Spectrophotometric Determination of the p Ka of Bromothymol Blue

​Analysis of Iron Supplement Tablet

Experiment: Spectrophotometric Determination of the

pKa of Bromothymol Blue

Russybel Richiez

S. Tewani

Date Submitted: May 4, 2020

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

1 5 0
2 10 0.
3 5 1
4 10 5
5 5 5
6 5 10
7 1 5
8 1 10
9 0 10
10 0 5
  1. The pH meter was used to measure the pH of the pH 1, hydrochloric acid solution, the pH 13, sodium hydroxide solution, and the 10 solutions. The pH of each solution was recorded.
  2. Using the scanning spectrophotometer, the spectra between 300 and 800 nm of the pH 1 and the pH 13 bromothymol blue solutions were obtained and recorded. From the recorded spectra one wavelength ( λ 1 ) was chosen at which the pH 1 solution absorbs strongly but the pH 13 solution absorbs weakly, and a second wavelength ( λ 2 ) at which the pH 13 solution absorbs strongly but the pH 1 solution absorbs weakly.
  3. The absorbance of each of the 12 solutions at the two chosen wavelengths were measured and tabulated.

Figure A1: Dependence of absorbance HIn and In- on pH

pH Vs ABSORBANCE

PHAbs at 435 nm Abs at 620 nm