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?

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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?