Acid-base indicators are dyes that change color depending on the pH of a solution. They are useful because color gives a quick visual estimate of acidity or basicity without needing a pH meter. In laboratories, indicators help students identify acids, bases, neutral solutions, and the endpoint of titrations.
A good indicator makes an invisible chemical change easy to see.
Key Facts
- pH = -log[H+]
- For water at 25°C, pH + pOH = 14
- An acid-base indicator is usually a weak acid or weak base with different colors in its protonated and deprotonated forms.
- Indicator equilibrium can be written as HIn ⇌ H+ + In-
- The visible color change usually occurs over about pH = pKa ± 1 for the indicator.
- Choose an indicator whose transition range overlaps the steep vertical part of the titration curve near the equivalence point.
Vocabulary
- Indicator
- An indicator is a substance that changes color over a specific pH range.
- Transition range
- The transition range is the pH interval over which an indicator visibly changes from one color to another.
- Endpoint
- The endpoint is the point in a titration when the indicator changes color and signals that the titration should stop.
- Equivalence point
- The equivalence point is the point in a titration where stoichiometric amounts of acid and base have reacted.
- pKa
- pKa is a measure of acid strength and is the pH at which an indicator has equal amounts of its acid and base forms.
Common Mistakes to Avoid
- Choosing an indicator because the final color looks bright is wrong because the transition range must match the equivalence point region of the titration curve.
- Treating the endpoint and equivalence point as always identical is wrong because the endpoint is observed by color change while the equivalence point is based on reaction stoichiometry.
- Using too much indicator is wrong because indicators are weak acids or bases and large amounts can slightly change the solution being titrated.
- Reading a pH value from a single indicator color too precisely is wrong because most indicators change over a range and give only an approximate pH.
Practice Questions
- 1 An indicator has pKa = 5.0. Estimate the pH range over which it changes color using pKa ± 1.
- 2 A titration has an equivalence point at pH 8.8. Which indicator is better: bromothymol blue with transition range pH 6.0 to 7.6 or phenolphthalein with transition range pH 8.2 to 10.0? Explain briefly.
- 3 A weak acid is titrated with a strong base. Explain why phenolphthalein is often a better indicator than methyl orange for this titration.