Question

# The reagent used for the identification of ammonium cation in the analysis of a simple salt is:$A.\;\;\;\;\;KI \\ B.\;\;\;\;\;{K_2}Hg{I_4} \\ C.\;\;\;\;\;{K_2}Cr{O_4} \\ D.\;\;\;\;\;{K_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right] \\$

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Hint:
Reagents are used in a chemical reaction to detect, measure, or make other substances. Ammonium ions can be identified in a solution by adding dilute sodium hydroxide solution and gently heating. If ammonium ions are present they will convert to ammonia gas.

-The ammonia gas formed by the reaction of ammonium ions with NaOH reacts with Nessler’s reagent to form a brown precipitate of H2N, HgO, HgI
$2{K_2}\left[ {Hg{I_4}} \right] + N{H_3} + 3KOH \to {H_2}N.HgO.HgI + 7KI + 2{H_2}O$
${K_2}{\left[ {HgI} \right]_4}$ is an organic compound consisting of a potassium cation and the tetraiodomercurate(II) anion. It is mainly used as nessler’s reagent, used to detect ammonia.
-It is an aqueous solution of potassium iodide, mercuric chloride, and potassium hydroxide.
Ammonium ion is formed by the reaction between acids and aqueous ammonia. The ammonium ion behaves chemically like the ions of the alkali metals, particularly potassium ion, which is almost the same size. All ammonium salts are white and soluble.

Hence option B is correct.