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Why is $\ce{[HIn] = [In-]}$ at equivalence point of a titration? I know that at equivalence point moles of known solution is equal to the moles of unknown solution, but I'm not able to relate that fact with the above conclusion. Perhaps I'm don't know something.

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  • $\begingroup$ Please no mathjax in titles. meta.chemistry.stackexchange.com/questions/261/… $\endgroup$ – orthocresol Aug 27 '16 at 9:34
  • $\begingroup$ @Shuvam at equivalence point the reductant and oxidant are equal in their mole stoichiometric. $\endgroup$ – JM97 Aug 27 '16 at 9:37
  • $\begingroup$ @Shuvam hope this helps chemguide.co.uk/physical/acidbaseeqia/indicators.html $\endgroup$ – JM97 Aug 27 '16 at 9:56
  • $\begingroup$ What is $\mathrm{HIn}$? The indicator? If so, this statement is false. The equivalence point is by definition when you have added a stoichiometric amount of the titrant to the analyte. $\endgroup$ – Zhe Oct 27 '16 at 17:35
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Ideally, the concentration of indicator is much, much lower than the concentration of the material to be titrated and of that used to titrate. Therefore, less than a drop of standard solution is needed to make the indicator entirely cross the equivalent point.

For most titrations, the equivalence point of the indicator is not the end point. It’s just somewhere near. And the only thing you can actually see and record is said indicator’s colour change. So it’s the only thing you can resort to.

The important choice in a titration is choosing the correct indicator. If you are titrating a strong acid with a strong base (so the titration’s equivalence point is approximately $7$), you want an indicator whose equivalence point is just behind that (at ca. $8$), so that the tiniest of drops added additionally will cause the colour change.

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As you might already know, $\ce{[In^-]}$ is the concentration of the indicator that was added to the solution, which basically shows whether the solution is basic or acidic or neutral. The equivalence point of a titration is when the titrant has reacted with the unknown chemical solution so that it reaches an endpoint of the indicator. The equalization of the concentration $\ce{[HIn] = [In^-]}$ is when the color is at its neutral state: neither colors of the indicator is dominant over the other.

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    $\begingroup$ I have downvoted this answer because it is not true that the equivalence point of a titration is when a solution is neutral; consider the titration of a weak base with a strong acid, for example. The equivalence point of a titration occurs when all the titrant has reacted. $\endgroup$ – a-cyclohexane-molecule Aug 28 '16 at 3:44
  • $\begingroup$ Oh, right. Stupid mistake. I do believe the rest of the answer should be right though. Is it? $\endgroup$ – phi2k Aug 28 '16 at 14:41
  • $\begingroup$ Yes, I believe so. $\endgroup$ – a-cyclohexane-molecule Aug 28 '16 at 19:21

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