Titration Curves, Poured Out
Add base to acid a little at a time and watch the pH curve build — through the buffer region, up the steep jump, to the equivalence point where the right indicator changes colour.
Add base to acid a drop at a time and the pH barely moves… then leaps… then flattens again. That distinctive titration curve is a fingerprint of the acid and base involved — and reading its four features tells you the equivalence point, the , and which indicator to use.
What a titration curve shows
A titration curve plots pH (vertical axis) against the volume of base added (horizontal axis). It records the whole journey from pure acid, through the point of exact neutralisation, into excess base. The shape depends on whether each reagent is strong or weak.
The steep vertical cliff is the giveaway: near the equivalence point, a single drop of base can swing the pH by several units because almost no acid is left to resist it.
Run the titration
Drag along the curve to add base, and watch the pH climb. Switch between a strong acid and a weak acid to compare the shapes: the weak-acid curve has a gentle buffer region and its equivalence point sits above pH 7. The shaded band shows the pH range over which a suitable indicator changes colour.
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An interactive titration curve plotting pH (0–14) against volume of base added. A draggable point moves along the curve, reading out the volume, the pH, and the current region (before equivalence / buffer region / equivalence point / excess base). Two presets are provided. Strong acid with strong base: pH starts near 1, rises slowly, jumps almost vertically through the equivalence point at pH 7 (at 25 mL), then levels near pH 12–13. Weak acid with strong base: pH starts higher, has a shallow buffer region where pH = pKa at half-equivalence, a smaller vertical jump, and an equivalence point above pH 7 (around pH 8.7). A shaded horizontal band shows a suitable indicator’s colour-change range, and a dashed vertical line marks the equivalence volume.
The four features to know
The equivalence point is not always pH 7. It is neutral only for strong acid–strong base. For a weak acid with a strong base, the salt formed is basic, so equivalence lies above 7; for a strong acid with a weak base, it lies below 7.
Choosing an indicator
An indicator changes colour over a narrow pH range. The rule is simple: its colour-change range must fall within the steep vertical section of the curve, so the colour flips with a single drop right at equivalence.
- Phenolphthalein (changes ~pH 8.3–10) suits weak acid–strong base titrations.
- Methyl orange (changes ~pH 3.1–4.4) suits strong acid–weak base titrations.
- Strong acid–strong base has such a long vertical jump that either indicator works.
Worked example
A weak acid is titrated with sodium hydroxide. The equivalence point is reached at of base. At half that volume, , the measured pH is . Find .
At half-equivalence, exactly half the weak acid has been converted to its conjugate base, so their concentrations are equal. The Henderson–Hasselbalch equation then gives . Therefore — you can read it straight off the curve.
Your turn
You titrate ethanoic acid (a weak acid) with sodium hydroxide (a strong base). Which indicator — methyl orange or phenolphthalein?
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Weak acid + strong base puts the equivalence point above pH 7, in the range where phenolphthalein changes colour. Methyl orange would change far too early, in the buffer region.
Common mistakes
Frequently asked questions
What is the equivalence point of a titration?+
Why is the equivalence point not always pH 7?+
What is the buffer region on a titration curve?+
How do you choose the right indicator for a titration?+
The ScholarsGate Chemistry Team
Oxbridge & Russell Group chemistry tutors
Written and reviewed by ScholarsGate tutors who teach A-Level and undergraduate chemistry. Every explainer is checked against the AQA, Edexcel, OCR and CIE specifications.
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