Potassium tetraiodomercurate(II)
# Potassium tetraiodomercurate(II)

Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula $K_2[HgI_4].$ It consists of potassium cations and tetraiodomercurate(II) anions. It is the active agent in Nessler's reagent, used for detection of ammonia.

## Preparation

The compound crystallizes from a heated solution of mercuric iodide, potassium iodide, and precisely 2% water in acetone. Attempted synthesis in concentrated aqueous solution will give the pale orange monohydrate $K[Hg(H_2O)I_3]$ instead.

## Applications

$K_2[HgI_4]$ is a precursor to analogous copper and silver salts $M_2[HgI_4]$ (M=Cu, Ag).

### Nessler's reagent

Nessler's reagent, named after Julius Neßler (Nessler), is a 0.09 mol/L solution of potassium tetraiodomercurate(II) in 2.5 mol/L potassium hydroxide. This pale solution becomes deeper yellow in the presence of ammonia $(NH_3).$ At higher concentrations, a brown precipitate derivative of Millon's base $(HgO·Hg(NH_2)Cl)$ may form. The sensitivity as a spot test is about 0.3 μg $NH_3$ in 2 μL.

    NH+4 + 2 $[HgI_4]^{2−}$ + 4 $OH^−$ → $HgO·Hg(NH_2)I↓$ + 7 $I^−$ + 3 $H_2O$

The brown precipitate is not fully characterized and may vary from $HgO·Hg(NH_2)I$ to $3HgO·Hg(NH_3)_2I_2.$

