Would using a spectroscopic method to determine equilibrium constant of a solution work if there was...

Would using a spectroscopic method to determine equilibrium constant of a solution work if there was no isosbestic point in the absorption spectra?Why or why not?

Expert Solution

Solution.

An isobestic point corresponds to the conditions when there is no change in the total absorbance of the sample. For example, if we would determine the pKa of indicator the spectra may reveal one wavelength, noted by a star, where the absorbance is essentially independent of pH:

The both forms of the indicator (HIn and In^-) having the same molar absorptivity at this wavelength, so the measurement at this wavelength will result in failed determination of pKa.

Hovewer, at other wavelength, varying a pH, the ratio HIn and In^- will be changed resulting in the absorbance change, and equilibrium constant can be easily found using the equation

.

Therefore, it is possible to use a spectroscopic method to determine equilibrium constant of a solution if there is no isosbestic point in the absorption spectra. The presence of isosbestic point has no action on the absorbances at maximal wavelength for the forms HIn and In^-. Therefore, it will not interfere in the pKa measurment.

queen_honey_blossom answered 2 years ago

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