Magnesium chloride: Wikis

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Magnesium chloride
Magnesium chloride.jpg
Magnesium-chloride-3D-polyhedra.png
IUPAC name
Other names Magnesium dichloride
Identifiers
CAS number 7786-30-3 Yes check.svgY,
7791-18-6 (hexahydrate)
PubChem 24584
RTECS number OM2975000
Properties
Molecular formula MgCl2
Molar mass 95.211 g/mol (anhydrous)
203.31 g/mol (hexahydrate)
Appearance white or colourless crystalline solid
Density 2.32 g/cm3 (anhydrous)
1.569 g/cm3 (hexahydrate)
Melting point

714 °C (987 K) (on rapid heating: slow heating leads to decomposition from 300 °C)

Boiling point

1412 °C (1685 K)

Solubility in water anhydrous
54.3 g/100 ml (20 °C)
72.6 g/100 mL (100 °C)
hexahydrate
157 g/100 mL (20 °C)
Solubility in ethanol 7.4 g/100 mL (30 °C)
Refractive index (nD) 1.675 (anhydrous)
1.569 (hexahydrate)
Structure
Crystal structure CdCl2
Coordination
geometry
(octahedral, 6-coordinate)
Hazards
MSDS ICSC 0764
EU Index Not listed
Main hazards Irritant
Flash point Non-flammable
Related compounds
Other anions Magnesium fluoride
Magnesium bromide
Magnesium iodide
Other cations Calcium chloride
Strontium chloride
Barium chloride
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Magnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2(H2O)x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water. Magnesium chloride as the natural mineral bischofite is also extracted (solution mining) out of ancient seabeds, for example the Zechstein seabed in NW Europe or the Permian Period seabed in the central parts of the US. Anhydrous magnesium chloride is the principal precursor to magnesium metal, which is produced on a large scale.

Contents

Structure, preparation, and general properties

MgCl2 crystallizes in the cadmium chloride motif, which features octahedral Mg. A variety of hydrates are known with the formula MgCl2(H2O)x, and each loses water with increasing temperature: x = 12 (-16.4 °C), 8 (-3.4 °C), 6 (116.7 °C), 4 (181 °C), 2 (ca. 300 °C).[1] In the hexahydrate, the Mg2+ remains octahedral, but is coordinated to six water ligands.[2] The thermal dehydration of the hydrates MgCl2(H2O)x (x = 6, 12) does not occur straightforwardly.[3]

As suggested by the existence of some hydrates, anhydrous MgCl2 is a Lewis acid, although a relatively weak one.

In the Dow process, magnesium chloride is regenerated from magnesium hydroxide using hydrochloric acid:

Mg(OH)2(s) + 2 HCl → MgCl2(aq) + 2 H2O(l)

It can also be prepared from magnesium carbonate by a similar reaction.

In most of its derivatives, MgCl2 forms octahedral complexes. Derivatives with tetrahedral Mg2+ are less common. Examples include salts of (tetraethylammonium)2MgCl 4 and adducts such as MgCl2(TMEDA).[4]

Applications

Magnesium chloride serves as precursor to other magnesium compounds, for example by precipitation:

MgCl2(aq) + Ca(OH)2(aq) → Mg(OH)2(s) + CaCl2(aq)

It can be electrolysed to give magnesium metal:[5]

MgCl2(l) → Mg(l) + Cl2(g)

This process is practiced on a substantial scale.

Magnesium chloride is used for a variety of other applications besides the production of magnesium: the manufacture of textiles, paper, fireproofing agents, cements and refrigeration brine,[5] and dust and erosion control. Mixed with hydrated magnesium oxide, magnesium chloride forms a hard material called Sorel cement.

Magnesium ion Mg2+ (usually added as the chloride) is an important component in the polymerase chain reaction, a procedure used to amplify DNA fragments. It is generally used in experimental biology whenever RNA and DNA and their enzymes are to function in vitro, since Mg2+ is a necessary associate ion for nucleotides in biology, such as ATP.

Magnesium chloride is also used in several medical and topical (skin related) applications. It has been used in pills as supplemental sources of magnesium, where it serves as a soluble compound which is not as laxitive as magnesium sulfate, and more bioavailable than magnesium hydroxide and magnesium oxide, since it does not require stomach acid to produce soluble Mg2+ ion. It can also be used as an effective anaesthetic for cephalopods, some species of crustaceans,[6] and several species of bivalve, including oysters [7]

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Culinary use

Magnesium chloride is an important coagulant used in the preparation of tofu from soy milk. In Japan it is sold as nigari (にがり, derived from the Japanese word for "bitter"), a white powder produced from seawater after the sodium chloride has been removed, and the water evaporated. In China it is called lushui (卤水). Nigari or lushui consists mostly of magnesium chloride, with some magnesium sulfate and other trace elements. It is also an ingredient in baby formula milk. There is also 40 mg magnesium chloride in the bottled drink Vitamin Water, in the flavour 'Multi-v', lemonade [a-zinc].

Use as an anti-icer

A number of state highway departments throughout the United States have decreased the use of rock salt and sand on roadways and have increased the use of solutions of magnesium chloride (often called "liquid magnesium chloride") as a de-icer or anti-icer. Magnesium chloride is much less toxic to plant life surrounding highways and airports, and is less corrosive to concrete and steel (and other iron alloys) than sodium chloride. The liquid magnesium chloride is sprayed on dry pavement (tarmac) prior to precipitation or wet pavement prior to freezing temperatures in the winter months to prevent snow and ice from adhering and bonding to the roadway. The application of anti-icers is utilized in an effort to improve highway safety. Magnesium chloride is also sold in crystal form for household and business use to de-ice sidewalks and driveways. In these applications, the compound is applied after precipitation has fallen or ice has formed, instead of previously.

The use of this compound seems to show an improvement in driving conditions during and after freezing precipitation, but it can damage electric utilities. This occurs in two ways: contamination of insulators, causing tracking and arcing across them, and corrosion of steel and aluminium poles and pole hardware.

Use in dust and erosion control

Road departments and private industry may apply liquid or powdered magnesium chloride to control dust and erosion on unimproved (dirt or gravel) roads and dusty job sites such as quarries. Its hygroscopy makes it absorb moisture from the air, controlling the number of small particles which become airborne. Similarly, owners of indoor arenas (e.g. for horse riding) may apply magnesium chloride to sand or other floor materials to control dust.

Use in hydrogen storage

Magnesium chloride has shown promise as a storage material for hydrogen. Ammonia, which is rich in hydrogen atoms, is used as an intermediate storage material. Ammonia can be effectively adsorbed onto solid magnesium chloride, forming Mg(NH3)6Cl2. Ammonia is released by mild heat, and is then passed through a catalyst to give hydrogen gas.

Medical use

MgCl2 was one of the earlier antiseptics, first used for that purpose by Dr. Pierre Delbet in 1915. One veterinary study in 1989 indicated some effectiveness against tumors when used as a feed additive.[8]..

References

  1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  2. ^ Wells, A. F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  3. ^ see notes in Rieke, R. D.; Bales, S. E.; Hudnall, P. M.; Burns, T. P.; Poindexter, G S. “Highly Reactive Magnesium for the Preparation of Grignard Reagents: 1-Norbornane Acid” Organic Syntheses, Collected Volume 6, p.845 (1988). http://www.orgsyn.org/orgsyn/pdfs/CV6P0845.pdf
  4. ^ N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984.
  5. ^ a b Hill, Petrucci, McCreary, Perry, "General Chemistry", 4th ed., Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA.
  6. ^ http://webs.lander.edu/rsfox/invertebrates/homarus.html
  7. ^ Culloty, S.C. & Mukahy, M.F.. 1992. An evaluation of anaesthetics for Ostrea edulis (L.). Aquaculture. 107: 249-252.
  8. ^ Veterinary Medical Research Institute, Iowa State University
  • Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.

External links


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