Written by Adeel Abbas
Acid-base titration is an important technique used in chemistry to determine the concentration of an unknown acid or base. The titration involves slowly adding a standardized solution of either an acid or a base to the unknown solution until the reaction is complete. To determine the end-point of the reaction, an indicator is used. The indicator changes color at a specific pH, which indicates that the reaction has reached its endpoint.
Choosing the right indicator is crucial for obtaining accurate results in an acid-base titration. Here are some key factors to consider when selecting an indicator:
- pH range: The indicator must have a pH range that overlaps with the pH range of the titration reaction. For example, if the titration involves a strong acid and a weak base, the pH at the equivalence point will be acidic. Therefore, an indicator that changes color in an acidic pH range should be used. Some commonly used indicators and their pH ranges are:
- Phenolphthalein: pH range of 8.2-10.0
- Methyl orange: pH range of 3.1-4.4
- Bromothymol blue: pH range of 6.0-7.6
- Color change: The indicator must have a clear and distinct color change that is easily recognizable. The color change should also be sharp, which means that it should occur over a narrow pH range.
- Sensitivity: The indicator should be sensitive to small changes in pH. This is important because the endpoint of the reaction is usually very close to the equivalence point. If the indicator is not sensitive enough, it may not show a clear color change at the endpoint.
- Chemical stability: The indicator should be chemically stable and not react with either the acid or the base. If the indicator reacts with the acid or base, it may affect the accuracy of the titration.
- Availability and cost: Some indicators may be more readily available and less expensive than others. However, the availability and cost should not compromise the accuracy of the titration.
In summary, choosing the right indicator is crucial for obtaining accurate results in an acid-base titration. The indicator should have a pH range that overlaps with the pH range of the titration reaction, a clear and distinct color change, sensitivity to small changes in pH, chemical stability, and be readily available and affordable.