A chemist performs an acid–base titration using 0.176 M aqueous NaOH as the titrant with 26.15 mL of 0.102 M H2SO4. The endpoint of the titration is detected using phenolphthalein as the indicator.
Some acid–base indicators are listed along with their pKa values. Which indicators would be most suitable for monitoring a titration involving a strong acid and a strong base?
Choose one or more:
bromothymol blue; pKa = 6.8
methyl yellow; pKa = 3.5
phenol red; pKa = 7.2
alizarine yellow; pKa = 11.0
phenolpthalein; pKa = 9.2
I would say bromothymol blue and phenol red but i am not sure any thoughts?
Yes, either of those indicators would be suitable for a pH of 7.0, that is, titration of strong acid with strong base.
To determine which indicators would be most suitable for monitoring a titration involving a strong acid and a strong base, we need to consider the pKa values of the indicators and compare them to the pH range over which the titration occurs.
In a titration involving a strong acid and a strong base, the pH at the equivalence point will be around 7 (neutral), since both the acid and base are fully dissociated. Therefore, indicators with pKa values closest to 7 would be most suitable, as they will undergo a color change around the equivalence point.
Let's analyze the given indicators:
1. Bromothymol blue; pKa = 6.8:
- This indicator has a pKa value close to neutral (7), making it suitable for monitoring the titration involving a strong acid and a strong base. It can be considered a good option.
2. Methyl yellow; pKa = 3.5:
- Although this indicator has a lower pKa value, it is not suitable for monitoring the titration of a strong acid with a strong base near neutral pH. It will undergo a color change at a lower pH, which is far from the expected equivalence point.
3. Phenol red; pKa = 7.2:
- This indicator also has a pKa value close to neutral (7), making it suitable for monitoring the titration involving a strong acid and a strong base. It can be considered a good option.
4. Alizarine yellow; pKa = 11.0:
- This indicator has a high pKa value, indicating that it will undergo a color change at a basic pH. It is not suitable for monitoring the titration involving a strong acid and a strong base near neutral pH.
5. Phenolphthalein; pKa = 9.2:
- This indicator has a pKa value higher than neutral pH. Therefore, it is not ideal for monitoring a titration involving a strong acid and a strong base near neutral pH.
By considering the pKa values, the most suitable indicators for monitoring a titration involving a strong acid and a strong base near neutral pH are bromothymol blue (pKa = 6.8) and phenol red (pKa = 7.2).