We let the chemical equation for the weak acid indicator propyl red HPr be HPr + H2O ↔ H3O+ + Pr- Therefore, the acid dissociation constant Ka is Ka = [H3O+][Pr-] / [HPr] The color of this indicator turns from red to yellow or the other way around at its turning point at which [HPr] = [Pr-] Substituting this to the equation for Ka, we now have Ka = [H3O+] The pH of the solution at its turning point is pH = -log[H3O+] = -log(Ka) = -log 3.3×10^-6 = 5.48 On this account, the pH range is pH 5.0 to 6.0.
