Question

The acid form of bromophenol blue, HIn, is yellow; the base form, In^- , is blue. What wavelength of visible light do you think each of these forms will absorb? Briefly explain.

Answer 1

For bromophenol blue: pH 3 = yellow; pH 4.6 = blue

As Visible Light ranges from 400 nm to 700 nm

Thus the wavelength of visible light of each of these forms(.i.e. HIn and In^-) will absorb a lot of light that had a wavelength of 640nm (orange light) in case of In^- (aq) which is blue in color, but very little amount of light that had a wavelength of 430 nm (blue-violet light) On the other hand, a sample that contains mostly yellow color HIn would do the opposite – it would absorb very little orange 640 nm light and a lot of blue-violet 430 nm light.

In order to measure the amount of HIn and Inˉ very precisely, you will convert the absorbances measured using the spectrophotometer to the [HIn] and [Inˉ] in the solutions using a calibration curve.