Title: Spectrophotometric Determination of the pKa of Bromothymol Blue
Introduction: The purpose of this lab was to find the value of pKa of Bromothymol Blue Indicator. Bromothymol Blue Indicator is a ph indicator which
The equation below allows for the pKa to be found given the pH, In- which is the
Products and the HIn which is the reactants
By monitoring the pH of a solution one can find the Ka of the indicator in use. For this experiment the acidic and basic forms of the indicator absorb more strongly at different wavelengths. The first wavelength λ 1 , is where the acidic form of the indicator absorbs the radiation strongly. The second wavelength λ 2 , is where the basic absorbs the radiation strongly. From Beers law we know that for pH changes, concentrations of each form will change according to the Ka of the indicator meaning the absorbance for each form will also change. In solutions of low pH, essentially all of the indicator is in the acid form. In highly basic solutions essentially all of the indicator is in the basic form
It was found that
PHAbs at 435 nm Abs at 620 nm