A chemist is selecting an acid–base indicator for a titration whose equivalence point…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A chemist is selecting an acid–base indicator for a titration whose equivalence point is expected at pH 8.3. The two candidate indicators are:
• Phenol red, pK_a = 7.9 (yellow when protonated, red when deprotonated)
• Thymol blue (first transition), pK_a = 1.8 (red when protonated, yellow when deprotonated)
The chemist tests each indicator separately in three standard buffer solutions of pH 5.0, pH 8.3, and pH 10.0.
(1) Describe which color phenol red will display in each of the three buffer solutions. Use chemical reasoning based on the relationship between pH and pK_a.
(2) Describe which color thymol blue will display in each of the three buffer solutions, again using chemical reasoning.
(3) Explain which of the two indicators is more appropriate for the titration with an equivalence point at pH 8.3, and justify your choice using your results from parts (a) and (b).
(4) Many titrations employ a mixed-indicator system that changes color over a broader pH range. Propose, with justification, whether combining phenol red and thymol blue would offer any advantage for identifying the equivalence point at pH 8.3.