Indicators
Definition and meaning of Indicators in chemistry.
Indicators are substances, often weak organic acids or bases, that change color (or another visible property) in response to a change in their chemical environment, most commonly pH, allowing chemists to visually signal the endpoint of a titration or reaction.
In more detail
Most acid-base indicators are weak acids (HIn) whose protonated and deprotonated forms have different colors due to differing conjugate structures. The ratio of the two forms shifts with pH according to the Henderson-Hasselbalch relationship, so each indicator changes color over a characteristic pH transition range approximately equal to its pKa plus or minus one unit. Choosing an indicator whose transition range brackets a titration's equivalence point pH ensures an accurate endpoint. Other types include redox indicators (change color with electrode potential) and adsorption indicators (used in precipitation titrations).
Key facts
| Field | Analytical Chemistry |
|---|---|
| Common example | Phenolphthalein, C20H14O4 |
| Typical transition range | pH ≈ pKa ± 1 |
| Main types | Acid-base, redox, adsorption indicators |
Phenolphthalein is colorless in acidic and neutral solutions but turns pink above about pH 8.2, making it useful for titrating a strong acid with a strong base, where the equivalence point occurs near pH 7 but the color change becomes sharp and visible just past it.
Frequently asked questions
Why do indicators change color gradually rather than at a single exact pH?
Because both the protonated (HIn) and deprotonated (In-) forms are present together near the pKa, the observed color is a mixture; the human eye only detects a clear color shift once one form dominates, typically over about a two pH unit range centered on the pKa.
Can an indicator be used for any titration?
No, the indicator's transition range must overlap the sharp pH change occurring near the titration's equivalence point, so weak acid or weak base titrations require indicators chosen to match their specific equivalence point pH.