Problem: Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color.What other indicator or indicators would you want to use to determine the pH of the solution more precisely?

FREE Expert Solution

We have to determine a suitable indicator to determine the pH of the given solution.

Indicators are weak acids/bases. The pH at which an indicator changes color is determined by its pKIn value.


The pH range for color change of phenolphthalein is 8.2 – 10.0.

Phenolphthalein has pKIn = 9.4 and it changes to red at pH values higher than 9.4.

Bromothymol blue has pKIn = 7.0 and it changes to from yellow to blue in the pH range of 6.0 – 7.6.

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Problem Details

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color.

What other indicator or indicators would you want to use to determine the pH of the solution more precisely?

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Acid Base Indicators concept. If you need more Acid Base Indicators practice, you can also practice Acid Base Indicators practice problems.

What professor is this problem relevant for?

Based on our data, we think this problem is relevant for Professor Kruger's class at UNG.