Indicator colour change

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Indicator colour change

Post by bohojekek »

The answer key to the question below states that when pH=pKa, the phenol red colour change occurs, but I thought that when pH=pKa, this is the half equivalence point, where the volume of titrant added is exactly enough to neutralise half the volume of analyte.

And the half equivalence point is also halfway to the equivalence point which is where the colour change occurs? (or occurs close to it because I know that the endpoint is where the colour change occurs and the endpoint is an estimation of the equivalence point). So can I please get an explanation of why the colour change is stated to occur at pH=pKa, thanks in advance.
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Re: Indicator colour change

Post by ChenBeier »

Here we talking about the indicator. Not the analyt and titrant, etc.
The indicator is an acid and an Anion itself

Hind => H+ + Ind-

Ka = H+ * Ind- / Hind

pKa = pH + log(Ind-/Hind)

The colour of Ind- is different from Hind

The change takes place if ind- = Hind, what means pH = pKa
Close to this point you will have mix colour of both species.

What you talking about is the titration of the acid or base but not the indicator. Example if pH =pks of acetic acid then half acid and acetate is present. But this has nothing to do with indicator. It indicates if its own pKs is equal a pH that colour change.
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