# Question about acid and base indicators

• RoboNerd
In summary,Methyl red undergoes a color change from red to yellow as a solution gets more basic. This color change is used to identify a pH range where the methyl red is useful. The methyl red is a possible indicator for the weak base and strong acid titration.
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.

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

mfb said:
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.

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?

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?

RoboNerd said:
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.

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!

Looks correct.

RoboNerd
mfb said:
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.

## 1. What is an acid and base indicator?

An acid and base indicator is a substance that changes color in the presence of an acid or base. It is used to determine the pH (acidity or basicity) of a solution.

## 2. How do acid and base indicators work?

Acid and base indicators work by changing color when they come into contact with hydrogen ions (H+) or hydroxide ions (OH-) in a solution. These ions are present in different amounts depending on the pH of the solution, causing the indicator to change color accordingly.

## 3. What are some common acid and base indicators?

Some common acid and base indicators include litmus, phenolphthalein, and universal indicator. Litmus turns red in the presence of an acid and blue in the presence of a base, while phenolphthalein is colorless in acidic solutions and pink in basic solutions. Universal indicator is a mixture of several indicators and can change to a variety of colors depending on the pH.

## 4. How are acid and base indicators used in the laboratory?

In the laboratory, acid and base indicators are used to determine the pH of a solution. A small amount of the indicator is added to the solution and the resulting color change is compared to a color chart to determine the approximate pH. Indicators can also be used to determine the endpoint of a titration, where a known amount of acid or base is added to a known volume of solution until the indicator changes color, indicating that the reaction is complete.

## 5. Can acid and base indicators be used to determine the strength of an acid or base?

No, acid and base indicators only provide information about the pH of a solution, not the strength of an acid or base. To determine the strength of an acid or base, other methods such as titration or pH measurement using a pH meter must be used.

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