Selecting The Proper Indicator

[capnamerica]

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I don't understand why the indicator you select has to change color at a pH near the pKa of the acid being titrated. In this case, wouldn't it change color at the half-equivalence point, because pH=pKa of the acid at this point? Please help! Thanks!

Edit: Is it the pKa of the INDICATOR that has to match the expected/predicted pH at the EQUIVALENCE POINT?

 

[fizzgig]

i think your edit was more correct. you want your indicator pka to be around the target's equivalence point.

when the titration's over, you want indicator to change color. it does so at 1:10 or 10:1 ratio of concentration, and contained within that range is a 1:1 ratio of concentration, which is the indicator's pka. so if pka of indicator is centered over the target's equivalence point, indicator will be changing color near that equivalence point. hope that makes sense, anyway.

 

[ilovemcat]

I don't understand why the indicator you select has to change color at a pH near the pKa of the acid being titrated. In this case, wouldn't it change color at the half-equivalence point, because pH=pKa of the acid at this point? Please help! Thanks!

Edit: Is it the pKa of the INDICATOR that has to match the expected/predicted pH at the EQUIVALENCE POINT?

No, the first part of what you said is wrong. 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

Indicator's tell us when the equivalence point has been reached - the region where you'll see a color change. That's why it's crucial to use an indicator as close as possible to the pH at equivalence point.

 

[Deeder03]

i think your edit was more correct. you want your indicator pka to be around the target's equivalence point.

when the titration's over, you want indicator to change color. it does so at 1:10 or 10:1 ratio of concentration, and contained within that range is a 1:1 ratio of concentration, which is the indicator's pka. so if pka of indicator is centered over the target's equivalence point, indicator will be changing color near that equivalence point. hope that makes sense, anyway.

General rule of thumb, you want it to be +/- 1 log unit from the equivalence point. There are a few good questions to illustrate this in the AAMC practice exams, 7,8,10 have them I believe.