Indicators in Titrations HL Only
Quick Notes
- The equivalence point is when stoichiometric amounts of acid and base have reacted (no original acid or base remains).
- The end point is when the indicator changes colour.
- A suitable indicator has an end point pH range that matches the equivalence point pH.
- Choose indicators based on acid and base strength:
- Strong acid + strong base: most indicators work (e.g. phenolphthalein, methyl orange)
- Strong acid + weak base: need a low-pKa indicator (e.g. methyl orange)
- Weak acid + strong base: need a high-pKa indicator (e.g. phenolphthalein)
- Weak acid + weak base: no sharp pH change, so no suitable indicator
Full Notes
In a titration, there is an equivalence point and an end point. It is very important you understand what these are and, most importantly, how they are different!
Equivalence Point
The equivalence point is the volume of titrant needed to completely react with the analyte – no original acid or base remains, only the salt formed.
The pH at equivalence depends on the acid–base strength and the nature of the salt produced:
- Strong acid + strong base → neutral salt → pH ≈ 7
- Strong acid + weak base → acidic salt → pH < 7
- Weak acid + strong base → basic salt → pH > 7
- Weak acid + weak base → variable pH (~5–9), not sharply defined
End Point
The end point is when the indicator changes colour.
It occurs when just the right volume of titrant is added to cause the indicator to change colour. It should match the equivalence point volume as closely as possible for accuracy.
It is really important to remember that equivalence point and end point are two separate things – we just want them to overlap for a given titration. This means the volume needed to change the colour of the indicator happens to be the same as the volume needed to perfectly react the acid or base being titrated. This is why choosing the right indicator in a titration is important.
Choosing the Right Indicator
We can use the salt formed in a titration to select an appropriate indicator. The salt determines the pH at the equivalence point, and an indicator should be chosen that has a pH range that covers the pH of the solution at the equivalence point.
Examples
- Strong acid + strong base → salt is neutral → use phenolphthalein or methyl orange
- Weak acid + strong base → salt is basic → use phenolphthalein (pH transition ~8–10)
- Strong acid + weak base → salt is acidic → use methyl orange (pH transition ~3–4)
Indicator Selection Summary
| Acid–Base Combination | Salt Produced | pH at Equivalence | Suitable Indicator |
|---|---|---|---|
| Strong acid + strong base | Neutral | ≈ 7 | Phenolphthalein or Methyl orange |
| Weak acid + strong base | Basic | > 7 | Phenolphthalein (pH ~8–10) |
| Strong acid + weak base | Acidic | < 7 | Methyl orange (pH ~3–4) |
| Weak acid + weak base | Variable | ~5–9 | No suitable indicator |
Summary
- The equivalence point is where stoichiometric neutralisation occurs.
- The end point is the indicator colour change.
- A suitable indicator matches its pH transition range to the equivalence point pH.
- Weak acid–weak base titrations have no sharp end point and need a pH meter.