Magnesium hydroxide is an inorganic compound with the chemical formula Mg(OH)₂. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water. Magnesium hydroxide is a common component of antacids, such as milk of magnesia.
Magnesium Hydroxide (Mg(OH)₂) Structure
The structure of magnesium hydroxide, Mg(OH)₂, can be described in terms of its crystal lattice.
Crystal Structure:
- Type: Magnesium hydroxide crystallizes in the brucite structure, which is named after the mineral brucite, where magnesium hydroxide occurs naturally.
- Crystal System: The brucite structure falls under the trigonal crystal system, specifically in the space group P3m1.
- Lattice Parameters:
- a = b ≈ 3.148 Å
- c ≈ 4.770 Å
- α = β = 90°
- γ = 120°
Atomic Arrangement:
- In the brucite structure, magnesium ions (Mg²⁺) are octahedrally coordinated by six hydroxide ions (OH⁻). Each hydroxide ion is bonded to three magnesium ions, forming layers.
- Layer Structure:
- The structure consists of stacked layers of octahedrally coordinated Mg²⁺ ions. Each layer has Mg²⁺ at the center of an octahedron formed by six OH⁻ ions.
- These layers are held together by weak van der Waals forces between the hydroxide layers, which explains why magnesium hydroxide can easily cleave along these planes, giving it a flaky or layered appearance in its natural form.
- Hydrogen Bonding:
- There’s also hydrogen bonding between the hydrogen atoms of one layer and the oxygen atoms of an adjacent layer, which contributes to the stability of the structure but still allows for the layers to slide past each other, accounting for its soft nature.
Physical properties of Magnesium Hydroxide (Mg(OH)₂)
Appearance:
- Color: White.
- Form: Typically appears as a white, amorphous powder or as fine crystalline powder.
Density:
- Bulk Density: Approximately 2.36 g/cm³.
Melting Point:
- Decomposition: It does not melt in the traditional sense; instead, it decomposes at around 350°C (662°F) into magnesium oxide (MgO) and water:Mg(OH)2→MgO+H2O
Solubility:
- In Water: Very low solubility in water; approximately 0.00064 g/100 mL at 25°C. This low solubility is why it can be used as an antacid without dissolving too quickly in the stomach.
- In Acids: Soluble in dilute acids, where it acts as a base.
pH:
- Of a Suspension: An aqueous suspension of magnesium hydroxide has a pH around 10, which is why it’s used as an antacid.
Thermal Properties:
- Heat Capacity: Specific heat capacity at constant pressure (Cp) is about 77.03 J/mol·K.
Refractive Index:
- Not commonly listed due to its typical use in forms where refractive index isn’t relevant, but it would be around 1.559 for the crystal.
Molecular Weight:
- 58.3197 g/mol.
Crystal Structure:
- Crystal System: Hexagonal crystal system.
Particle Size:
- Can vary widely based on manufacturing; for pharmaceutical use, it’s often micronized to increase surface area for better reactivity as an antacid.
Other Physical Properties:
- Hardness: On the Mohs scale, it’s relatively soft, but exact hardness can vary based on form and purity.
- Hygroscopicity: Not particularly hygroscopic, but it will slowly absorb carbon dioxide from the air to form magnesium carbonate, especially in moist conditions.
- Electrical Properties: Poor conductor of electricity due to its ionic nature and insolubility in non-polar solvents or when dry.
Bulk Properties:
- Flowability: As a powder, its flow properties can be poor, often requiring flow aids when used in industrial processes.
- Surface Area: Can have a high surface area when finely divided, which enhances its reactivity as an antacid or in flame-retardant applications.
Chemical properties of Magnesium Hydroxide (Mg(OH)₂)
Basic Nature:
- Neutralization: As a base, it reacts with acids to form salts and water:
Mg(OH)2+2HCl→MgCl2+2H2O
Decomposition:
- Thermal Decomposition: Upon heating, magnesium hydroxide decomposes to form magnesium oxide and water vapor:
Mg(OH)2→ΔMgO+H2O
- This property is utilized in its use as a flame retardant since the decomposition process is endothermic, cooling the surrounding environment.
Solubility and Precipitation:
- Insolubility in Water: Its very low solubility in water leads to its use as a suspension in water (milk of magnesia) for medicinal purposes. The suspension provides a steady release of OH⁻ ions.
- Precipitation: It can be precipitated from a solution of a magnesium salt by adding a strong base like NaOH: MgCl2+2NaOH→Mg(OH)2↓+2NaCl
Reactivity with Carbon Dioxide:
- Carbonate Formation: In the presence of carbon dioxide, especially in moist environments, it slowly reacts to form magnesium carbonate:
Mg(OH)2+CO2→MgCO3+H2O
- This reaction is slow under normal conditions but can be significant in scenarios where CO₂ absorption is desired.
Amphoteric Nature:
- While primarily basic, magnesium hydroxide can exhibit slight amphoteric behavior, although this is much less pronounced compared to hydroxides like aluminum hydroxide. It can react with very strong bases in certain conditions, but this is not commonly observed in typical applications.
Complex Formation:
- With Ligands: Magnesium ions from Mg(OH)₂ can form complexes with various ligands, although this property is more relevant when Mg(OH)₂ is dissolved or in reaction systems where ligands are present.
Buffer Capacity:
- pH Regulation: Its suspension in water acts as a buffer, maintaining a pH around 10, which is why it’s effective in treating acid indigestion.
Reduction Reactions:
- With Strong Reducing Agents: Although not typically involved in redox reactions in its common uses, when subjected to high temperatures or in certain chemical environments, magnesium from magnesium hydroxide could theoretically be involved in reduction reactions.
Environmental Applications:
- Wastewater Treatment: Used for neutralizing acidic waste streams and precipitating heavy metals as hydroxides due to its basic nature.
Flame Retardant Properties:
- When used in polymers, its decomposition into MgO and water not only cools the combustion process but also dilutes the flammable gases with water vapor, reducing flammability.
Biological Activity:
- In biological systems, magnesium hydroxide can influence enzyme activity where magnesium ions are a co-factor, although this is more about the magnesium ion than the hydroxide itself.
Preparation of Magnesium Hydroxide (Mg(OH)₂)
Magnesium hydroxide can be prepared through several methods, both in laboratory settings and on an industrial scale.
Precipitation from Salts
From Magnesium Salts with a Base:
- One of the most common methods involves the reaction of a soluble magnesium salt (like magnesium chloride or magnesium nitrate) with a strong base (such as sodium hydroxide or ammonia):
MgCl2+2NaOH→Mg(OH)2↓+2NaCl
Mg(NO3)2+2NH4OH→Mg(OH)2↓+2NH4NO3
Procedure:
- Dissolve the magnesium salt in water.
- Slowly add the base to the solution while stirring. Magnesium hydroxide will precipitate out as a white solid.
- Filter the precipitate, wash it to remove any soluble salts, and then dry it.
From Magnesium Oxide
Hydration of Magnesium Oxide:
Magnesium oxide (MgO) can be directly hydrated to form magnesium hydroxide:
- MgO+H2O→Mg(OH)2
Procedure:
- Mix magnesium oxide with water. The reaction might need some stirring or slight heating to proceed effectively, as MgO is not very soluble in water.
Electrolysis
Electrolytic Process:
- In industrial settings, magnesium hydroxide can be produced by electrolysis of a magnesium salt solution, although this method is less common for direct synthesis due to energy costs.
From Seawater or Brine
Extraction from Natural Sources:
- Seawater or Brine: Magnesium ions can be precipitated from seawater or magnesium-rich brines by adding a base (lime – Ca(OH)₂ or dolomitic lime – which contains both Ca(OH)₂ and Mg(OH)₂):
Mg2++Ca(OH)2→Mg(OH)2↓+Ca2+
Procedure:
- Treat the seawater or brine with lime. Magnesium hydroxide will precipitate out.
- The precipitate is then separated, washed, and dried.
Commercial Production:
- Industrial Scale: Often involves the reaction of magnesium chloride (which can be obtained from seawater or brine) with lime or another base in large reactors. The process might include steps for purification to remove impurities like calcium salts.
N/B
- Purity: For medicinal or high-purity applications, further purification might be necessary, involving recrystallization or washing with deionized water to remove soluble impurities.
- Particle Size: The particle size can be controlled by the conditions of precipitation (like the rate of base addition, temperature, and stirring speed), which is crucial for its applications, especially in pharmaceuticals.
- Safety: When performing these reactions, particularly with strong bases, appropriate safety measures should be taken, including the use of protective gear due to the caustic nature of the reactants.
Uses of Magnesium Hydroxide (Mg(OH)₂)
- Known commercially as Milk of Magnesia, it’s used to neutralize stomach acid, providing relief from heartburn, indigestion, and upset stomach: Mg(OH)2+2HCl→MgCl2+2H2O
- It acts as an osmotic laxative, drawing water into the intestine, which helps to induce bowel movements.
- Used in plastics and polymers. When heated, magnesium hydroxide decomposes endothermically (absorbs heat), releasing water vapor which dilutes flammable gases and cools the combustion zone: Mg(OH)2→MgO+H2O
- Employed to neutralize acidic waste streams and can precipitate heavy metals from industrial wastewater as hydroxides.
- Used in power plants to remove sulfur dioxide from exhaust gases, helping to reduce acid rain: SO2+Mg(OH)2→MgSO3+H2O
- Some deodorants use magnesium hydroxide as an alternative to aluminum compounds for its antibacterial properties and pH regulation.
- It can be found in toothpaste as a mild abrasive or polishing agent.
- Serves as an intermediate in the production of other magnesium compounds.
- Used as a food additive for its pH regulating properties, sometimes found in sugar processing.
- Although less common than lime, it can be used to adjust the pH of acidic soils. However, magnesium carbonate or oxide might be more commonly used for this purpose due to cost and handling.