POTASSIUM TETRAIODOMERCURATE(II) OR NESSLER'S REAGENT[K2[HgI4]]

Potassium tetraiodomercurate(II) is an inorganic compound consisting of potassium cations and the tetraiodomercurate(II) anion. It is mainly used as Nessler's reagent, a 0.09 mol/L solution of potassium tetraiodomercurate(II) (K₂[HgI₄]) in 2.5 mol/L potassium hydroxide, used to detect ammonia.

Detailed description

Preparation and structure

Crystallizing from a concentrated aqueous solution of mercuric iodide with potassium iodide is the monohydrate KHgI₃^.H₂O, which is pale orange.^[3] In aqueous solution this triodido complex adds iodide to give the tetrahedral tetraiodo dianion.^[4]

Solutions of K₂HgI₄ react with Cu(I) salts to give Cu₂HgI₄.^[5]

Nessler's reagent

Named after Julius Nessler, an alkaline solution of K₂HgI₄ is called Nessler's reagent. This pale solution becomes deeper yellow in the presence of ammonia. At higher concentrations, a brown precipitate may form. The sensitivity as a spot test is about 0.3 μg NH₃in 2 μL.

NH₄⁺ + 2[HgI₄]²⁻ + 4OH⁻ → HgO·Hg(NH₂)I ↓ + 7I⁻ + 3H₂O

The formula for the brown precipitate is given as 3HgO.Hg(NH₃)₂I₂ and as NH₂.Hg₂I₃^[6]

Nessler's reagent is generally prepared by combining potassium iodide and mercury(II) chloride.^[7] Nessler's reagent may be used with Nessler tubes.

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