Mercury(II) thiocyanate

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Mercury(II) thiocyanate
Hg(SCN)2 Xray.jpg
Mercury thiocyanate powder.png
Names
Other names
Mercuric thiocyanate
Mercuric sulfocyanate
Identifiers
3D model (JSmol)
ECHA InfoCard 100.008.886 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 209-773-0
PubChem CID
UNII
  • InChI=1S/2CHNS.Hg/c2*2-1-3;/h2*3H;/q;;+2/p-2
    Key: GBZANUMDJPCQHY-UHFFFAOYSA-L
  • C(#N)[S-].C(#N)[S-].[Hg+2]
Properties
Hg(SCN)2
Molar mass 316.755 g/mol
AppearanceWhite 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
Occupational safety and health (OHS/OSH):
Main hazards
highly toxic
GHS labelling: [1]
GHS-pictogram-skull.svg GHS-pictogram-silhouette.svg GHS-pictogram-pollu.svg
Danger
H300, H310, H330, H373, H410
P260, P262, P270, P271, P273, P280, P284, P301+P316, P302+P352, P304+P340, P316, P319, P320, P321, P330, P361, P364, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
3
1
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).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

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. [2]

Contents

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. [2] 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. [3] Most syntheses are achieved by precipitation:

Hg(NO3)2 + 2 KSCN → Hg(SCN)2 + 2 KNO3

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. [4]

Reactions

Mercury thiocyanate has a few uses in chemical synthesis. It is the precursor to other thiocyanate complexes such as 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

MMercury thiocyanate improve detection limits in the determination of chloride ions in water by UV-visible spectroscopy. This technique was a standard method for the determination of chloride ions in laboratories worldwide. The method involves the addition of mercury thiocyanate to a solution with an 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 the concentration of the iron complex. This value allows one to calculate the concentration of chloride. [7]

Pharaoh's serpent

Pharaoh's serpent demonstration Weze faraona.png
Pharaoh's serpent demonstration

Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. The fireworks versions were a mixture with a small amount of potassium nitrateand gum arabic as a binder. [8] This use ceased in most countries in the early 20th century due to the toxicity of mercury, and the existence of a superior alternatives.

Chemistry of the reaction

When heated, mercury(II) thiocyanate decomposes in an exothermic reaction that can produce a large mass of coiling, serpent-like solid. An inconspicuous flame, which is usually the blue color of burning carbon disulfide but which can be yellow from impurities or incidental combustion of flammable materials on the surface it is ignited on. The resulting solid can range from dark graphite gray to light tan in colour with the inside generally much darker than the outside. This was found to be due to decomposition of the produced β-HgS (black mercury sulfide) and vaporization of the resulting mercury from the othermost and hottest layers of the solid. [9]

The decomposition of Hg(SCN)2 is exothermic on its own, and the CS2 produced ignites easily and burns off. The C3N4 product is a simplification; the actual product contains 0.5% hydrogen and is likely to consist of sheets of triazine rings linked by −N= and −NH− groups similar to g−C3N4 and was found to contain nano-particles of β-HgS (black mercury sulfide). [9]

The number of resonance structures of heptazine and triazine, varying molecular weights of samples, and the fluorescense of the product made acquiring spectra difficult even by relatively exotic methods of NMR (with one spectrum acquisition being run for 12 days straight to get a mostly clean reading). Because of this, a heptazine-based structure similar to Liebig's melon, a compound initially prepared around the same time that the pharoah's snake reaction was discovered, was not ruled out by the authors as a partial component of the solid material. [9] The generalized reaction is as follows:

C3N4 is not a product of this decomposition. Cyanogen is generally only produced when Hg(CN)2 or similar is heated to decomposition, and early attempts to form (SCN)2 via the same route starting at this compound failed and only generated SO2, CO2 and N2. [9]

Related Research Articles

Carbon compounds are defined as chemical substances containing carbon. More compounds of carbon exist than any other chemical element except for hydrogen. Organic carbon compounds are far more numerous than inorganic carbon compounds. In general bonds of carbon with other elements are covalent bonds. Carbon is tetravalent but carbon free radicals and carbenes occur as short-lived intermediates. Ions of carbon are carbocations and carbanions are also short-lived. An important carbon property is catenation as the ability to form long carbon chains and rings.

<span class="mw-page-title-main">Thiocyanate</span> Ion (S=C=N, charge –1)

Thiocyanates are salts containing the thiocyanate anion [SCN]−. [SCN]− is the conjugate base of thiocyanic acid. Common salts include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyrotechnics.

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.

<span class="mw-page-title-main">Mercury(II) oxide</span> Chemical compound

Mercury(II) oxide, also called mercuric oxide or simply mercury oxide, is the inorganic compound with the formula HgO. It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroydite is very rarely found.

<span class="mw-page-title-main">Heptazine</span> Chemical compound

Heptazine, or tri-s-triazine or cyamelurine, is a chemical compound with formula C
6N
7H
3, that consist of a planar triangular core group or three fused triazine rings, with three hydrogen atoms at the corners. It is a yellow, weakly fluorescent solid with melting point over 300 °C. It is soluble in organic solvents such as acetonitrile, but is decomposed by water in the presence of light.

<span class="mw-page-title-main">Black snake (firework)</span> Type of firework which smokes and spews out ash resembling a snake, staying on the ground

"Black snake" is a term that can refer to at least three similar types of fireworks: the Pharaoh's snake, the sugar snake, or a popular retail composition marketed under various product names but usually known as "black snake". The "Pharaoh's snake" or "Pharaoh's serpent" is the original version of the black snake experiment. It produces a more impressive snake, but its execution depends upon mercury (II) thiocyanate, which is no longer in common use due to its toxicity. For a "sugar snake", sodium bicarbonate and sugar are the commonly used chemicals.

<span class="mw-page-title-main">Mercury(II) cyanide</span> Chemical compound

Mercury(II) cyanide, also known as mercuric cyanide, is a poisonous compound of mercury and cyanide. It is an odorless, toxic white powder. It is highly soluble in polar solvents such as water, alcohol, and ammonia, slightly soluble in ether, and insoluble in benzene and other hydrophobic solvents.

A solubility chart is a chart describing whether the ionic compounds formed from different combinations of cations and anions dissolve in or precipitate from solution.

<span class="mw-page-title-main">Mercury(II) acetate</span> Chemical compound

Mercury(II) acetate, also known as mercuric acetate is a chemical compound, the mercury(II) salt of acetic acid, with the formula Hg(O2CCH3)2. Commonly abbreviated Hg(OAc)2, this compound is employed as a reagent to generate organomercury compounds from unsaturated organic precursors. It is a white, water-soluble solid, but some samples can appear yellowish with time owing to decomposition.

<span class="mw-page-title-main">Potassium thiocyanate</span> Chemical compound

Potassium thiocyanate is the chemical compound with the molecular formula KSCN. It is an important salt of the thiocyanate anion, one of the pseudohalides. The compound has a low melting point relative to most other inorganic salts.

<span class="mw-page-title-main">Organomercury chemistry</span> Group of chemical compounds containing mercury

Organomercury chemistry refers to the study of organometallic compounds that contain mercury. Typically the Hg–C bond is stable toward air and moisture but sensitive to light. Important organomercury compounds are the methylmercury(II) cation, CH3Hg+; ethylmercury(II) cation, C2H5Hg+; dimethylmercury, (CH3)2Hg, diethylmercury and merbromin ("Mercurochrome"). Thiomersal is used as a preservative for vaccines and intravenous drugs.

Mercury(I) sulfide or mercurous sulfide is a hypothetical chemical compound of mercury and sulfur, with elemental formula Hg
2S. Its existence has been disputed; it may be stable below 0 °C or in suitable environments, but is unstable at room temperature, decomposing into metallic mercury and mercury(II) sulfide.

<span class="mw-page-title-main">Thiocyanogen</span> Chemical compound

Thiocyanogen, (SCN)2, is a pseudohalogen derived from the pseudohalide thiocyanate, [SCN]−, with behavior intermediate between dibromine and diiodine. This hexatomic compound exhibits C2 point group symmetry and has the connectivity NCS-SCN.

Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the Hg2+
2 ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studies in 1927 and Raman spectroscopy in 1934 making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.

<span class="mw-page-title-main">Ammonium thiocyanate</span> Chemical compound

Ammonium thiocyanate is an inorganic compound with the formula [NH4]+[SCN]−. It is an ammonium salt of thiocyanic acid. It consists of ammonium cations [NH4]+ and thiocyanate anions [SCN]−.

<span class="mw-page-title-main">Mercury(I) nitrate</span> Chemical compound

Mercury(I) nitrate is an inorganic compound, a salt of mercury and nitric acid with the formula Hg2(NO3)2. A yellow solid, the compound is used as a precursor to other Hg22+ complexes. The structure of the hydrate has been determined by X-ray crystallography. It consists of a [H2O-Hg-Hg-OH2]2+ center, with a Hg-Hg distance of 254 pm.

<span class="mw-page-title-main">Copper(I) thiocyanate</span> Chemical compound

Copper(I) thiocyanate is a coordination polymer with formula CuSCN. It is an air-stable, white solid used as a precursor for the preparation of other thiocyanate salts.

<span class="mw-page-title-main">Copper(II) thiocyanate</span> Chemical compound

Copper(II) thiocyanate (or cupric thiocyanate) is a coordination polymer with formula Cu(SCN)2. It is a black solid which slowly decomposes in moist air. It was first reported in 1838 by Karl Ernst Claus and its structure was determined first in 2018.

Lithium thiocyanate is a chemical compound with the formula LiSCN. It is an extremely hygroscopic white solid that forms the monohydrate and the dihydrate. It is the least stable of the alkali metal thiocyanates due to the large electrostatic deforming field of the lithium cation.

<span class="mw-page-title-main">Sulfur dicyanide</span> Chemical compound

Sulfur dicyanide is an inorganic compound with the formula S(CN)2. A white, slightly unstable solid, the compound is mainly of theoretical and fundamental interest given its simplicity. It is the first member of the dicyanosulfanes Sx(CN)2, which includes thiocyanogen ((SCN)2) and higher polysulfanes up to S4(CN)2. According to X-ray crystallography, the molecule is planar, the SCN units are linear, with an S-C-S angle of 95.6°.

References

  1. ↑ "Mercuric thiocyanate (Compound)". pubchem.ncbi.nlm.nih.gov. Retrieved 31 May 2023.
  2. 1 2 Davis, T. L. (1940). "Pyrotechnic Snakes". Journal of Chemical Education. 17 (6): 268–270. doi:10.1021/ed017p268.
  3. ↑ 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". Bulletin of the Chemical Society of Japan. 43 (8): 2422–2429. doi: 10.1246/bcsj.43.2422 .
  4. ↑ Beauchamp, A.L.; Goutier, D. "Structure cristalline et moleculaire du thiocyanate mercuric" Canadian Journal of Chemistry 1972, volume 50, p977-p981. doi : 10.1139/v72-153
  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. ↑ 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.
  8. ↑ Weingart, George W. (1947). "Part III. Products of Manufacture and Formulas". Pyrotechnics (2d, rev. and enl ed.). Brooklyn: Chemical Pub. Co. pp. 182–183. ISBN   9780820601120.
  9. 1 2 3 4 Miller, Thomas S.; d'Aleo, Anita; Suter, Theo; Aliev, Abil E.; Sella, Andrea; McMillan, Paul F. (17 November 2017). "Pharaoh's Serpents: New Insights into a Classic Carbon Nitride Material". Zeitschrift für anorganische und allgemeine Chemie (Journal of Inorganic and General Chemistry). 643 (21): 1572–1580. doi: 10.1002/zaac.201700268 .
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