In the analysis of food, the nitrogen from the protein is sometimes converted into ammonia which is titrated against a strong acid. Given that the final NH3 concentration (approximate) is 0.2M and that pKa for NH4+ is 9.2, choose the best indicator for such a titration
phenolphthalein - pKa 9.2
thymol blue - pKa 8.9
methyl orange - pKa 3.7
bromothymol blue - pKa 7.0
methyl red - pKa 5.1
What is the mass of 1 mL of water at 4 deg celsius?
To choose the best indicator for a titration, you need to consider the pH range over which the indicator changes color. In this case, the final concentration of NH3 is approximate 0.2M, which means the resulting solution will be basic.
When choosing an indicator for a basic solution, you want to select one that changes color around or above the pH of the equivalence point. The equivalence point is the point at which the acid and base are stoichiometrically equivalent.
Looking at the pKa values of the indicators given:
- phenolphthalein has a pKa of 9.2 (suitable for basic to slightly acidic solutions)
- thymol blue has a pKa of 8.9 (suitable for basic solutions)
- methyl orange has a pKa of 3.7 (suitable for acidic solutions)
- bromothymol blue has a pKa of 7.0 (suitable for slightly acidic to slightly basic solutions)
- methyl red has a pKa of 5.1 (suitable for slightly acidic solutions)
Based on the given pKa values, the best indicator for this titration would be phenolphthalein, as it has a pKa value closest to 9.2, which is the pKa of NH4+. Phenolphthalein changes color in the pH range of 8.2 to 10.0, making it suitable for this titration.