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find the term isosbestic point.? [a. Explain why the isobestic point would be a poor choice...

find the term isosbestic point.?

[a. Explain why the isobestic point would be a poor choice of wavelength for determiningpKa?

5. Phenolphthalein is an indicator often used for weak acid-strong base titrations. The acid form of phenolphthalein is colorless and the base form is pink. Do you think the pKa of phenolphthalein would be determined using the same the method from this experiment to find the pKa for bromothymol blue? Explain why or why not?

I dont need the answer for question 4 , i just need the answer for question 5 . Well i think we have used pka determination by isobestic point .

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Answer #1

The Henderson-Hasselbulch equation, which is shown below.

pH = pKa + Log[base]/[acid])

For phenolphthalein (HIn), pH = pKa + Log([In-]/[HIn])

If you know the concentration of the acid and base species and pH of the solution, you can easily determine the pKa value of the phenolphthalein indicator.

The isosbestic point (a wavelength at which the absorption of light by a solution remains constant, as the equilibrium between the components changes in the solution) may also be used for such purpose.

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