Mercury(II) thiocyanate

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]

Mercury(II) thiocyanate
Names
Other names
Mercuric thiocyanate
Mercuric sulfocyanate
Identifiers
3D model (JSmol)
ECHA InfoCard 100.008.886
EC Number
  • 209-773-0
UNII
Properties
Hg(SCN)2
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
Hazards
Main hazards highly toxic
T+ N
NFPA 704 (fire diamond)
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 code
1
3
1
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).
N verify (what is YN ?)
Infobox references

Synthesis and structure

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]

Uses

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.[4]

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.[5]

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 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.[6]

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.[6]

Pharaoh's serpent
Main article: Black snake (firework)
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:[7] 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.

References
  1. 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.
  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. 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.
  5. 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.
  6. 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.
  7. "Make a Pharaoh's Snake Firework". About.com Education. Retrieved 2016-02-08.
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