indicators titration

[destroythemcat]

Could anyone explain this question to me:

Q: Below are common indicators. Which of the following would be useful for determining the equivalence point (to within 1%) in a titration of 1 N strong acid with a strong base?

a. thymol blue (pH 8-9)
b. litmus (pH 6-8)
c. methyl red (pH 4-6)
d. all the above

The answer is D all the above. I thought that the answer would be (b) because it has a pH in the range of 7, which would also be the pH of the equiv. point?

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[Saxappeal1]

You mean 1M right? The slope change for let's say, 6 to 7 is very minimal if any at all, as is the same for 8 to 7.Just picture the graph of a titration curve of NaOH into HCl, the slopes are very similar at those pHs that are only 1 unit away anyway at the appropriate ends. Actually AAMC 10 has a similar question on it, it asks what indicator would best be used to indicate the equivalence point of both a titration of NaOH into HCl vs into HOAc (acetic acid; a weak acid) and even though the pH equivalence point is 7 of this type of curve and the pH range for the correct indicator for the weak acid titration curve is 8-10, the slope of the strong acid curve doesnt change really from ph 7 to ph 8, so it is said to be close enough because this is pretty accurate with respect to both titrations.

 

[AspiringDoc2015]

Pretty much what Sax said, but that seems like incredibly ambiguous question writing....this isn't an AAMC question, is it? An quick and dirty rule of thumb with any indicators is that they are effective and accurate within +/- 1 so since each of these contain the desired range they could all technically be used. Visualizing it the way Sax said works well also. Still, terrible question writing....