Magnesium Hydroxide

Magnesium hydroxide is an inorganic compound with the chemical formula <u>Mg(OH)₂</u>. It occurs naturally as the mineral brucite and appears as a white, crystalline solid with very low solubility in water. Owing to its basic properties and relative chemical stability, magnesium hydroxide has widespread applications in medicine, industry, environmental protection, and materials science. It is best known to the general public as the active ingredient in antacid and laxative preparations commonly marketed as milk of magnesia.

Chemical nature and properties

Magnesium hydroxide consists of divalent magnesium cations (Mg²⁺) bonded to hydroxide anions (OH⁻) in a layered crystal structure. The compound is only sparingly soluble in water, producing a mildly alkaline suspension rather than a true solution. This low solubility limits the concentration of hydroxide ions released into aqueous systems, giving magnesium hydroxide a gentler alkalinity compared with stronger bases such as sodium hydroxide or calcium hydroxide.
Thermally, magnesium hydroxide decomposes endothermically at elevated temperatures, forming magnesium oxide (MgO) and water vapour. This decomposition absorbs heat, a property that underpins its use as a flame retardant and smoke suppressant.

Occurrence in nature

In nature, magnesium hydroxide occurs as brucite, a relatively soft mineral found in metamorphic and ultramafic rocks, as well as in association with serpentinite and dolomite. Brucite is also present in certain clay minerals, particularly within the chlorite group, where it occupies interlayer positions and prevents swelling. Variants in which magnesium is partially substituted by aluminium form the basis of layered double hydroxides, such as hydrotalcite, which are notable for their anion-exchange properties.
Brucite may also form as a secondary mineral in concrete and cement exposed to seawater, where it contributes to volumetric expansion and cracking due to its swelling behaviour.

Preparation and industrial production

In the laboratory, magnesium hydroxide is commonly prepared by the precipitation reaction between a soluble magnesium salt and an alkaline solution:
Mg²⁺ + 2 OH⁻ → Mg(OH)₂
Industrially, magnesium hydroxide is most often produced from seawater, which contains magnesium as the second most abundant cation after sodium. On a large scale, seawater is treated with calcium hydroxide, which raises the pH and causes magnesium hydroxide to precipitate due to its low solubility. The precipitate is then separated, washed, and processed further.
Alternative precipitating agents, such as sodium hydroxide or ammonium hydroxide, can also be used. Sodium hydroxide often yields higher purity and better recovery, though it may be more expensive. Advanced methods employ electrolysis with ion-exchange membranes, allowing continuous production while generating useful by-products such as hydrogen and oxygen gases. Careful degasification of seawater is essential to minimise co-precipitation of calcium compounds.

Conversion to magnesium oxide

The majority of industrially produced magnesium hydroxide is converted into magnesium oxide (magnesia) by controlled heating. Magnesium oxide is valued for its high melting point, excellent thermal conductivity, and electrical insulating properties. It is widely used in refractory materials, electrical insulation, and as a component of specialised ceramics.

Medical and pharmaceutical uses

Magnesium hydroxide has long been used in medicine, primarily as an antacid and laxative.
As an antacid, it neutralises excess gastric acid by reacting with hydrochloric acid in the stomach, forming magnesium chloride and water. Typical adult doses range from approximately 0.5 to 1.5 grams. Its low solubility results in a gradual neutralisation effect, reducing the risk of acid rebound.
As a laxative, magnesium hydroxide acts osmotically. Magnesium ions are poorly absorbed in the intestine and draw water into the intestinal lumen, softening stools and increasing intraluminal volume, which stimulates peristalsis. Additionally, magnesium ions promote the release of cholecystokinin, enhancing intestinal motility and fluid secretion.
Only a small fraction of ingested magnesium is absorbed systemically, and it is normally excreted by the kidneys. However, prolonged use in individuals with renal impairment can theoretically lead to hypermagnesaemia.

Milk of magnesia: historical development

The medicinal use of magnesium hydroxide dates back to the early nineteenth century. In 1818, American inventor Koen Burrows received a patent related to its preparation. In 1829, physician James Murray developed a condensed solution known as “fluid magnesia”, which gained medical recognition in the British Isles.
The term “milk of magnesia” was first introduced in 1872 by Charles Henry Phillips, referring to a suspension of magnesium hydroxide with a characteristic milky appearance. The product became widely marketed for digestive ailments and remains a common over-the-counter remedy today. In the United Kingdom, the non-branded preparation is traditionally known as cream of magnesia.

Food additive and regulatory status

Magnesium hydroxide is approved for use as a food additive and is designated as E528. It is recognised as generally safe when used within regulated limits and functions primarily as an acidity regulator. In pharmaceutical products, it is available as chewable tablets, capsules, powders, and liquid suspensions, often flavoured to improve palatability.

Industrial and environmental applications

Beyond medicine, magnesium hydroxide has a range of important industrial applications. It is used in wastewater treatment to neutralise acidic effluents, offering a safer and more environmentally compatible alternative to stronger alkalis. Its buffering action allows for controlled pH adjustment, particularly in systems sensitive to excessive alkalinity.
In marine engineering and environmental restoration, magnesium hydroxide plays a role in the Biorock method of artificial reef construction, where it helps maintain seawater-compatible pH conditions.

Flame retardant and fire safety uses

Magnesium hydroxide is widely employed as a flame retardant in plastics, cable insulation, roofing materials, and coatings. Upon heating, it decomposes endothermically, absorbing heat and releasing water vapour. This process cools the material, dilutes combustible gases, and suppresses smoke formation. Unlike halogenated flame retardants, magnesium hydroxide does not release toxic or corrosive gases, making it attractive for safety-critical applications.

Role in construction and cement chemistry

In cementitious systems exposed to seawater, magnesium hydroxide can form through chemical reactions involving magnesium ions and alkaline pore solutions. Because brucite is a swelling mineral, its formation can generate internal stresses, leading to cracking and accelerated deterioration of concrete structures. For this reason, aggregates rich in dolomite or reactive magnesium compounds are unsuitable for use in marine concrete.