Question about acid and base indicators

[RoboNerd]

Homework Statement

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?

Homework Equations

Henderson Hasselbalch equation

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!

 

[mfb]

Where do the factors of 10 or 1/10 come from for the ratios?

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.

Why?

 

[RoboNerd]

Where do the factors of 10 or 1/10 come from for the ratios?
Why?

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?

 

[RoboNerd]

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?

 

[mfb]

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.

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.

 

[RoboNerd]

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!

 

[mfb]

Looks correct.

 

[Borek]

So it is just a rough estimate?

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.