CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
An indicator, In-A, exhibits a yellow color in its protonated form and a blue color in its deprotonated form. The pKa of the indicator is 8.0.
During a titration of a weak base with a strong acid, the solution in the flask is observed to turn a distinct green—the color produced when yellow and blue are present in approximately equal intensities.
Answer the following questions about this observation and about selecting indicators for titrations.
(1) Determine the pH of the titration mixture at the instant the green color first appears. Explain your reasoning.
(2) If the experimenter instead wishes to see the midpoint color one pH unit lower than in part (a), what pKa value should the experimenter look for in a different indicator? Justify your answer.
(3) Describe qualitatively how the color of In-A will change as the pH is gradually lowered from 9.0 to 7.0 during the course of the titration. Your description should link specific pH intervals to the dominant molecular forms and resulting colors.