Phenolphthalein is a commonly used acid-base indicator in titrations, and its effective pH range is approximately 8.2 to 10.0. This means that it changes color from colorless to pink as the pH of the solution transitions from slightly acidic to slightly basic.
Suitable Titration Trials for Phenolphthalein: Phenolphthalein is particularly suitable for titrations involving:
-
Strong Acid vs. Strong Base: The endpoint of a strong acid (e.g., hydrochloric acid, HCl) titrated with a strong base (e.g., sodium hydroxide, NaOH) will typically fall within the effective pH range of phenolphthalein.
-
Weak Acid vs. Strong Base: Titrations of weak acids (e.g., acetic acid, CH₃COOH) with strong bases (e.g., NaOH) will also yield a suitable endpoint for phenolphthalein since the endpoint will approach the basic side of the pH scale, making it suitable for visualizing the reaction.
-
Weak Base vs. Strong Acid: Titrations of weak bases (e.g., ammonia, NH₃) with strong acids (e.g., HCl) may not be suitable, as the endpoint generally falls outside the visible color change range of phenolphthalein.
Examples of Suitable Trials:
- Titrating hydrochloric acid (HCl) with sodium hydroxide (NaOH). The color change will occur around the equivalence point (pH 7), but a significant color change (from colorless to pink) will be seen as the solution exceeds pH 8.2.
- Titrating acetic acid (CH₃COOH) with sodium hydroxide (NaOH), where the equivalence point will be reached at a pH above 7, highlighting the change in color.
In summary, phenolphthalein is best used for titrations involving strong bases with either strong or weak acids where the pH at equivalence improves visibility within its effective range.
1 answer