The indicator propyl red has K_a = 3.3 × 10^-6. What would be the approximate pH range over which it would change color?

Question

Quizplus · Accepted Answer

The color change of an indicator occurs around its pKa value, which is the negative logarithm of its acid dissociation constant (Ka). For propyl red with a Ka of 3.3 × 10^-6, its pKa is -log(3.3 × 10^-6) ≈ 5.48. The color change typically spans a pH range of about ±1 pH unit around the pKa, so for propyl red, this would be approximately from 4.5 to 6.5.

The Indicator Propyl Red Has Ka = 3

The Indicator Propyl Red Has K_a = 3