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

Question

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