Question

1. You have three indicators available for your use:
   
   Bromocresol green (pK_a = 4.7)
   Thymol blue (pK_a,1 = 65, pK_a,2 = 8.9)
   p-nitrophenol (pK_a = 7.2)
   
   Without performing calculations, which indicator would you select for each of the following titrations. Explain your reasoning.
   1. NH₃ with HNO₃
   2. Ba(OH)₂ with HBr
   3. LiOH with HCOOH

Answer 1

To be an efficient indicator the pKa value of the indicator must be close to the medium pH or equivalent pH (pH=pKa+1 to pKa-1).

So for a) NH3 which is a weak base and HNO3 which is a strong acid,the medium pH will be less than 7(slightly acidic). So I will select bromocresol green(pKa=4.7) for these titration.

b) For Ba(OH)2 which is a strong base and HBr which is a strong acid, the medium pH will be almost close to 7( almost neutral). So I will select p-nitrophenol(pKa=7.2) for these titration.

c) For LiOH which is a strong base and HCOOH which is a weak acid, the medium pH will be greater than 7(slightly basic). So I will select Thymol blue (pka2=8.9) for these titration.