Aluminium Phosphate

Aluminium phosphate is an amorphous form of aluminium hydroxy phosphate, where phosphate groups replace some hydroxyl groups in aluminium hydroxide. It serves various purposes, including acting as a flame retardant, antacid, fungicide, catalyst, and food additive. Its chemical formula is AlPO_4​..Aluminium phosphate commonly exists as an anhydrous salt but can also be found in crystalline forms such as pentahydrate and dihydrate.

1.0Occurrence and Structure 

Naturally found in minerals such as berlinite, which is isostructural with quartz.

• It is rare in pure form but occurs in various geological and biological systems.
• It is an inorganic compound.
• It consists of aluminium (Al³⁺) and phosphate (PO₄³⁻​) ions.

Aluminium phosphate dihydrate comprises phosphate anions, aluminium cations, and water molecules. It contains one phosphorus atom, four oxygen atoms (three are bonded with single bonds and one with a double bond), and one aluminium atom.

The structure of aluminium phosphate dihydrate generally appears to have two water molecules, while in pentahydrate form, five water molecules are present along with aluminium phosphate. The dihydrate form is AlPO₄⋅2H₂O, and the pentahydrate form is AlPO₄⋅5H₂O. Similar to silica (SiO₂) structures, it forms a framework structure when crystallised in quartz.

2.0Formation  

Phosphate groups replace hydroxyl in aluminium hydroxy phosphate to form aluminium phosphate (AlPO₄). The chemical reaction is as follows:

$Al(OH{)}_3+H_{3}P{O}_4\to AlP{O}_4+3H_{2}O$

This reaction results in the formation of aluminium phosphate along with water as a byproduct.

3.0Physical Properties of Aluminium Phosphate

4.0Chemical Properties of Aluminium Phosphate

• Thermal Stability: Stable at room temperature but decomposes on heating.
• Reactivity with Acids: Dissolves in strong acids, forming aluminium salts and phosphoric acid.
• Insolubility in Alkaline Solutions: Resistant to bases, making it useful in certain applications.
• Reaction with Hydrochloric Acid: Aluminium phosphate reacts with hydrochloric acid (HCl) to form aluminium trichloride (AlCl₃) and phosphoric acid H₃​PO₄​):

$AlP{O}_4+3HCl\to AlC{l}_3+H_{3}P{O}_4$

• Reaction with Magnesium Chloride: Aluminium phosphate (AlPO₄) reacts with magnesium chloride (MgCl₂​) to produce magnesium phosphate(Mg₃(PO₄)₂   and aluminium trichloride(AlCl_3​).

$2AlP{O}_4+3MgC{l}_2\to M{g}_3(P{O}_4)2+2AlC{l}_3$

5.0Uses of Aluminium Phosphate

1. Molecular Sieves (AlPOs):

• Utilised in aluminium phosphate molecular sieves, which resemble aluminium phosphate structures with microporous cavities.
• Composed of alternating aluminium and phosphate tetrahedra.

2. Immunological Adjuvants:

• Combined with aluminium hydroxide as an efficiency enhancer in vaccines.
• Preferred for their low cost, long-standing safety, and high efficacy with antigens.

3. Antacid:

• Neutralises stomach acid, forming aluminium chloride.
• Safe for use during pregnancy and breastfeeding.
• Absorbs up to 20% aluminium from the gastrointestinal tract without reported health risks.

4. Additional Applications:

• Used as a white colourant in pigments and corrosion inhibitors.
• Incorporated in cement, dental cement, and refractory binders.
• Incorporated in ceramic materials to improve heat resistance and stability.
• Enhances the durability and chemical resistance of glass products.
• Investigated for use in bone repair and implants due to its biocompatibility.
• Used in chromatography and other adsorption processes.
• Incorporated in materials to enhance fire resistance.