Question

How do indicators work?

Answer 1

It is done by putting the indicator in the Acid or Base.

Explanation:

pH indicators detect the presence of `H^+ and OH^-`
. They do this by reacting with `H^+ and OH^-`
: they are themselves weak acids and bases. If an indicator is a weak acid and is colored and its conjugate base has a different
color, deprotonation causes a color change.

Answer 2

Well, see this old answer.

Explanation:

An indicator is a large, weak organic acid whose acid and base forms have distinctive colours. And we can represent this species by `HIn`. As with any aqueous acid we may assess the acid base equilibrium:

`HIn(aq) + H_2O(l) rightleftharpoonsH_3O^+ + In^(-)`

The point is that `HIn` and `In^-` have quite DISTINCTIVE colours, and depending on the choice of indicator, you can signal the stoichiometric endpoint of a reaction by a macroscopic colour change.

imgarcarde.com

The graph shows (poorly) the titration curve of a strong acid, when titrated by a strong base. Because the rise in `pH` is so precipitous, i.e. so steep, just a drop of titrant (approx. `0.01*mL`) will effect a dramatic change in `pH`, and in these scenarios is does not really matter which indicator you use.