Aluminium Hydroxide

Aluminium hydroxide occurs naturally as the mineral gibbsite. It is an inorganic basic salt that neutralises hydrochloric acid in gastric secretions. Aluminium hydroxide dissolves slowly in the stomach and reacts with hydrochloric acid to produce aluminium chloride and water. It also increases pH levels, thereby inhibiting pepsin activity and enhancing adsorption.

Other names for aluminium hydroxide include aluminic acid, aluminic hydroxide, and aluminium (III) hydroxide. These terms are also used for aluminium-based compounds like carbonates, sulfates, and hydroxides. The chemical formula of aluminium hydroxide is Al(OH)₃.

1.0Nature And Structure 

Aluminium hydroxide is an amphoteric compound that can act as an acid and a base. It dissociates into aluminium ions (Al³⁺) and hydroxide ions (OH⁻) in acidic solutions. In basic solutions, it forms the aluminium hydroxide ion, Al(OH)₄⁻, which has a tetrahedral structure with a bond angle of 116.5° between Al-O-Al bonds.

The structure of aluminium hydroxide consists of two layers of hydroxyl groups and two layers of aluminium ions. These aluminium ions occupy two-thirds of the octahedral holes formed between the hydroxyl layers, stabilised by hydrogen bonding. This arrangement gives aluminium hydroxide the typical structure of a metal hydroxide.

2.0Preparation of Aluminum Hydroxide

The Bayer process is the primary industrial method for producing aluminium hydroxide. This process involves dissolving bauxite in a sodium hydroxide solution from 0°C to 270°C. After removing impurities, the resulting sodium aluminate solution is allowed to precipitate, forming aluminium hydroxide as the final product.

Aluminium hydroxide can be further processed through calcination to produce alumina (aluminium oxide).

Reactions in the Bayer Process

1. Dissolution of Bauxite:
   Al₂O₃⋅2H₂O + 2NaOH→2NaAlO₂ + 3H₂O
2. Precipitation of Aluminum Hydroxide:
   NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH
3. Calcination to Alumina:
   2Al(OH)₃ → Al₂O₃ + 3H₂O

3.0Physical Properties of Aluminum Hydroxide

4.0Chemical Properties of Aluminum Hydroxide

Amphoteric Nature: Aluminium hydroxide is amphoteric, meaning it can act as both an acid and a base.

Reaction with Acids

As a Bronsted-Lowry Base: Aluminium hydroxide reacts with acids, neutralising them to form salt and water.

Reaction with Sulfuric Acid:

$2Al(OH{)}_3+3H_{2}S{O}_4\Rightarrow A{l}_2(S{O}_4{)}_3+6H_{2}O$

Reaction with Nitric Acid: 

$3HN{O}_3+Al(OH{)}_3\to Al(N{O}_3{)}_3+3H_{2}O$

Reaction with Carbonic Acid:

$Al(OH{)}_3+H_{2}C{O}_3\to H_{2}O+A{l}_2(C{O}_3{)}_3$

Decomposition at High Temperatures: At elevated temperatures, aluminium hydroxide decomposes to form aluminium oxide and water:

$2Al(OH{)}_3\to A{l}_2{O}_3+3H_{2}O$

5.0Uses of Aluminum Hydroxide

1. Production of Aluminum Salts: Due to its purity and solubility, aluminium hydroxide is a key raw material in the manufacture of aluminium salts like aluminium sulfate and barium aluminate.
2. Flame Retardant: Used as a flame retardant filler in plastics, polyester, and rubber. It decomposes above 200°C, releasing water and reducing flammability.
3. Ceramic Manufacturing: Converts to alumina, a vital material for ceramics due to its thermal stability, strength, and low thermal expansion.
4. Water Purification: Removes heavy metals and organic impurities via adsorption and coprecipitation in sewage treatment.
5. Pharmaceutical Applications: It neutralises gastric acid and is used in stomach treatments. It also serves as an adjuvant to boost vaccine effectiveness by slowing antigen release.
6. Catalyst Carrier: Tailored for use in hydrogenation reactions and preparing compounds like fullerenes by controlling its surface properties.
7. Paper Coating: It enhances paper whiteness, opacity, smoothness, and ink absorption when used as an additive in coatings and resins.