Question

1 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 could be determined from isosbestic point to find the pKa for bromothymol blue? Explain why or why not

Answer 1

An isosbestic point is defined as the wavelength where two species have the same molar absorptivity. At the isosbestic point the total absorbance of a solution of the two ions is independent of their relative concentrations but is dependent only upon the total dye concentration. The appearance of an isosbestic point is evidence that only two species are involved. This technique is not restricted to indicators, and can be used with any substance whose absorption spectrum changes with pH.

It would not be possible to determine the equilibrium constant without an isobestic point. This is the point where the spectrum of the acid and basic solution have the same pH as well as a constant wavelength and absorbance. The accuracy of the isosbestic point and the intersection point gives us more accurate reading of the λ_max of both acidic/basic spectrum. The determination of the equilibrium constant will be possible with other indicators such as thymol blue, cresol red, and phenolphthalein only if there is an isosbestic point.

In view of the above, YES the pKa of phenolphthalein can be determined from isosbestic point to find the pKa for bromothymol blue.

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