Mercury(II) thiocyanate
Encyclopedia
Mercury thiocyanate (Hg
(S
C
N
)2) is an inorganic chemical compound
, the salt of Hg2+ and the thiocyanate
anion. It is a stable solid at room temperature that has the appearance of white powder with chunks; it can also be grey in color, depending on purity. Mercury compounds are extremely toxic and protective equipment should be used whenever working with mercury thiocyanate. However, it is commercially available, though expensive. Mercury
thiocyanate is best known for its former use in pyrotechnics
, as it will produce a large, winding “snake” when set on fire. This is known as the Pharaoh’s Serpent. Though some people still use it for this purpose, it is generally avoided because of the production of toxic gases when this reaction occurs.
, hexanes, and methyl isobutyl ketone
. Most syntheses are achieved by precipitation. The two early syntheses achieved separately by Berzelius and Friedrich Wöhler
were completed using the following reactions:
This property was discovered soon after the first synthesis of mercury thiocyanate by Wohler in 1821: “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.
A similar, though less extreme, effect to the Pharaoh’s Serpent can be achieved using a firework known as a black snake. These are generally benign products consisting usually of sodium bicarbonate
or a mixture of linseed oil
and napthalenes.
, Raman spectroscopy
and solid state NMR.
It can also be used in organic synthesis, to replace a halide on an organic compound with a thiocyanate group through a Sn2 mechanism. However, this reaction does not create one pure product as the SCN- can react on either end with the organohalide. This means that such a reaction would yield two distinct products, one with the sulfur bound to the organic compound and one with the nitrogen bound to the organic compound.
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 irons cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to complex with Fe(III), producing Fe(SCN)2+, which absorbs visible light at 450 nm. This absorption allows for the measurement of concentration of Fe(SCN)2+, produced as a result of the reaction between chloride ion and mercury thiocyanate. From this value the concentration of chloride can then be calculated.
In 1995, a new method for determining the concentration of chloride ions in aqueous solution using mercury thiocyanate was discovered. 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 wavelength of 254 nm, allowing more accurate measurements of concentration than the aforementioned technique using iron.
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
(S
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
N
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
)2) is an inorganic chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
, the salt of Hg2+ and the thiocyanate
Thiocyanate
Thiocyanate is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...
anion. It is a stable solid at room temperature that has the appearance of white powder with chunks; it can also be grey in color, depending on purity. Mercury compounds are extremely toxic and protective equipment should be used whenever working with mercury thiocyanate. However, it is commercially available, though expensive. Mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
thiocyanate is best known for its former use in pyrotechnics
Pyrotechnics
Pyrotechnics is the science of using materials capable of undergoing self-contained and self-sustained exothermic chemical reactions for the production of heat, light, gas, smoke and/or sound...
, as it will produce a large, winding “snake” when set on fire. This is known as the Pharaoh’s Serpent. Though some people still use it for this purpose, it is generally avoided because of the production of toxic gases when this reaction occurs.
Synthesis
The first synthesis of mercury thiocyanate was probably completed in 1821 by the chemist Jöns Jacob Berzelius with evidence for the first pure sample occurring in 1866 prepared by a chemist named Hermes. Because of its ionic nature, there are several ways to synthesize the compound. Mercury(II) thiocyanate is made by reacting solutions containing mercury(II) and thiocyanate ions. The low solubility product of mercury thiocyanate causes it to precipitate. It is also soluble in several solvents including benzeneBenzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
, hexanes, and methyl isobutyl ketone
Methyl isobutyl ketone
Methyl isobutyl ketone is the organic compound with the formula 2CHCH2CCH3. This colourless liquid, a ketone, is widely used as a solvent.-Production:...
. Most syntheses are achieved by precipitation. The two early syntheses achieved separately by Berzelius and Friedrich Wöhler
Friedrich Wöhler
Friedrich Wöhler was a German chemist, best known for his synthesis of urea, but also the first to isolate several chemical elements.-Biography:He was born in Eschersheim, which belonged to aau...
were completed using the following reactions:
- 2 HSCN + HgO → Hg(SCN)2 + H2O (Berzelius)
- Hg(NO3)2 (aq) + 2 KSCN (aq) → Hg(SCN)2 (s) + 2KNO3 (aq) (Wohler)
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 is started which produces a large mass of coiling serpent-like solid. An inconspicuous flame which is often blue but can also occur in yellow/orange accompanies the combustion. The resulting solid can range from dark graphite grey to light tan in color with the inside generally much darker than the outside.This property was discovered soon after the first synthesis of mercury thiocyanate by Wohler in 1821: “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.
A similar, though less extreme, effect to the Pharaoh’s Serpent can be achieved using a firework known as a black snake. These are generally benign products consisting usually of sodium bicarbonate
Sodium bicarbonate
Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...
or a mixture of linseed oil
Linseed oil
Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...
and napthalenes.
Uses and Chemical Properties
Mercury thiocyanate has a few uses in chemical synthesis. It is often involved in the synthesis of related compounds that contain mercury (II) and/or thiocyanates. These compounds include, but are not limited 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 reacted to form the compounds above amongst others. These compounds can be examined analytically using infrared spectroscopyInfrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
, Raman spectroscopy
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
and solid state NMR.
It can also be used in organic synthesis, to replace a halide on an organic compound with a thiocyanate group through a Sn2 mechanism. However, this reaction does not create one pure product as the SCN- can react on either end with the organohalide. This means that such a reaction would yield two distinct products, one with the sulfur bound to the organic compound and one with the nitrogen bound to the organic compound.
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
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...
. The chloride irons cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to complex with Fe(III), producing Fe(SCN)2+, which absorbs visible light at 450 nm. This absorption allows for the measurement of concentration of Fe(SCN)2+, produced as a result of the reaction between chloride ion and mercury thiocyanate. From this value the concentration of chloride can then be calculated.
In 1995, a new method for determining the concentration of chloride ions in aqueous solution using mercury thiocyanate was discovered. 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 wavelength of 254 nm, allowing more accurate measurements of concentration than the aforementioned technique using iron.