Question

In Nessler's reagent for detection of ammonia, the

active species is :

• A. $H{g}_{2}C{l}_2$
• B. $H{g}^{2+}$
• C. $H{g}_2{I}_2$
• D. $Hg{I}_4^{2-}$

Answer 1

Answer: (D)

$Hg{I}_4^{2-}$

Active specie is $Hg{I}_4^{2-}$
Nessler's reagent - Potassium tetraiodomercurate(II) is an inorganic compound consisting of potassium cations and the tetraiodomercurate(II) anion. It is mainly used as Nessler's reagent.
An alkaline solution of $K_{2}Hg{I}_4$ is called Nessler's reagent.
In Nessler's reagent for the detection of ammonia the active species is $Hg{I}_4^{2-}$. Nessler's reagent gives brown ppt. of iodide of Milion's base with ammonium salt.

Solution of potassium tetraiodomercurate(II) ($K_{2}Hg{I}_4$) in potassium hydroxide, to detect ammonia or ammonium ions present in the solution. This pale solution becomes deeper yellow in the presence of ammonia. At higher concentrations, a brown precipitate may form.
Reaction: $2Hg{I}_4^{2-}+N{H}_{4}Cl+4O{H}^{-}⟶N{H}_2-Hg-O-Hg-I+7I^{-}+C{l}^{-}+3H_{2}O$