Question

Nesslefs reagent is used to detect

• A. $PO _{4}^{3-}$
• B. $MNO _{4}^{-}$
• C. $ NH_{4}^{+} $
• D. $ CrO_{4}^{2-} $

Answer 1

Answer: (C)

Nessler's reagent is prepared by the reaction of mercuric chloride in the excess of KI.
$HgCl _{2}+2 KI \longrightarrow \underset{\text { scarlet ppt. }}{ HgI _{2}}+2 KCl$
$HgI _{2}+2 KI \longrightarrow \underset{\text{potassium tetraiodo mercurate(II) colourless complex}}{K _{2}\left[ HgI _{4}\right]}$
Alkaline solution of $K _{2}\left[ HgI _{4}\right]$ is called Nessler's reagent. It is used to test $NH _{3}$ or $NH _{4}^{+}$ions which give brown precipitate or colouration with ammonia due to the formation of the iodide of Millon's base $Hg _{2} NI \cdot H _{2} O$. It has the structure