Indicators
- An acid-base indicator is a weak acid which dissociates to give an anion of a different colour
- Consider a weak acid, HIn:
HIn (aq) + H2O (l) ⇌ H3O+ (aq) + In– (aq)
colour 1 ⇌ colour 2
- HIn and its conjugate base In– are different colours
- The colour of the solution depends on the relative concentrations of the two species
- If the solution is acidic, the above equilibrium will be shifted to the left and more HIn will be present
- Colour 1 will dominate
- If the solution is alkaline, the above equilibrium will shift to the right and more In– will be present
- Colour 2 will dominate
- The colour of the indicator depends on the pH of the solution
- The colour does not change suddenly at a certain pH, but changes gradually over a pH range
- The colour of the indicator depends on the ratio of [HIn] to [In–]
- The pH at which these transitions will occur depends on the Ka of the indicator
- The endpoint of the reaction is where there is a balance between [HIn] and [In–]. At this point these two concentrations are equal:
- Taking negative logs of both sides:
pKa = pH
- This means the pKa of an indicator is the same as the pH of its endpoint
- The colour change for most indicators takes place over a range of pH = pKa ± 1
Weak bases as indicators
- An indicator can also be a weak base:
BOH (aq) ⇌ B+ (aq) + OH- (aq)
colour 1 ⇌ colour 2
- For such indicators:
- Colour 1 is observed in alkaline conditions
- Colour 2 is observed in acidic conditions
Choosing a suitable indicator
- Around the equivalence point of a titration, the pH changes very rapidly
- Indicators change colour over a narrow pH range, approximately centred around the pKa of the indicator
- An indicator will be appropriate for a titration if the pH range of the indicator falls within the rapid pH change for that titration
Common Indicators and their colours table
Strong acid - strong base
- In strong acid – strong base titrations, the pH changes from 4 to 10 at the end-point so a suitable indicator must change colour within this range
- Methyl red and phenolphthalein are suitable indicators for these titrations
- Methyl orange is not ideal but it shows a significant enough colour change at the end point so is widely used
- In weak acid – strong base titrations, the pH changes from 7 to 10 at the end-point so a suitable indicator must change colour within this range
- Phenolphthalein is the only suitable indicator for weak acid – strong base titrations that is widely available
- In strong acid – weak base titrations, the pH changes from 4 to 7 at the end-point so a suitable indicator must change colour within this range
- Methyl red is the most suitable indicator for these titrations
- However, methyl orange is often used since it shows a significant enough colour change at the end-point and is more widely available than methyl red
Weak acid - weak base
- In weak acid - weak alkali titrations, there is no sudden pH change at the end-point and thus there are no suitable indicators for these titrations
- The end-points of these titrations cannot be easily determined
The overlay on the graph shows that both phenolphthalein and methyl orange would change colour outside the point of inflection in a weak acid-weak base titration so they would not be able to show the equivalence point of the titration
