High Quality Content by WIKIPEDIA articles! Nessler's reagent is a reagent named after Julius Nessler and is used to detect small amounts of ammonium ions.. It is a 0.09 mol/L solution of potassium tetraiodomercurate(II) (K2[HgI4]) in 2.5 mol/L potassium hydroxide. A yellow coloration indicates the presence of ammonia: at higher concentrations, a brown precipitate may form. The sensitivity as a spot test is about 0.3 ?g NH3 in 2 ?L. It is toxic if swallowed, inhaled or absorbed through the skin. It presents a neurological hazard and may act as a carcinogen and be a reproductive hazard. It is corrosive and causes burns. Nessler's reagent is generally prepared from potassium iodide and mercury(II) iodide. A hot concentrated solution of mercury(II) chloride is added to concentrated solution of potassium iodide, until the precipitate of mercury(II) iodide stops dissolving. The liquid is filtered, and potassium hydroxide and a further bit of mercury(II) chloride solution are added. The resulting solution is then cooled and diluted to required concentration. Nessler's reagent may be used with Nessler tubes.

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