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.

Nesseler's reagent is an alkaline solution of Potassium tetraiodomercurate(II) ($K_2\left[Hg{I}_4\right]$). It is prepared by combining potassium iodide (KI) and mercuric chloride ($HgC{l}_2$)

It is made slightly alkaline by adding KOH or NaOH.

On being reacted with gaseous ammonia it produces brown fumes and on being passed through a solution of ammonia, it gives a dirty brown precipitate. The reaction involved is:

$2K_2\left[Hg{I}_4\right]+3KOH+N{H}_3\to [OH{g}_2.N{H}_2]I+7KI+2H_{2}O$

The product formed is brown in colour and is used in test for ammonium ion.

It is a qualitative test for ammonia.

$[OHg_2.NH_2]I$ is called Iodide of Million's base and has structure as:

$H_{2}N-Hg-O-Hg-I$ which is an insoluble/ brown ppt.