What does a titration curve tell us and how do we choose an indicator?
Interpret titration curves for the four acid-base combinations and select a suitable indicator.
The shape of titration curves for strong and weak acid-base combinations, equivalence point versus end point, and choosing an indicator by pKa.
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What this dot point is asking
You must read titration curves, distinguish equivalence point from end point, and select an appropriate indicator.
Equivalence point and end point
The end point is where the indicator changes colour. A well chosen indicator gives an end point that coincides with the equivalence point. These are separate ideas: equivalence is a chemical fact about moles, while end point is what the indicator shows.
The four combinations
Strong acid with strong base: the curve starts low, rises gently, then jumps steeply through a wide pH range, with the equivalence point at pH 7. The neutral salt formed does not affect pH.
Strong acid with weak base: the equivalence point is below 7 (acidic), because the salt formed contains the conjugate acid of the weak base.
Weak acid with strong base: the equivalence point is above 7 (basic), because the salt contains the conjugate base of the weak acid. There is also a buffer region with a flat section before the jump.
Weak acid with weak base: the jump is small and gradual, so there is no sharp equivalence point and indicators are unreliable.
Choosing an indicator
The indicator must change colour within the steep vertical part of the curve, so that one drop of titrant takes the pH through the colour change. Match the indicator's (and hence its working range) to the equivalence pH.
- Phenolphthalein (range about 8.3 to 10) suits weak acid with strong base.
- Methyl orange (range about 3.1 to 4.4) suits strong acid with weak base.
- Either suits strong acid with strong base, because the jump spans both ranges.
In the exam, identify which partner is weak, predict whether the equivalence point is acidic, neutral or basic, then select an indicator whose range sits inside the steep part of the curve.