Spectroscopic Determination of the pKa of Bromothymol Blue Andrea Myer May 27, 2023 Kevin Huang, CHEM 241L, Section 402 I pledge that I have not used someone else’s old or current lab when writing this lab report. I pledge that I did not collaborate with any other students, except where allowed, and that the report I submitted here contains my own ideas, thoughts, observations, calculations, data, conclusions, and answers. Lastly, I pledge that the data represented in this report was my own collected during lab or provided to me by my TA. Introduction Bromothymol blue is an indicator used in titration-based experiments and transitions color as the ratio of protonation, and ultimately pH, of a given solution changes. This ratio is commonly illustrated by the Henderson-Hasselback equation (pH=pKa+log[A^-/HA]), where A^- represents the concentration of conjugate base and HA is the concentration of acid in the observed sample. The concentration of a species in solution can be determined using Beer’s Law, A=Ɛbc, where absorbance of the solution at the λ_max is equal to the product of the cuvette path length, molar coefficient, and solution concentration. Therefore, absorbance is directly correlated with concentration. But how is absorbance determined?^1 UV-Visible spectroscopy utilizes a light …show more content…
This can be corroborated by the solvent-solvent extraction data detailed in Figure 4. Since organic-phase bromothymol blue (which rises to the top of the flask in a solvent-solvent extraction) is a neutral molecule, it must be the acidic form. Since the aqueous phase easily dissolves charged molecules, the deprotonated, negatively charged form of bromothymol blue is found here. Because the first peak on Figure 1 occurs at a lower pH, this must correspond to the organic-phase, protonated version of bromothymol