Mg(OH)₂ — Magnesium Hydroxide

Magnesium hydroxide (Mg(OH)₂) is an inorganic compound that appears as a white, sparingly soluble solid. It occurs naturally as the mineral brucite and is commonly known in its suspension form as milk of magnesia. This compound is widely used in medicine as an antacid to neutralize stomach acid and as a laxative to relieve constipation. Industrially, it serves as a flame retardant, wastewater treatment agent, and a precursor for magnesium oxide (MgO) production.

Due to its mild alkalinity and low solubility, magnesium hydroxide acts as a safe and effective base for neutralization processes without posing severe corrosive risks. It is an essential compound in both household and industrial chemistry.

Magnesium hydroxide consists of magnesium cations (Mg²⁺) and hydroxide anions (OH⁻) held together by ionic bonds. The compound adopts a layered crystal structure similar to that of brucite, where each magnesium ion is surrounded octahedrally by six hydroxide ions.

Because of the strong lattice forces and hydrogen bonding between hydroxide ions, Mg(OH)₂ is only slightly soluble in water. However, even the small amount that dissolves dissociates completely, providing hydroxide ions that give it weakly basic properties. This low solubility makes it an ideal antacid, as it neutralizes stomach acid without causing excessive alkalinity.

Magnesium hydroxide can be prepared in the laboratory or industrially by precipitation reactions between soluble magnesium salts and hydroxide ions. A common preparation method is by reacting magnesium chloride (MgCl₂) with sodium hydroxide (NaOH):

The white precipitate formed is magnesium hydroxide. Another natural source is seawater, where magnesium ions are abundant. The addition of lime (Ca(OH)₂) to seawater results in the precipitation of magnesium hydroxide:

Industrially, this method is used to extract large quantities of Mg(OH)₂ from seawater for use in refractories and flame-retardant materials.

• Appearance: White, fine crystalline powder or suspension (milk of magnesia).
• Solubility: Sparingly soluble in water (0.009 g/100 mL at 25°C). The solution is weakly basic.
• Thermal Decomposition: On heating, magnesium hydroxide decomposes to magnesium oxide (MgO) and water.

\(Mg(OH)_2 \xrightarrow{Heat} MgO + H_2O\)

• Reaction with Acids: Reacts readily with acids to form corresponding magnesium salts and water.

\(Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O\)

• Reaction with Carbon Dioxide: Slowly reacts with CO₂ to form magnesium carbonate.

\(Mg(OH)_2 + CO_2 \rightarrow MgCO_3 + H_2O\)

• Basic Nature: Weakly basic, capable of neutralizing strong acids without creating high pH conditions.

• Medical Use (Antacid and Laxative): Suspensions of Mg(OH)₂, known as milk of magnesia, are used to neutralize excess gastric acid and relieve constipation. It acts gently on the stomach, making it a preferred over-the-counter medicine.
• Flame Retardant: Used as a non-toxic flame retardant in plastics and rubber. When heated, it decomposes endothermically, releasing water and reducing the spread of flames.
• Environmental Applications: Applied in wastewater treatment to neutralize acidity and remove heavy metals through precipitation reactions.
• Chemical Industry: Used as an intermediate in the production of magnesium oxide, magnesium salts, and fertilizers.
• Pulp and Paper Industry: Employed as a neutralizing and bleaching agent to improve paper whiteness and reduce acidity.
• Cosmetics and Personal Care: Found in deodorants and skincare formulations as an absorbent and pH adjuster.

Magnesium hydroxide is considered safe for medical and cosmetic use at controlled doses. It is listed as a generally recognized safe (GRAS) substance by the FDA. However, excessive ingestion may cause diarrhea, dehydration, or electrolyte imbalance. When used externally, it is non-toxic and non-irritating.

In industrial settings, Mg(OH)₂ dust may cause mild respiratory irritation if inhaled. It should be handled with protective gloves, masks, and eyewear to avoid prolonged contact. Despite its low toxicity, magnesium hydroxide should be stored in sealed containers away from moisture and acids to prevent degradation.