MAGNESIUM HYDROXIDE: STRUCTURE, PROPERTIES, NOMENCLATURE, USES - CHEMISTRY - 2025

The magnesium hydroxide is an inorganic compound having the chemical formula Mg (OH) ₂. 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) ₂ 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

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. Their positive charges play an important role in these layers due to the substitution of Mg ²⁺ by trivalent cations, and to 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 is kept constant for Mg ²⁺: OH ^- ions, reflected in the same formula Mg (OH) ₂.

Structure

Formula and octahedron

Ions that make up magnesium hydroxide. Source: Claudio Pistilli

The upper image shows the ions that make up Mg (OH) ₂. As can be seen, there are two OH ^- anions for each Mg ²⁺ cation, 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 ²⁺ and OH ^- ions. 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) _{6 is formed}, where the oxygen atoms evidently come from the OH ^-; and the crystal structure now rests on considering such octahedra and how they interact with each other.

In fact, the Mg (OH) ₆ units 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

The upper image shows the magnesium hydroxide (LDH) double-layer structure. The green spheres represent the Mg ²⁺ ions, which could be replaced by others with a higher charge to generate a positive charge in the layer.

Note that around each Mg ²⁺ 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 Mg ²⁺ of different planes, which makes the layers intertwine.

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) ₆ units together.

Neutral molecules (such as alcohols, ammonia and nitrogen) or even anions can be housed between these layers, depending on how positive they are (if there are Al ³⁺ or Fe ³⁺ ions replacing Mg ²⁺). The "filler" of these species is confined by the surfaces composed of the OH ^- anions.

Morphologies

Double-layered, hexagonal glass grows slowly or rapidly. Everything 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) ₂ (s) => MgO (s) + H ₂ O (g)

Water solubility

0.004 g / 100 mL at 100 ° C; that is to say, 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 (OH ₂) ₆.

On the other hand, if Mg (OH) ₂ has absorbed CO ₂, it will release the trapped gas as effervescence when dissolved in an 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

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 Mg ²⁺, such as Al ³⁺, Fe ³⁺, Zn ²⁺ and Mn ²⁺. 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 CO ₂, 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) ₂ has up to three IUPAC accepted names (outside of mineralogy or medicine). These are very similar to each other, since the way they end 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', indicating with the Greek numerator prefix the number of OH ions ^- 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 should not be referred to when it comes to its purest solid, or as an inorganic compound (reagent, raw material, etc.).

Applications

Neutralizer

Mg (OH) ₂ owes to its low solubility in water the fact that it is an excellent neutralizer of acidity; otherwise, it would basify the medium by providing large concentrations of OH ^- ions, as do other bases (strong electrolytes).

Thus, Mg (OH) ₂ barely releases OH ^-, at the same time it reacts with H ₃ O ⁺ ions to form the complex aqueous magnesium, also mentioned above. 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 (it 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) ₂ 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.

Brucite mineral is often 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

The Mg (OH) ₂ synthesized as nanoplates has been shown to be efficient to catalyze chemical reductions; for example, 4-nitrophenol (Ph-NO ₂) to 4-aminophenol (Ph-NH ₂). Likewise, these have antibacterial activity, so it could be used as a therapeutic agent.

Adsorbent

Some Mg (OH) ₂ solids can be quite porous, depending on the method of their preparation. Therefore, they find application as adsorbents.

In aqueous solutions, dye molecules can adsorb (on their surfaces), 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 RK, Muhammad RS (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