Nessler's reagent is a 2% solution of potassium tetraiodomercurate(II) in aqueous potassium hydroxide. Its chemical formula is $$\mathrm{K}_2[\mathrm{HgI}_4]$$. The elements present in it are Potassium (K), Mercury (Hg), and Iodine (I). Nitrogen (N) is not present in Nessler's reagent.