# How do indicators work?

May 27, 2017

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

#### Explanation:

pH indicators detect the presence of ${H}^{+} \mathmr{and} O {H}^{-}$
. They do this by reacting with ${H}^{+} \mathmr{and} O {H}^{-}$
: 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.

May 27, 2017

An indicator is a large, weak organic acid whose acid and base forms have distinctive colours. And we can represent this species by $H I n$. As with any aqueous acid we may assess the acid base equilibrium:
$H I n \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + I {n}^{-}$
The point is that $H I n$ and $I {n}^{-}$ 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.
The graph shows (poorly) the titration curve of a strong acid, when titrated by a strong base. Because the rise in $p H$ is so precipitous, i.e. so steep, just a drop of titrant (approx. $0.01 \cdot m L$) will effect a dramatic change in $p H$, and in these scenarios is does not really matter which indicator you use.