Bromothymol blue is a common acid–base indicator that exists in two interconvertible…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Bromothymol blue is a common acid–base indicator that exists in two interconvertible molecular forms: HIn (yellow) and In⁻ (blue). A student records the following macroscopic observations.
• A buffer solution of known pH = 6.0 appears distinctly yellow.
• A buffer solution of known pH = 7.6 appears distinctly blue.
• A third buffer of unknown pH appears green, suggesting an approximately equal mixture of the two colored forms.
Use these observations to analyze the acid–base behavior of bromothymol blue.
(a) For each of the two reference buffers (pH = 6.0 and pH = 7.6), identify which molecular form (HIn or In⁻) is predominant. Explain your reasoning in terms of relative concentrations.
(b) Using only the qualitative information provided, determine the approximate pH range (lower limit and upper limit) over which the color of bromothymol blue changes from yellow to blue. Justify your answer.
(c) Based on the green color of the unknown buffer, estimate how the pH of this solution compares with the pKa of bromothymol blue. Provide a brief explanation relating protonation state to your conclusion.