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Question about acid and base indicators

  1. Jan 31, 2016 #1
    1. The problem statement, all variables and given/known data

    Methyl red has the following structure [not listed here on this post]

    Ka = 5.0 * 10 E -5

    It undergoes a color change from red to yellow as a solution gets more basic.

    • Calculate an approximate pH range for which the methyl red is useful.
    • What is the color change and the pH at the color change when a weak acid is titrated with a strong base using methyl red as an indicator?
    • For which of these two types of titrations is methyl red a possible indicator?

    2. Relevant equations

    Henderson Hasselbalch equation

    3. The attempt at a solution

    OK. I tried to figure this problem out, but I have no idea if I am right or not.
    If anybody could check, that would be great. Thanks.

    For the approximate pH range:
    • For the pH range, I took the pH range to be within range of pKa plus/minus 1. So did: -log(5.0 * 10E-6) and then I added and subtracted 1.
    • I got the answer to be [4.30, 6.30]
    For the weak acid with strong base titrated:
    • Color changes from red to yellow. Then, the pH at the color change will be [second bullet]
    • pH = pKa + log( [In-] / [HIn] ) = -log (5.0 E -6) + log ( 1/10 ) = 4.30
    For the weak base titrated with strong acid:
    • Color change: yellow to red
    • pH? I did the same thing as in the previous step only my log expression had log(10/1) . I got my pH to be 6.30.

    For which of these two types of titrations is methyl red a possible indicator?
    The methyl red is a possible indicator for the weak base and strong acid titration.

    Any input on whether my solutions are right or not are greatly appreciated.

    Thanks in advance!
     
  2. jcsd
  3. Jan 31, 2016 #2

    mfb

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    Staff: Mentor

    Where do the factors of 10 or 1/10 come from for the ratios?
    Why?
     
  4. Jan 31, 2016 #3
    These factors relate to when the indicator will have a color change.

    If I have HIn <----> H+ + In-, and I am titrating an acid with a base, then the ratio of [In-] to [HIn] will be 1/10 when the first visible color change occurs.
    And vice versa for a base being titrated with an acid.


    Because the pH of the equivalence point for a weak base and strong acid titration is going to be greater than 7.

    Ohh. I see. My bad.

    It should be a possible indicator for a weak acid and strong base titration as then the pH would be less than 7 at equivalence point and the indicator's range is less than 7.

    Right?
     
  5. Jan 31, 2016 #4
    Actually, I am staying with my thought that it would be a possible indicator for a weak base and strong acid titration.

    The equivalence point of a weak base and strong acid titration will have a pH that is acidic ( < 7) and the indicator's effective pH range is in an acidic solution.

    Riight?
     
  6. Jan 31, 2016 #5

    mfb

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    So it is just a rough estimate? Okay.

    My "why" question was more out of curiosity. Good point with the equivalence point, and I agree that weak base plus strong acid is better.
     
  7. Jan 31, 2016 #6
    So do you think that my answers happen to be correct, or I need to fix something with my answers/my reasoning?

    Thanks for the help!!
     
  8. Jan 31, 2016 #7

    mfb

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    Looks correct.
     
  9. Jan 31, 2016 #8

    Borek

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    Staff: Mentor

    Yes, one that is commonly used in this context. Real value is an individual thing and depends a bit on the color change as eye reacts differently to different hues.
     
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