- You use and indicator to tell you when you reach the endpoint of the reaction
- This occurs when the indicator changes colour
However, in reality, you should only stop when the solutions are mixed in exactly the right proportions
i.e. all H+and OH– ions from the reagent solutions have reacted to form H2O
- This is called the molar equivalence point
- In an ideal world, the endpoint and the equivalence point are the same, or near to the same
This point is not necessarily the neutral point (pH 7); due to hydrolysis, the resultant solution may have different a pH.
How indicators work
Indicators are usually weak acids that ionise. For example, litmus, or phenolphthalein.
- Indicators do not change colour at any one particular pH.
- Instead, they change over a narrow range of pH.
- There is a gradual smooth change from one colour to another
- You need to choose an indicator that changes colour as close as possible to the equivalence point
- Since in most cases, the graph around the EP is quite steep, it usually does not matter as long as the colour change occurs at the steep point
Strong acid added to strong base
- You could use either of the two in this experiment, since they both change at the steep point
- There will be virtually no difference in the volume of acid added, so any one would be fine
- Phenolphthalein does not change colour near the EP, so you must use methyl orange
Weak acid added to strong base
- You would use phenolphthalein, since it changes colour near the EP
- You would not use methyl orange, as it obviously does not change colour near the equivalence point
Weak acid added to weak base
- Neither indicator would work in this case
- This is why titrating weak acids/bases against each other is not advisable