(c) Mercury (II) Iodide

Preparation

It is prepared by treating HgCl₂ with KI.

        HgCl₂ + 2Kl ————→ Hgl₂ + 2KCl

                                          ^{(scorlet red)}

1. Mercuric iodide exists in two forms, i.e. red and yellow. The yellow form is stable above 400 K white the red form is stable below this temperature.

                >400K

        Hgl₂    ?      Hgl₂

         ^(red)  <400K ^(yellow)

 

2. It readily dissolves in KI forming a complex

        HgI₂ + 2KI ————→ K₂[HgI₄]

An alkaline solution of K₂HgI₄ is called Nessler’s reagent and is used to detect the presence of NH₄⁺ with which it gives a brown precipitate due to the formation of iodide of Million’s base.

Uses  

It is used to prepare Nessler’s reagent and for making ointments for treating skin infections.