Question

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

• A. Brown ppt.
• B. Blue ppt.
• C. Black ppt.
• D. Green ppt.

Answer 1

The correct option is A

Brown ppt.

Explanation for correct option

(A) Brown ppt.

Nessler's reagent

• For the qualitative test of ammonia, Nessler's reagent is used.
• Nessler's reagent is potassium tetraiodomercurate (II) $\left({\mathrm{K}}_2{\mathrm{HgI}}_4\right)$.
• It is an inorganic compound made up of potassium cations and the anion tetraiodomercurate (II).

Test for ammonia

• The color of the complex generated will be determined by the color of the precipitate formed after the reaction of Nessler's reagent with ammonia $\left({\mathrm{NH}}_3\right)$.
• Potassium hydroxide $\left(\mathrm{KOH}\right)$ is added to make the solution of ammonia make slightly alkaline.
• Potassium tetraiodomercurate(II) generates dirty brown precipitate when passed through an ammonia solution.
• The reaction is as follows:

$\underset{(\mathrm{Potassium}\mathrm{tetraiodomercurate}(\mathrm{II}\left)\right)}{2{\mathrm{K}}_2{\mathrm{HgI}}_4}+\underset{\left(\mathrm{Potassium}\mathrm{Hydroxide}\right)}{3\mathrm{KOH}}+\underset{\left(\mathrm{Ammonia}\right)}{{\mathrm{NH}}_3}\to \underset{\left(\mathrm{Brown}\mathrm{ppt}.\right)}{\underset{\left(\mathrm{Iodide}\mathrm{of}\mathrm{Millon}\text{'}\mathrm{s}\mathrm{base}\right)}{\left[{\mathrm{OHg}}_2.{\mathrm{NH}}_2\right]\mathrm{I}}}+\underset{\left(\mathrm{Potassium}\mathrm{Iodide}\right)}{7\mathrm{KI}}+\underset{\left(\mathrm{Water}\right)}{2{\mathrm{H}}_2\mathrm{O}}$

Explanation for incorrect options

• The brown ppt. is formed when Nessler's reagent reacts with ammonia.
• Hence, all other options (B), (C) and (D) are incorrect.

Therefore the correct option is (A) Brown ppt​.