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Question #8c06a

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Answer 1 · 2014-12-01T18:14:14

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 $H I n$ $HIn$ :

$H I n_{(a q)} ⇌ H_{(a q)}^{+} + I n_{(a q)}^{-}$ $HIn_((aq))rightleftharpoonsH_((aq))^(+)+In_((aq))^(-)$
$H I n$ $HIn$ is colourless.
$I n^{-}$ $In^-$ is pink.
We can write an expression for $K$ $K$ which I will call $K_{(I n)}$ $K_((In))$

$K_{(I n)} = \frac{{[H]}^{+} {[I n d]}^{-}}{[H I n]}$ $K_((In))=([H]^(+)[Ind]^(-))/([HIn])$

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

$K_{(I n)} = {[H]}^{+}$ $K_((In))=[H]^+$

So $p K_{(I n)} = p H$ $pK_((In))=pH$

By consulting a table of $p K_{(I n)}$ $pK_((In))$ values we can get the $p H$ $pH$ at the end-point which enables us to select a suitable indicator for a titration.

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

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Question #8c06a

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