Mercury(II) thiocyanate

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Mercury(II) thiocyanate
Mercury(II) thiocyanate in crystal
Mercury(II) thiocyanate
Other names
Mercuric thiocyanate
Mercuric sulfocyanate
3D model (JSmol)
ECHA InfoCard 100.008.886
EC Number 209-773-0
Molar mass 316.755 g/mol
Appearance White monoclinic powder
Odor odorless
Density 3.71 g/cm3, solid
Melting point 165 °C (329 °F; 438 K) (decomposes)
0.069 g/100 mL
Solubility Soluble in dilute hydrochloric acid, KCN, ammonia
slightly soluble in alcohol, ether
−96.5·10−6 cm3/mol
Main hazards highly toxic
Very Toxic T+Dangerous for the Environment (Nature) N
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oilHealth code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gasReactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calciumSpecial hazards (white): no codeNFPA 704 four-colored diamond
Lethal dose or concentration (LD, LC):
46 mg/kg (rat, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Mercury(II) thiocyanate (Hg(SCN)2) is an inorganic chemical compound, the coordination complex of Hg2+ and the thiocyanate anion. It is a white powder. It will produce a large, winding “snake” when ignited, an effect known as the Pharaoh's serpent.[1]

Synthesis and structure[edit]

The first synthesis of mercury thiocyanate was probably completed in 1821 by Jöns Jacob Berzelius:

HgO + 2 HSCN → Hg(SCN)2 + H2O

Evidence for the first pure sample was presented in 1866 prepared by a chemist named Otto Hermes.[1] It is prepared by treating solutions containing mercury(II) and thiocyanate ions. The low solubility product of mercury thiocyanate causes it to precipitate from solution.[2] Most syntheses are achieved by precipitation:

Hg(NO3)2 + 2 KSCN → Hg(SCN)2 + 2KNO3

The compound adopts a polymeric structure with Hg2+ centers 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.[3]

Pharaoh's serpent[edit]

Pharaoh's serpent demonstration

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 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 color with the inside generally much darker than the outside.[1]

The reaction has several stages as follows:[4]

Igniting mercury thiocyanate causes it to form an insoluble brown mass that is primarily carbon nitride, C3N4. Mercury sulfide and carbon disulfide are also produced.

2Hg(SCN)2 → 2HgS + CS2 + C3N4

Carbon disulfide combusts to carbon dioxide and sulfur dioxide:

CS2 + 3O2 → CO2 + 2SO2

The heated C3N4 partially breaks down to form nitrogen gas and cyanogen:

2C3N4 → 3(CN)2 + N2

Mercury sulfide reacts with oxygen to form mercury vapor and sulfur dioxide. If the reaction is performed inside a container, a gray film of mercury coating on its inner surface can be observed.

HgS + O2 → Hg + SO2

This reaction was discovered by Wöhler in 1821, soon after the first synthesis of mercury thiocyanate: "winding out from itself at the same time worm-like processes, to many times its former bulk, a very light material the color of graphite...". For some time, a firework product called "Pharaoschlangen" was available to the public in Germany, but was eventually banned when the toxic properties of the product were discovered through the death of several children mistakenly eating the resulting solid.[1]

A similar, although less extreme, effect to the Pharaoh's serpent can be achieved using a firework known as a black snake. These are generally benign products, usually consisting of sodium bicarbonate or a mixture of linseed oil and naphthalenes.[citation needed]


Mercury thiocyanate has a few uses in chemical synthesis. It is the precursor to potassium tris(thiocyanato)mercurate(II) (K[Hg(SCN)3]) and caesium tris(thiocyanato)mercurate(II) (Cs[Hg(SCN)3]). The Hg(SCN)3 ion can also exist independently and is easily generated from the compounds above, amongst others.[5]

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.[6]

Use in chloride analysis[edit]

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 common method for determination of chloride ions in laboratories worldwide ever since. An automated system was invented in 1964 and then a commercial chloroanalyzer was made available in 1974 by Technicon (Tarrytown, NY, USA). The basic mechanism involves the addition of mercury thiocyanate to a solution with unknown concentration of chloride ions and iron as a reagent. 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 concentration of the iron complex. This value allows one to calculate the concentration of chloride.[7]

It can be used for determining the concentration of chloride ions in 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 concentration than the aforementioned technique using iron.[7]


  1. ^ a b c d Davis, T. L. (1940). "Pyrotechnic Snakes". Journal of Chemical Education. 17 (6): 268–270. doi:10.1021/ed017p268.
  2. ^ Sekine, T.; Ishii, T. (1970). "Studies of the Liquid-Liquid Partition systems. VIII. The Solvent Extraction of Mercury (II) Chloride, Bromide, Iodide and Thiocyanate with Some Organic Solvents" (pdf). Bulletin of the Chemical Society of Japan. 43 (8): 2422–2429. doi:10.1246/bcsj.43.2422.[permanent dead link]
  3. ^ Beauchamp, A.L.; Goutier, D. "Structure cristalline et moleculaire du thiocyanate mercurique" Canadian Journal of Chemistry 1972, volume 50, p977-p981. doi:10.1139/v72-153
  4. ^ "Make a Pharaoh's Snake Firework". Education. Retrieved 2016-02-08.
  5. ^ Bowmaker, G. A.; Churakov, A. V.; Harris, R. K.; Howard, J. A. K.; Apperley, D. C. (1998). "Solid-State 199Hg MAS NMR Studies of Mercury(II) Thiocyanate Complexes and Related Compounds. Crystal Structure of Hg(SeCN)2". Inorganic Chemistry. 37 (8): 1734–1743. doi:10.1021/ic9700112.
  6. ^ Kitamura, T.; Kobayashi, S.; Taniguchi, H. (1990). "Photolysis of Vinyl Halides. Reaction of Photogenerated Vinyl Cations with Cyanate and Thiocyanate Ions". Journal of Organic Chemistry. 55 (6): 1801–1805. doi:10.1021/jo00293a025.
  7. ^ a b Cirello-Egamino, J.; Brindle, I. D. (1995). "Determination of chloride ions by reaction with mercury thiocyanate in the absence of iron(III) using a UV-photometric, flow injection method". Analyst. 120 (1): 183–186. doi:10.1039/AN9952000183.

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