Question

Question #8c06a

Answer 1

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 `HIn`:

`HIn_((aq))rightleftharpoonsH_((aq))^(+)+In_((aq))^(-)`
`HIn` is colourless.
`In^-` is pink.
We can write an expression for `K` which I will call `K_((In))`

`K_((In))=([H]^(+)[Ind]^(-))/([HIn])`

At the end point of a titration `[HIn]=[In]^-` so we can write:

`K_((In))=[H]^+`

So `pK_((In))=pH`

By consulting a table of `pK_((In))` values we can get the `pH` at the end-point which enables us to select a suitable indicator for a titration.

So for phenol phthalein `pK_((In))` = 9.3. This also is equal to the `pH` 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