Mercury(II) thiocyanate (Hg(SCN)₂) is an inorganic

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

, the

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. M ...

of Hg²⁺ and the

thiocyanate Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyr ...

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

. It is a white powder. It will produce a large, winding "snake" when ignited, an effect known as the Pharaoh's serpent.

Synthesis and structure

The first synthesis of mercury thiocyanate was probably completed in 1821 by

Jöns Jacob Berzelius Baron Jöns Jacob Berzelius (; by himself and his contemporaries named only Jacob Berzelius, 20 August 1779 – 7 August 1848) was a Swedish chemist. Berzelius is considered, along with Robert Boyle, John Dalton, and Antoine Lavoisier, to be o ...

: :HgO + 2 HSCN → Hg(SCN)₂ + H₂O Evidence for the first pure sample was presented in 1866 prepared by a chemist named Otto Hermes. It is prepared by treating solutions containing mercury(II) and thiocyanate ions. The low solubility product of mercury thiocyanate causes it to precipitate from the solution. Most syntheses are achieved by precipitation: :Hg(NO₃)₂ + 2 KSCN → Hg(SCN)₂ + 2KNO₃ The compound adopts a polymeric structure with Hg²⁺ centres linearly coordinated to two S atoms with a distance of 2.381 Å. Four weak Hg²⁺--N interactions are indicated with distances of 2.81 Å.

Uses

Mercury thiocyanate has a few uses in chemical synthesis. It is the precursor to potassium tris(thiocyanato)mercurate(II) (K g(SCN)₃ and caesium tris(thiocyanato)mercurate(II) (Cs g(SCN)₃. The Hg(SCN)₃⁻ ion can also exist independently and is easily generated from the compounds above, amongst others. Its reactions with organic halides yield two products, one with the sulfur bound to the organic compound and one with the nitrogen bound to the organic compound.

Use in chloride analysis

It was discovered that mercury thiocyanate can improve detection limits in the determination of chloride ions in water by UV-visible spectroscopy. This technique was first suggested in 1952 and has been a standard method for the determination of chloride ions in laboratories worldwide ever since. An automated system was invented in 1964 and then a commercial colour analyzer was made available in 1974 by Technicon (

Tarrytown, NY Tarrytown is a village in the town of Greenburgh in Westchester County, New York. It is located on the eastern bank of the Hudson River, approximately north of Midtown Manhattan in New York City, and is served by a stop on the Metro-North ...

, USA). The basic mechanism involves the addition of mercury thiocyanate to a solution with an unknown concentration of chloride ions and iron as a

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

. The chloride ions cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to bind Fe(III), which absorbs intensely at 450 nm. This absorption allows for the measurement of the concentration of the iron complex. This value allows one to calculate the concentration of chloride. It can determine the concentration of chloride ions in an aqueous solution. Mercury thiocyanate without iron (III) is added to a solution with an unknown concentration of chloride ions, forming a complex of the mercury thiocyanate and chloride ion that absorbs light at a 254 nm, allowing more accurate measurements of attention than the aforementioned technique using the iron.

Pharaoh's serpent

Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. When the compound is in the presence of a strong enough heat source, a rapid,

exothermic reaction In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...

that produces a large mass of coiling, serpent-like solid is started. An inconspicuous flame, which is often blue but can also be yellow/orange, accompanies the combustion. The resulting solid can range from dark graphite gray to light tan in colour with the inside generally much darker than the outside. The reaction has several stages as follows: Igniting mercury thiocyanate causes it to form an insoluble brown mass that is primarily

carbon nitride In organic chemistry, carbon nitrides are compounds consisting only of carbon and nitrogen atoms. Covalent network compounds * Beta carbon nitride - a solid with a formula β-, which is predicted to be harder than diamond. * Graphitic carb ...

, C₃N₄. Mercury sulfide and carbon disulfide are also produced.

References

External links

"Pharaoh's snake"

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. September 2, 2008.
"How to make the Pharaoh's Serpent (Mercury (II) Thiocyanate)"

. March 24, 2014 {{Mercury compounds Thiocyanates Mercury(II) compounds