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[Nikunj Gangar]

Recently i performed an experiment in chem. lab i.e. analysis of NH₂ in given aniline sample. It was done in the following manner:
1. For blank titration :
add brominating soln. in a bottle, add HCl. Then cool for 30 mins. Then add KI soln. and finally titrate with Na₂S₂O₃
2. For back titration:
similar procedure to be followed, just that the aniline sample before titration.

What I dont understand is that why is bromination of aniline is done for titration? Secondly what is the role of brominating soln? Can't bromine be used directly.
It would be great if somebody would explain me the above with reaction mechanism
Thanks,
Nikunj.