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) (K
2 gI4 in 2.5 mol/L potassium hydroxide, used to detect
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
.
Preparation and structure
Crystallizing from a concentrated
aqueous solution of
mercuric iodide with potassium iodide is the
monohydrate K
2HgI
4.H
2O, which is pale orange.
[F. Wagenknecht, R. Juza, "Potassium Triiodomercurate(II)" in ''Handbook of Preparative Inorganic Chemistry'', 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1100.] In aqueous solution this triodido complex adds iodide to give the tetrahedral tetraiodo dianion.
Solutions of K
2HgI
4 react with Cu(I) salts to give Cu
2HgI
4.
Nessler's reagent
Named after
Julius Neßler (Nessler), an alkaline solution of K
2HgI
4 is called Nessler's reagent. This pale solution becomes deeper yellow in the presence of ammonia. At higher concentrations, a brown
precipitate
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
may form. The sensitivity as a
spot test
Spot analysis, spot test analysis, or spot test is a chemical test, a simple and efficient technique where analytic assays are executed in only one, or a few drops, of a chemical solution, preferably in a great piece of filter paper, without using ...
is about 0.3 μg NH
3 in 2 μL.
:NH
4+ + 2
gI4sup>2− + 4 OH
− → + 7 I
− + 3 H
2O
The formula for the brown precipitate, a derivative of
Millon's base, is given as 3HgO·Hg(NH
3)
2I
2 and as NH
2·Hg
2I
3.
References
External links
IARC Monograph: "Mercury and Mercury Compounds"
{{potassium compounds
Iodo complexes
Potassium compounds
Mercury(II) compounds
IARC Group 3 carcinogens
Coordination complexes
Chemical tests