Acid/ Base Indicators

The choice of indicator depends on the type of acid / base titration that is being carried out. A strong base with a strong acid has a large pH range at the equivalence point (long vertical section of the graph). Therefore, many indicators would change colour at the equivalence point and would work. However, if there is a weak acid/ strong base or weak base/ strong acid, the pH change at the equivalence point is less and the pH at the equivalence point is not 7. Therefore, care needs to be taken with the choice of indicator, so that it changes colour at the equivalence point and not before or after it. Weak acid with weak bases, don't have clear distinct equivalence points and no indicator would work well. The following notes from chem guide, explain clearly how indicators work. They are simply weak acids where the acid and its conjugate base have different colours, and so the colour will be different in acidic or basic conditions. The indicator should be chosen so that the PkIn value is at or close to the pH at the equivalence point.

Indicators

Here are the acid/base colours that we saw of some indicators;