What is Nesseler's reagent ? What is the action of ammonia on Nesseler's reagent?

Explanation

Nesseler's reagent is an alkaline solution of `K_(2)HgI_(4)` obtained by mixing the solutions of `HgI_(2)` and `KI` in excess.
`HgI_(2(aq))+underset("excess")(2KI_((aq)))rarr K_(2)HgI_(4(aq))`
Ammonia or ammonium salts give orange brown precipitate with Nesseler's reagent due to the formation of Million's base.