Which is the most appropriate acid-base indicator to use in the titration of 0.1

Question

Which is the most appropriate acid-base indicator to use in the titration of 0.100 molar acetic acid(aq) by 0.100 molar NaOH (aq)? This is the titration of a weak acid by a strong base. a. phenol red, for which pK_In = 7.3, also Hind is yellow and Ind is red b. bromothymol blue, for which pK_In = 6.8, also Hind is yellow and Ind is blue c. phenolphthalein, for which pK_In = 9.1, also Hind is colorless and Ind is pink d. cresol red, for which pK_In = 7.9, also Hind is yellow and Ind is red e. These indicators will all work equally well for the titration of a weak acid by a strong base.

Explanation / Answer

C) Phenolphthalein is a weak acidcommonly used acid base indicator for titrations.In the given case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink. The half-way stage happens at pH 9.3. Since a mixture of pink and colourless is simply a paler pink. As the equivalence point for the titration (pH =9.3) occurs within the pH range for the visible colour change of the indicator, this indicator can be used for the given titration.

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