Potassium tetraiodomercurate(II)

Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula K₂[HgI₄]. 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(H₂O)I₃] instead.

Applications

K₂[HgI₄] is a precursor to analogous copper and silver salts M₂[HgI₄] (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 (NH₃). At higher concentrations, a brown precipitate derivative of Millon's base (HgO·Hg(NH₂)Cl) may form. The sensitivity as a spot test is about 0.3 μg NH₃ in 2 μL.

NH+4 + 2 [HgI₄]²⁻ + 4 OH⁻ → HgO·Hg(NH₂)I↓ + 7 I⁻ + 3 H₂O

The brown precipitate is not fully characterized and may vary from HgO·Hg(NH₂)I to 3HgO·Hg(NH₃)₂I₂.

References

External links

• IARC Monograph: "Mercury and Mercury Compounds"
• National Pollutant Inventory - Mercury and compounds fact sheet