- Joined
- Jul 22, 2011
- Messages
- 1,507
- Reaction score
- 318
Which of the following indicators would be BEST for the titration of p-nitrophenol (pka 7.2) by NaOH?
A. Penolphthalein (pH range of color change is 8 and 9.6)
B. Thymol blue - pH 1.2 ~ 2.8
C. Methyl Red - pH 4.6 ~ 5.8
D. Bromthymol Blue pH 6 ~ 7.6
Their answer says "To predict the pH near equivalence point, which is what you are doing when you choose an indicator, you should use the Henderson-Hasselbalch equation. At 2 pH units beyond the Pka of the acid, the ratio of conjugate base is 100:1, which means that the rxn is almost at equivalence. This is the point at which the indicator should start to show some color change. In this question, the pka for the weak acid is 7.2. The best indicator is therefore around pH = 9.2, which is between 8 ~ 9.6"
So, my reasoning is that since a weak acid is titrated by a strong base, we know that pH at eq will be larger than 7. Also, we know that pH = 7.2 at half point. That means pH at eq should be larger than 7.2 as well. So, we can eliminate B and C. it is pretty "easy" to assume that A is the best answer, but how can we tell if the pH at eq is lower/higher than 8?
A. Penolphthalein (pH range of color change is 8 and 9.6)
B. Thymol blue - pH 1.2 ~ 2.8
C. Methyl Red - pH 4.6 ~ 5.8
D. Bromthymol Blue pH 6 ~ 7.6
Their answer says "To predict the pH near equivalence point, which is what you are doing when you choose an indicator, you should use the Henderson-Hasselbalch equation. At 2 pH units beyond the Pka of the acid, the ratio of conjugate base is 100:1, which means that the rxn is almost at equivalence. This is the point at which the indicator should start to show some color change. In this question, the pka for the weak acid is 7.2. The best indicator is therefore around pH = 9.2, which is between 8 ~ 9.6"
So, my reasoning is that since a weak acid is titrated by a strong base, we know that pH at eq will be larger than 7. Also, we know that pH = 7.2 at half point. That means pH at eq should be larger than 7.2 as well. So, we can eliminate B and C. it is pretty "easy" to assume that A is the best answer, but how can we tell if the pH at eq is lower/higher than 8?
Last edited:
