So Ideal Indicator has a pKa 2 units above acid's pKa??

[johnwandering]

Pertaining to titration of a weak acid with a strong base...

I always thought that the ideal indicator would have a pKa as close as possible to the equivalence point.
But one of the answer explanations in my practice test states that the Ideal pKa for an indicator is 2 units above...


Is this correct?


Edit***
Oh, now I understand~~ equivalence Pt is above pKa...

Sorry!!!

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[Phantastic]

Yes, you are correct.

I'd like to point out that the whole "idea" of an indicator is just an extension of what you already know. An indicator is a weak acid/conj base combination that each exhibit different "indications" (e.g. colors). Since at the pKa of this weak acid, the [weak acid] = [conj base], the only range of pH that we can see this indication is the pKa +/- 1. This corresponds to ratios of 10:1 or 1:10 of [weak acid]:[conj base], and will likely show differing ranges of colors. Outside this range, we will only see the color of the now dominant species:

pH < pKa = color of weak acid
pH > pKa = color of weak base

See how intuitive this concept is? Almost impossible to forget.