Magnesium chloride

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. Anhydrous magnesium chloride is the principal precursor to magnesium metal, which is produced on a large scale.

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Structure, preparation, basic 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]

As suggested by the existence of several 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)₂MgCl₄ and adducts such as MgCl₂(TMEDA.^[3]

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:^[4]

MgCl₂(l) → Mg(l) + Cl₂(g)

This process is practiced on a substantial scale: In 1990, US production was around one million tonnes, with a bulk price around $180 per tonne.^{[citation needed]}

The thermal dehydration of the hydrates MgCl₂(H₂O)_x (x = 6, 12) does not occur straightforwardly.^[5]

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,^[4] and dust and erosion control. Mixed with hydrated magnesium oxide, magnesium chloride forms a hard material called Sorel cement. Magnesium chloride is also used as a reaction component in Polymerase Chain Reaction, a procedure used to amplify DNA fragments.

Culinary use

Magnesium chloride is an important coagulant used in the preparation of tofu from soy milk. In Japan it is sold as nigari (the term is derived from the Japanese word for "bitter"), a white powder produced from seawater after the sodium chloride has been removed, and the water evaporated. Nigari consists mostly of magnesium chloride, with some magnesium sulfate and other trace elements. It is also an ingredient in baby formula milk.

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 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 yet it seems to be negatively affecting electric utilities. Two main issues have been raised regarding the anti-icer magnesium chloride as it relates to electric utilities: 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 absorbed 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.

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. ^ N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984.
4. ^ ^{a b} Hill, Petrucci, McCreary, Perry, "General Chemistry", 4th ed., Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA.
5. ^ 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

• Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.