Question

In Nessler's reagent, the ion present is :

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

Answer 1

The correct option is B

$Hg{I}_4^{2-}$
The basic solution of potassium tetraiodomercurate(II) $\left(K_{2}Hg{I}_4\right)$ is known as Nessler’s reagent.
When $HgC{l}_2$ is added with $KI$ solution, a red ppt. of mercuric iodide $\left(Hg{I}_2\right)$ is formed. Later if excessive $KI$ solution is added to it and then a soluble compound of potassium mercuric iodide is formed. Then if this solution is alkaline in $KOH$ solution the resulting basic solution produced is called Nessler’s reagent.

It is used in identifying $N{H}_3$ or $N{H}_4^{+}$ radical or ammonium salt.