Question

In the titration of ammonia with Hcl which of the following indicators would be best. This iis a strong-acid weak base system.

A. Bromocresol green, Ka=2.19x10^-5

B. Cresol purple, Ka= 3.98x10^-9

Answer 1

pKa of bromocresol green = -log (2.19*10^-5)

                                      = 4.66

pKa of Cresol purple = -log(3.98*10^-9

                              = 8.40

Weak base has initial pH greater than 7, titration with HCl causes to decrease the pH value and finally we get pH below 2. That is is besed on concentration of HCl. At equivalence point it is around 1 to 2 because HCl is strong acid. Weak base may have initial pH below 8.4 that means in between 7-8.40.

Indicator should have the pKa with in the range of pH change then only its color will be changed. So Cresol purple is not suitable as it has pKa value which is not in the range of pH change.

Bormocresol green has the pKa value which is with in the range of pH change value.

The best indicator is A) Bromocresol green.