Nessler's reagent is: 

A solution known as Nessler reagent $\left({\mathrm{K}}_2{\mathrm{Hlg}}_4\right)$ is created by mixing sodium hydroxide, potassium iodide, mercuric chloride, and/or mercury iodide.

${\mathrm{NH}}_4^{+}\stackrel{{\mathrm{H}}_2{\mathrm{PtCl}}_6}{⟶}{\left({\mathrm{NH}}_4\right)}_2$ $\left({\mathrm{PtCl}}_6\right)\downarrow$ (Yellow ppt.)

$\downarrow$

$\frac{{\displaystyle {\mathrm{Na}}_3\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]}}{ }$${\left({\mathrm{NH}}_4\right)}_3\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]\downarrow$$(\mathrm{Yellow} \mathrm{ppt}.)$