methyl orange: red at pH

Question

methyl orange: red at pH < 3.1: orange at pH 3.1-4.4: yellow-orange above pH 4.4
Litmus: red at pH < 4.5: purple at pH 4.5-8.3: blue above pH 8.3
Thymol blue: yellow at pH < 8.0: green at pH 8.0-9.6: blue above pH 9.6
Trinitrobenzene: colorless at pH < 12: yellow at pH 12.0-1: orange above pH 14.0
Which of the pH indicators from the list above would be most appropriate for the titration of 0.30 M aqueous acetic acid (K_a = 1.8 × 10^-5) with 0.15 M aqueous sodium hydroxide?

Quizplus · Accepted Answer

Thymol blue would be the most appropriate pH indicator for the titration of acetic acid with sodium hydroxide because the equivalence point of the titration occurs at pH 8.2, which is within the pH range at which thymol blue changes color (pH 8.0-9.6). Methyl orange would change color at a lower pH and trinitrobenzene would change color at a much higher pH, while litmus would not provide a sharp color change at the equivalence point.

Methyl Orange: Red at PH < 3