Phenolphthalein is an indicator that changes colour to pink/purple between pH 8-->10, however, neutralization occurs when pH=7. Why can phenolphthalein still be used as the indicator during a strong acid-strong base titration with virtually no error?
Because ih the titration of a strong acid with a strong base the titration curve is so steep near the equivalence point that one drop of the titrant will change the pH from about 3 or 4 to about 10 or 11; therefore, virtually no titration error occurs.
A student performing this experiment forgot to add phenolphthalein solution to the vinegar solution
before beginning the titration. After adding 27 mL of NaOH solution, he realized his error and added the
indicator. The solution turned bright pink. Suggest a procedure the student could follow to salvage the titration.
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Phenolphthalein is commonly used as an indicator in strong acid-strong base titrations because it undergoes a color change in a pH range close to the equivalence point, where neutralization occurs. Although phenolphthalein changes color between pH 8 and 10, it can still be used effectively during a titration with virtually no error for the following reasons:
1. Fast response time: Phenolphthalein rapidly responds to changes in pH. This means that even if the indicator changes color slightly before or after the equivalence point at pH 7, it will still occur within a narrow pH range during the titration.
2. Steep color change: Phenolphthalein provides a sharp transition from colorless to pink/purple within the pH range of 8 to 10. This makes it easier to detect when the reaction is nearing completion, allowing for more precise endpoint determination.
3. Buffered solution: During the titration, a buffered solution is often prepared to maintain a relatively constant pH. A buffer consists of a weak acid and its conjugate base (or a weak base and its conjugate acid) and helps resist changes in pH. By using a buffer, the pH range where phenolphthalein changes color can be adjusted to closely match the expected pH of the equivalence point at 7. This minimizes any potential error caused by the indicator's range.
4. pH sensitivity of the indicator: Despite its official color change range of pH 8-10, phenolphthalein actually begins to change color at a lower pH, around 8.2, and reaches maximum color intensity at pH 9.8. This means that the actual transition range overlaps with the expected pH at the equivalence point, making it suitable for strong acid-strong base titrations.
It is worth noting that if a more accurate or precise endpoint determination is required, other indicators or techniques such as pH meters can be used. These alternatives offer greater sensitivity and can provide more reliable results in specific cases.