Question

What is the action of Nessler’s reagent on ammonia?
A. Brown ppt.
B. Blue ppt.
C. Black ppt.
D. Green ppt.

Answer 1

We have to know that for qualitative analysis of ammonia, we use Nessler’s reagent. The color of the precipitate obtained after the reaction of ammonia with Nessler’s reagent would be the color of the formed complex.

Complete answer:
We have to know that Nessler’s reagent was named after Julius Nessler’s, a German chemist. The color of Nessler’s reagent is pale yellow. We have to know that it appears as yellow crystals and is highly soluble in water, moderately soluble in alcohol, acetone, and ether.
We have to know that Potassium tetraiodomercurate (II) is an inorganic compound consisting of potassium cations and the tetraiodomercurate (II) anion. It is basically utilized as Nessler's reagent, a $0.09M$ solution of potassium tetraiodomercurate (II) $\left( {{K_2}\left[ {Hg{I_4}} \right]} \right)$in $2.5M$ potassium hydroxide, used to recognize ammonia. When Nessler’s reagent is reacted with ammonia in gaseous form, brown fumes are obtained and when the fumes are passed through ammonia solution, a dirty brown precipitate is obtained. We can write the reaction as,
${\text{2}}{{\text{K}}_{\text{2}}}\left[ {{\text{Hg}}{{\text{I}}_{\text{4}}}} \right]{\text{ + 3KOH + N}}{{\text{H}}_{\text{3}}}\xrightarrow{{}}\left[ {{\text{OH}}{{\text{g}}_{\text{2}}}{\text{ X N}}{{\text{H}}_{\text{2}}}} \right]{\text{I + 7KI + 2}}{{\text{H}}_{\text{2}}}{\text{O}}$
We have to know that the product obtained from the reaction of ammonia and Nessler’s reagent is brown.
The action of Nessler’s reagent on ammonia is brown ppt.

So, the correct answer is “Option A”.

Note:
We have to remember the chemical formula of the reagent and formed complex after its reaction with ammonia. The brown precipitate is because of the formation of this complex. One should not get confused between the color of Nessler’s reagent and the complex color. The complex color is brown whereas Nessler’s reagent color is pale yellow.