->