Question

Which of these indicators would be the best choice for the titration of HBr(aq) with NH3(aq) ? Assume 0.1 M solutions

A) methyl orange, pKIn = 3.4 B) * bromocresol green, pKIn = 4.7 C) bromothymol blue, pKIn = 7.1 D) phenolphthalein, pKIn = 9.4

the answere is B but im not sure why

Answer 1

answer : B)

explanation :

for best selection of indicator pKa value and equivalence point pH should be almost equal

in the titration of weak base and strong acid . the pH will be less than 7 at equivalence point.

it is generally in between 4 to 6 . so the indicator which has pKa falls in this range will be best for titration.

here bromocresol green indicator pKa value 4.7 . it can show correct color change at equivalence point.

option C) and D ) are not at all useful for titration because those are used in strong base and strong acid titrrations and also strong base weak acid titrations.

option is also useful for weak base and strong acid titration but best one is B)