Practice: Chemistry student is using an indicator with a pKa of 4.7 for the titration of a strong acid with strong base. Calculate the pH range at which the indicator will change colors.
a) 2.7–3.5 b) 10–11 c) 4.7–8.7 d) 3.7–5.7 e) 2.7–5.8
Acid-Base Indicator is a weak acid or base that helps determine the pH at the endpoint of a titration.
Concept #1: Acid-Base Indicator
Indicators have a pH range of pKa+/- 1.
Example #1: Pick the best indicator for the following titration
a) bromothymol blue pKa = 7.2
b) thymolphthalein pKa = 9.2
c) phenolphthalein pKa = 9.5
d) methyl red pKa = 5.1
e) bromophenol blue pKa = 4.1
Practice: Bromophenol blue (pKa = 4.1) is a common acid-base indicator. It is yellow in its acidic form and blue in conjugate base form. If the solution being titrated has a pH = 4.0, what color would the bromophenol blue indicator possess?
a) yellow b) orange c) blue d) green e) purple
