AAMC 10 CBT: pH Indicators

[MedPR]

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This is number 26 (PS). Before I looked at the answer choices, I thought the answer was going to be Indicator D (pH 5-8) because the graphs both look like the equivalence point is around 7. After seeing it wasn't an option, I was torn between indicator C and indicator E (the correct answer).

I guess I just really don't understand how this works. Help please!


Edit: After reading the explanation over again, I think I might understand now. The equivalence point could really be anywhere on the steepest part of the graph so you need to use an indicator that covers as much of that pH range as possible..? If Indicator D had been one of the answer choices, would it have been a better choice than Indicator E?

 

[Arctangent]

The way I would have approached this would be like the way you did. The equivalence point is technically right at the middle of the steep section of the curve, assuming it was plotted based on Henderson-Hasselbach, so I would estimate 7 for titration 1 and 8-8.5 for titration 2. If D was an option I would have chosen it. So then between indicator C (4-6) and indicator E (8-10), based on my approximations of the equivalence point, I would choose indicator E, as it is closer to the estimated equivalence points, and also as you say, the 4-6 range for titration 4 does not actually cover much of the steep section, and you could actually argue misses the equivalence point by a good margin. I'd say that overall, this question is not that clear cut for a titration indicator question.

 

[pfaction]

I got this same one wrong too! Same answer choice as me as well. C is simply too acidic for realistic, and I was looking for Choice D as well.