# Question #8c06a

Dec 1, 2014

Acid-base indicators are coloured substances which they themselves are weak acids. They can work as indicators because the co-acid and base forms have different colours.

A common indicator used in acid/alkai titrations is phenol phthalein. This is a white organic solid. In its ionised form it is colourless in solution. Its co-base is pink in colour. If we represent its formula as $H I n$:

$H I {n}_{\left(a q\right)} r i g h t \le f t h a r p \infty n s {H}_{\left(a q\right)}^{+} + I {n}_{\left(a q\right)}^{-}$
$H I n$ is colourless.
$I {n}^{-}$ is pink.
We can write an expression for $K$ which I will call ${K}_{\left(I n\right)}$

${K}_{\left(I n\right)} = \frac{{\left[H\right]}^{+} {\left[I n d\right]}^{-}}{\left[H I n\right]}$

At the end point of a titration $\left[H I n\right] = {\left[I n\right]}^{-}$ so we can write:

${K}_{\left(I n\right)} = {\left[H\right]}^{+}$

So $p {K}_{\left(I n\right)} = p H$

By consulting a table of $p {K}_{\left(I n\right)}$ values we can get the $p H$ at the end-point which enables us to select a suitable indicator for a titration.

So for phenol phthalein $p {K}_{\left(I n\right)}$ = 9.3. This also is equal to the $p H$ at the end-point. In this case this makes phenol phthalein a suitable indicator for titrations involving weak acids.

Here is a video of an indicator lab!

video from: Noel Pauller