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Why do acid / base indicators not affect a reaction?

Osminog

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  1. Medical Student
Don't they protonate / deprotonate in order to change color, thus raising or lowering the pH of the solution (even just by a little)?

Yes. pH indicators are usually weak acids, so the introduction of a pH indicator would cause the solution’s pH to very, very, very slightly decrease. This minuscule decrease would have no practical effect on the reaction.

On a slightly tangential note, pH indicators are somewhat imprecise in that they depend on people’s subjective interpretations (“has the color fully changed?”). That’s why many scientists prefer to use a pH meter.

None of the information above will be helpful to you on the MCAT, though. On the MCAT, the pH indicator is just a tool for measuring a solution’s pH, with no meaningful effect on the solution’s pH or on the reaction. I hope that makes sense. Best of luck with your studies!
 
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PlsLetMeIn21

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The amount of indicator is so small that you don't consider it. It's a weaker acid than what you are titrating, so the proton of the indicator doesn't matter until the weak acid you're titrating has been fully deprotonated, which is the equivalence point you are looking for. Think of it like a diprotic acid. The second proton has no affect until you have removed the first one. The indicator's proton doesn't matter until the weak acid's proton is gone.
 
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