Nessler's reagent is an aqueous solution of potassium iodide, mercuric chloride, and potassium hydroxide. Its chemical formula is ${\mathrm{K}}_2{\mathrm{HgI}}_4$. It is used to detect the presence of ammonia. It turns into a pale brown colour in the presence of ammonia.

Nessler’s Reagent Reaction:

$2\underset{\mathrm{Nessler}\text{'}\mathrm{s} \mathrm{reagent}}{\underbrace{{\mathrm{K}}_2{\mathrm{HgI}}_4}}+{\mathrm{NH}}_3+3\mathrm{KOH}\stackrel{}{\to }\underset{\mathrm{Iodide} \mathrm{of} \mathrm{Millon}\text{'}\mathrm{s} \mathrm{base}\left(\mathrm{brown} \mathrm{precipitate}\right)}{\underbrace{\mathrm{HgO}.\mathrm{Hg}\left({\mathrm{NH}}_2\right)\mathrm{I}}}+7\mathrm{KI}+2{\mathrm{H}}_2\mathrm{O}$