Monoprotic vs Polyprotic Acids: The Difference
Acetic acid, CH₃COOH, contains four hydrogen atoms and donates exactly one. Phosphoric acid, H₃PO₄, contains three and donates all three — but not equally, and not at the same pH. Counting acidic hydrogens correctly is the difference between a titration answer that works and one that is out by a factor of two or three.
The short answer: a monoprotic acid can donate one proton per molecule; a polyprotic acid can donate more than one, losing them one at a time in separate steps with steadily smaller Ka values. Acidic hydrogens are the ones attached to an electronegative atom — most often O, sometimes N, S or a halogen — and never the ones on a plain C–H bond.
Quick comparison at a glance
| Feature | Monoprotic | Polyprotic |
|---|---|---|
| Protons donated | Exactly 1 | 2 or more — diprotic (2), triprotic (3), and beyond |
| Ionisation steps | One | One per proton, sequentially |
| Ka values | A single Ka | Ka₁ > Ka₂ > Ka₃ |
| Equivalence points in titration | One | Up to one per ionisable proton — only those whose pKa values are far enough apart |
| Moles of NaOH per mole of acid | 1 | 2 or 3 |
| Examples | HCl, HNO₃, CH₃COOH, HF | H₂SO₄, H₂CO₃, H₂S, H₃PO₄ |
What a monoprotic acid is
One ionisable proton, one step, one equilibrium:
HCl → H⁺ + Cl⁻ (strong, complete) CH₃COOH ⇌ H⁺ + CH₃COO⁻ (weak, partial)
Acetic acid is the useful case to stare at. Its formula has four hydrogens, but three of them sit on carbon in the CH₃ group, and a C–H bond is far too unreactive to release a proton in water. Only the hydrogen on the –COOH group is acidic, because the oxygen pulls electron density away from it and the resulting carboxylate ion is stabilised by resonance.
Rule of thumb: count the hydrogens on –COOH groups and on the O of an oxoacid — not hydrogens on carbon, and not the hydrogen of an alcohol –OH, which is far too weakly acidic to ionise in water.
What a polyprotic acid is
A polyprotic acid gives up protons in strict order. Phosphoric acid is the textbook triprotic example (H⁺ below is shorthand for H₃O⁺ — in solution the proton is always attached to a water molecule):
| Step | Equation | Ka (25 °C) | pKa |
|---|---|---|---|
| 1 | H₃PO₄ ⇌ H⁺ + H₂PO₄⁻ | 7.5 × 10⁻³ | 2.12 |
| 2 | H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻ | 6.2 × 10⁻⁸ | 7.21 |
| 3 | HPO₄²⁻ ⇌ H⁺ + PO₄³⁻ | 4.8 × 10⁻¹³ | 12.32 |
Ka₁ is always the largest, usually by several orders of magnitude — around 10⁵ per step for inorganic oxoacids such as H₃PO₄, though only 10¹ to 10³ per step for carboxylic acids such as citric and oxalic, whose acidic groups sit further apart. The reason is electrostatic: pulling a positive proton away from a neutral molecule is one thing; pulling it away from a particle that already carries a −1 charge is much harder, and harder still from a −2. Each departure leaves a more negative ion clinging more tightly to what remains.
A practical consequence: for most polyprotic acids, only the first ionisation matters for the pH. The second step contributes so little extra H⁺ that it can usually be ignored in a first-approximation calculation.
Two more you should recognise:
- Carbonic acid, H₂CO₃ — diprotic, Ka₁ = 4.5 × 10⁻⁷ (pKa 6.35), Ka₂ = 4.7 × 10⁻¹¹ (pKa 10.33) — that first value treats dissolved CO₂ and H₂CO₃ together, which is how every data book quotes it. This is the acid in fizzy drinks and the backbone of the blood's buffer system.
- Sulfuric acid, H₂SO₄ — the odd one out. Its first ionisation is complete, so H₂SO₄ is a strong acid, but the second is only weak, with Ka₂ = 1.2 × 10⁻² (pKa 1.92). So 0.10 M H₂SO₄ is not simply 0.20 M in H⁺.
How to tell them apart
- Draw or picture the structure.
- Count the hydrogens attached to an acidic oxygen — the O of a –COOH group or of an oxoacid — or to a halogen. An alcohol –OH does not count.
- That count is the number of protons the acid can donate — and the maximum number of equivalence points a titration can show.
Careful with formulas written in the organic style: ethanoic acid is often printed as CH₃COOH precisely so the one acidic H stands at the end. Oxalic acid, HOOC–COOH, has two.
Worked examples
1. How many moles of NaOH neutralise 1 mol of H₂SO₄? Two — one per ionisable proton. For H₃PO₄ it is three, and for HNO₃ just one.
2. What volume of 0.100 M NaOH neutralises 25.0 mL of 0.100 M H₂SO₄? Moles of acid = 0.0250 L × 0.100 = 2.50 × 10⁻³ mol. Moles of NaOH needed = 2 × 2.50 × 10⁻³ = 5.00 × 10⁻³ mol. Volume = 5.00 × 10⁻³ ÷ 0.100 = 50.0 mL — double what a monoprotic acid would need.
3. Is citric acid, C₆H₈O₇, polyprotic? Yes. It carries three –COOH groups, so it is triprotic; the fourth acidic-looking hydrogen is on an alcohol –OH and is not released in water.
4. Which is the stronger acid, H₂PO₄⁻ or HPO₄²⁻? H₂PO₄⁻, by about five orders of magnitude — 6.2 × 10⁻⁸ against 4.8 × 10⁻¹³.
Common mistakes to avoid
- Counting every hydrogen in the formula. CH₄ has four hydrogens and is not an acid at all; CH₃COOH has four and donates one. The bond the hydrogen sits on is what matters.
- Assuming polyprotic means strong. H₂S, H₂CO₃ and H₃PO₄ are all polyprotic and all weak. Strength and the number of protons are independent properties.
- Expecting all the equivalence points to show up. In water, phosphoric acid's third proton is far too weakly held (pKa 12.32) to give a clean endpoint, so a normal titration shows two usable ones, not three.
FAQ
What does "protic" mean?
It refers to protons — H⁺ ions — that the acid can donate. Mono = one, di = two, tri = three, poly = many.
Why is Ka₂ always smaller than Ka₁?
Because the second proton has to leave an ion that is already negatively charged, and the electrostatic attraction holding it back is much stronger.
Is sulfuric acid monoprotic or diprotic?
Diprotic — but only the first proton is released completely. The second is a weak-acid ionisation with Ka₂ = 1.2 × 10⁻².
How many equivalence points does a diprotic acid titration show?
Two, provided the two pKa values are separated by roughly 4 units or more. If they are too close, the steps blur into one.
The takeaway
Monoprotic acids donate one proton; polyprotic acids donate two or more, always sequentially and always with a sharply decreasing Ka. Count the hydrogens on acidic oxygens rather than every hydrogen in the formula, and remember that the number you get sets the stoichiometry of every neutralisation and the shape of every titration curve.
Come from [What Is Kw?] and see the stoichiometry in action in What Is a Titration? Finding Concentration. The constants themselves are in [What Is Ka?] and Strong vs Weak Acids: What's the Difference?. Next: [What Is an Amphoteric Substance?].
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