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
A. phenolphthalein - pKa 9.2
B. thymol blue - pKa 8.9
C. methyl orange - pKa 3.7
D. bromothymol blue - pKa 7.0
E. methyl red - pKa 5.1
To choose the best indicator for the titration of ammonia against a strong acid, we need to find an indicator that changes color within a pH range close to the pKa of the ammonium ion (NH4+).
The pKa of the ammonium ion is given as 9.2.
The best indicator for this titration would be one that changes color within a pH range close to 9.2.
Looking at the options given:
A. phenolphthalein - pKa 9.2
B. thymol blue - pKa 8.9
C. methyl orange - pKa 3.7
D. bromothymol blue - pKa 7.0
E. methyl red - pKa 5.1
Among these options, phenolphthalein has a pKa closest to 9.2, making it the best indicator choice for this titration. Therefore, the correct answer is A. phenolphthalein - pKa 9.2.
To choose the best indicator for such a titration, we need to consider the pH range over which the indicator undergoes a color change. This range should ideally cover the pH at which the reaction between the ammonia and the acid occurs.
The reaction involved in this titration is the neutralization of ammonia (NH3) with a strong acid, forming ammonium ions (NH4+). The pKa for NH4+ is given as 9.2.
Given this information, we need an indicator that changes its color within the pH range of approximately 9.2 to indicate the endpoint of the titration.
Looking at the options:
A. phenolphthalein (pKa 9.2) - This indicator has a pKa value that corresponds to the desired pH range, so it could be a suitable choice.
B. thymol blue (pKa 8.9) - This indicator's pKa value is slightly lower than the desired pH range, so it may not provide a clear color change at the endpoint.
C. methyl orange (pKa 3.7) - This indicator has a pKa value that is much lower than the desired pH range, so it is not suitable for this titration.
D. bromothymol blue (pKa 7.0) - This indicator's pKa value is significantly lower than the desired pH range, so it may not provide a clear color change at the endpoint.
E. methyl red (pKa 5.1) - This indicator's pKa value is significantly lower than the desired pH range, so it may not provide a clear color change at the endpoint.
Based on the analysis, the best indicator for this titration is A. phenolphthalein with a pKa value of 9.2.