Magnesium hydroxide structure, properties, nomenclature, uses

3296
Simon Doyle

The magnesium hydroxide is an inorganic compound whose chemical formula is Mg (OH)two. In its pure form it is a dull white solid with an amorphous appearance; However, with a small and exact content of impurities, it transforms into the crystalline solid brucite, a mineral found in certain deposits in nature, and is a rich source of magnesium..

It is a weak electrolyte or base, so its dissociation is low in water. This property makes Mg (OH)two a good acidity neutralizer for human consumption; remedy popularly known as milk of magnesia suspension. It is also a fire retardant by releasing water during its thermal decomposition..

Solid sample of magnesium hydroxide. Source: Chemicalinterest [Public domain]

In the upper image some magnesium hydroxide solids are shown, in which its opaque white color can be appreciated. The more crystalline they are, they develop glassy and pearly surfaces.

Its crystalline structure is peculiar since it establishes double-layered hexagonal crystals, which are promising designs for the design of new materials. In these layers their positive charges play an important role due to the substitution of Mgtwo+ by trivalent cations, and the species confined between the walls composed of OH anions-.

On the other hand, other applications derive depending on the morphology of the prepared particles or nanoparticles; as catalysts or adsorbents. In all of them, the 1: 2 ratio for Mg ions is kept constant.two+: OH-, reflected in the same formula Mg (OH)two.

Article index

  • 1 Structure
    • 1.1 Formula and octahedron
    • 1.2 Double layer
    • 1.3 Morphologies
  • 2 Properties
    • 2.1 Physical appearance
    • 2.2 Molar mass
    • 2.3 Density
    • 2.4 Melting point
    • 2.5 Solubility in water
    • 2.6 Refractive index
    • 2.7 pH
    • 2.8 Heat capacity
  • 3 Where is it located?
  • 4 Nomenclature
  • 5 Uses
    • 5.1 Neutralizer
    • 5.2 Antacid
    • 5.3 Fire retardant
    • 5.4 Catalyst
    • 5.5 Adsorbent
  • 6 References

Structure

Formula and octahedron

Ions that make up magnesium hydroxide. Source: Claudio Pistilli [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

The image above shows the ions that make up Mg (OH)two. As can be seen, there are two OH anions- for each Mg cationtwo+, which interact electrostatically to define a crystal with a hexagonal structure. The same formula indicates that the Mg: OH ratio is 1: 2.

However, the true crystal structure is a bit more intricate than assuming simple Mg ions.two+ and OH-. In fact, magnesium is characterized by having a coordination number of 6, so it can interact with up to six OH-.

Thus, the octahedron Mg (OH) is formed6, where the oxygen atoms evidently come from the OH-; and the crystal structure now rests on the consideration of such octahedra and how they interact with each other.

In fact, the units Mg (OH)6 end up defining double-layered structures that, in turn, are arranged in space to originate the hexagonal crystal.

Double layer

Double layer structure of magnesium hydroxide. Source: Smokefoot [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

The upper image shows the double layer structure of magnesium hydroxide (LDH): Layered double hydroxides). The green spheres represent Mg ionstwo+, which could be replaced by others with a higher charge to generate a positive charge in the layer.

Note that around each Mgtwo+ there are six red spheres connected to their respective white spheres; that is, the octahedral units Mg (OH)6. The OH- acts as a bridge to join two Mgtwo+ of different planes, which makes the layers are intertwined.

Likewise, it is observed that hydrogen atoms point up and down, and are primarily responsible for the intermolecular forces holding the two layers of Mg (OH) units together.6.

Neutral molecules (such as alcohols, ammonia and nitrogen) or even anions can lodge between these layers, depending on how positive they are (if there are Al3+ o Faith3+ replacing Mgtwo+). The “filler” of these species is confined by the surfaces composed of the OH anions.-.

Morphologies

Double-layered, hexagonal glass grows slowly or quickly. It all depends on the synthesis or preparation parameters: temperature, molar ratio, stirring, solvents, reagents as a source of magnesium, bases or precipitating agents, etc. As the crystal grows, it defines the microstructure or morphology of its nanoparticles or aggregates..

Thus, these nanoparticles can have cauliflower-like plate, platelet, or globule-like morphologies. Likewise, the distribution of their sizes can change, as can the degree of porosity of the resulting solids..

Properties

Physical appearance

It is a white, granular or powdered solid, and odorless.

Molar mass

58.3197 g / mol.

Density

3.47 g / mL.

Melting point

350 ° C. At this temperature it decomposes into oxide by releasing the water molecules contained in its crystals:

Mg (OH)two(s) => MgO (s) + HtwoO (g)

Water solubility

0.004 g / 100 mL at 100 ° C; that is, it barely dissolves in boiling water, making it an insoluble compound in water. However, as the pH decreases (or the acidity increases), its solubility increases due to the formation of the complex aqueous, Mg (OHtwo)6.

On the other hand, if the Mg (OH)two has absorbed COtwo, will release retained gas as effervescence when dissolved in acidic medium.

Refractive index

1,559

pH

An aqueous suspension thereof has a pH that varies between 9.5 and 10.5. Although these values ​​are normal, it reflects its low basicity compared to other metal hydroxides (such as NaOH).

Heat capacity

77.03 J / mol K

Where is it located?

Pastel blue vitreous crystal of the mineral brucite. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

Magnesium hydroxide can be found in nature as the mineral brucite, which is characterized by its transparent white color, with green or bluish tones depending on its impurities. Likewise, brucite is part of some clays, such as chlorite, as it is sandwiched between the layers of silicates, joined by metal ions..

In brucite there are other ions in addition to Mgtwo+, like Al3+, Faith3+, Zntwo+ and Mntwo+. Its ores can be found in different regions or lakes of Scotland, Canada, Italy and the USA..

Physically its crystals look like molten glass (upper image), with white, grayish, bluish or greenish colors, and transparent in rare specimens.

This mineral is one of the evils that affect cements and concrete, since it tends to expand and cause fractures in them. However, it does not absorb COtwo, so its calcination does not contribute to the greenhouse effect and, therefore, it is an appropriate mineralogical source (and the richest) to obtain magnesium, in addition to seawater.

Nomenclature

Mg (OH)two It has up to three IUPAC accepted names (outside of mineralogy or medicine). These are very similar to each other, since the way they end up hardly varies..

For example, 'magnesium hydroxide' corresponds to its name according to the stock nomenclature, omitting (II) at the end because +2 is almost by default the only oxidation state of magnesium.

'Magnesium dihydroxide', denoting the number of OH ions with the Greek numerator prefix- indicated in the formula according to the systematic nomenclature. And 'magnesium hydroxide', ending with the suffix -ico for being the maximum and “only” oxidation state of magnesium, according to the traditional nomenclature..

The other names, such as brucite or milk magnesia, although they are directly related to this compound, it is not advisable to refer to it when it comes to its purest solid, or as an inorganic compound (reagent, raw material, etc.).

Applications

Neutralizer

Mg (OH)two It owes to its low solubility in water the fact that it is an excellent acidity neutralizer; otherwise, it would basify the medium by providing large concentrations of OH ions-, just like other bases do (strong electrolytes).

Thus, Mg (OH)two barely releases OH-, while reacting with H ions3OR+ to form the aqueous complex of magnesium, also mentioned above. By being able to neutralize the acidity of aqueous media, it is intended for the treatment of wastewater.

It is also an additive to food, fertilizers, and certain personal hygiene products, such as toothpaste, as it reduces their acidity..

Antacid

Being slightly soluble in water, it can be ingested without risking the effects of its OH ions- (dissociates very little as a weak electrolyte).

This characteristic, linked to the subsection above, makes it an antacid to treat heartburn, gastrointestinal diseases, indigestion and constipation, sold under the formula of milk of magnesia.

On the other hand, milk of magnesia also helps fight annoying canker sores (the white and red sores that appear in the mouth).

Fire retardant

In the properties section it was mentioned that Mg (OH)two decomposes releasing water. Precisely, this water helps to stop the advance of the flames, since they absorb heat to vaporize and, in turn, the vapors dilute the combustible or flammable gases.

The mineral brucite is usually used industrially for this purpose, destined as a filler in certain materials, such as plastics of different polymers (PVC, resins, rubbers), cables or ceilings..

Catalyst

Mg (OH)two synthesized as nanoplates, it has been shown to be efficient in catalyzing chemical reductions; for example, that of 4-nitrophenol (Ph-NOtwo) to 4-aminophenol (Ph-NHtwo). Likewise, these have antibacterial activity, so it could be used as a therapeutic agent.

Adsorbent

Some solids of Mg (OH)two they can be quite porous, depending on the method of their preparation. Therefore, they find application as adsorbents.

In aqueous solutions they can adsorb (on their surfaces) the dye molecules, clarifying the water. For example, they are capable of adsorbing the indigo carmine dye present in streams of water..

References

  1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
  2. Wikipedia. (2019). Magnesium hydroxide. Recovered from: en.wikipedia.org
  3. National Center for Biotechnology Information. (2019). Magnesium hydroxide. PubChem Database. CID = 14791. Recovered from: pubchem.ncbi.nlm.nih.gov
  4. Amethyst Galleries. (2014). The mineral brucite. Recovered from: galleries.com
  5. Henrist et al. (2003). Morphological study of magnesium hydroxide nanoparticles
  6. precipitated in dilute aqueous solution. Journal of Crystal Growth 249, 321-330.
  7. Saba J., Shanza R. K., Muhammad R. S. (2018). Synthesis and structural analysis of mesoporous magnesium hydroxide nanoparticles as efficient catalyst.
  8. Thimmasandra Narayan Ramesh and Vani Pavagada Sreenivasa. (2015). Removal of Indigo Carmine Dye from Aqueous Solution Using Magnesium Hydroxide as an Adsorbent. Journal of Materials, vol. 2015, Article ID 753057, 10 pages. doi.org/10.1155/2015/753057

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