Aluminium Sulfate (Al₂(SO₄)₃): The Complete Guide

Aluminium sulfate is an important industrial chemical, commonly referred to as “alum” (though strictly, alums are double sulfates). It appears as a white crystalline solid that is highly soluble in water. It is the backbone of municipal water treatment, serving as the definitive flocculent that delivers clear drinking water.

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1. Basic Identification

• Chemical Formula: Al₂(SO₄)₃ (anhydrous); common hydrates: Al₂(SO₄)₃·18H₂O or Al₂(SO₄)₃·16H₂O
• Alternative Names: Filter alum, papermaker’s alum, dialuminum trisulfate
• Molecular Weight: 342.15 g/mol (anhydrous); ~666.4 g/mol (octadecahydrate)
• CAS Number: 10043-01-3 (anhydrous); 7784-31-8 (octadecahydrate)
• Appearance: Glossy white crystals, granules, or powder

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2. Physical Properties

Property	Aluminium Sulfate (Anhydrous)
Melting Point	770 °C (1418 °F) – decomposes
Boiling Point	Decomposes to Al₂O₃ + SO₃
Density	2.67 g/cm³
Aqueous pH	2 – 4 (acidic)
Solubility	31.2 g/100 mL (0 °C); 86.9 g/100 mL (100 °C); insoluble in ethanol

Aluminum sulfate is a heavy white granular material. It is stable, non‑fuming, and non‑volatile. It absorbs moisture from air to form hydrates but does so peacefully (no violent reaction).

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3. Chemical Behavior & Synthesis

3.1 Hydrolysis – The Coagulation Mechanism

In water, Al₂(SO₄)₃ dissociates to Al³⁺ and SO₄²⁻. The Al³⁺ ion hydrolyzes:

Al³⁺ + 3 H₂O ⇌ Al(OH)₃ ↓ + 3 H⁺

This produces a gelatinous precipitate of aluminum hydroxide that traps suspended particles (flocculation).

3.2 Synthesis

Al₂O₃ + 3 H₂SO₄ → Al₂(SO₄)₃ + 3 H₂O

Bauxite or pure alumina is digested in hot concentrated sulfuric acid. The solution is concentrated and crystallized.

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4. Industrial & Laboratory Applications

4.1 Water Purification (Primary Use)

As a coagulant, aluminium sulfate neutralizes the negative charge on suspended dirt particles, allowing them to clump into heavy flocs that settle out. This is the standard method for producing clear drinking water from turbid sources.

4.2 Paper Manufacturing

Used to “size” paper – making it resistant to ink bleed. Aluminum sulfate causes rosin sizing agents to bind to paper fibers under acidic conditions.

4.3 pH Regulation in Gardening

Aluminium sulfate lowers soil pH (makes it acidic). Gardeners use it to turn hydrangea flowers deep blue by freeing natural aluminum in the soil.

4.4 Antiperspirants & Medical Astringents

Its astringent properties block sweat glands and stop bleeding from shaving cuts (styptic pencils).

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5. Safety & Hazard Management

GHS Symbol	Meaning
❗ GHS05	Corrosive

Critical Warning: Solid is stable, but concentrated aqueous solutions are strongly acidic and can cause burns.

5.1 Health Effects

Route of Exposure	Effect
Inhalation	Dust causes stinging irritation to nose and throat.
Skin Contact	Prolonged contact leads to drying, irritation, mild acid burns.
Eye Contact	Dangerous – dust dissolves in eye moisture to form acid, causing severe conjunctivitis and potential tissue damage.
Ingestion	Irritation of mouth/stomach; sour taste; high doses cause vomiting.

5.2 PPE

• Respiratory: N95 dust mask (for dry powder)
• Hands: Nitrile or rubber gloves
• Eyes: Tight‑sealing chemical goggles
• Body: Lab coat or apron

5.3 First Aid

• Eyes: Flush 15‑20 minutes; seek ophthalmologist
• Skin: Wash with soap and water
• Ingestion: Drink water or milk to dilute; do not induce vomiting; seek medical attention

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6. Storage & Handling

• Container: Thick plastic or glass – not unlined metal (acidic)
• Atmosphere: Keep dry to prevent caking
• Location: Well‑ventilated, dry storage
• Disposal: Dilute and neutralize with lime or soda ash before landfilling

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7. Environmental Impact

Aluminium sulfate can lower pH of water bodies if dumped in large quantities, harming aquatic life. However, controlled doses are used to remove toxic phosphorus blooms (eutrophication) from lakes – the flocs settle and remove algae‑feeding nutrients.

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8. Comparison with Other Aluminium Compounds

Compound	Formula	Primary Nature	Primary Use
Aluminium Sulfate	Al₂(SO₄)₃	Acidic polyatomic salt	Water purification, pH adjustment
Aluminium Chloride	AlCl₃	Fuming Lewis acid	Petrochemical catalysis
Potassium Alum	KAl(SO₄)₂	Double sulfate salt	Pickling, crystal deodorant

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9. Frequently Asked Questions

Q: Is aluminum sulfate the same as the alum block for shaving?
A: No. Shaving alum blocks are typically potassium alum (KAl(SO₄)₂). Aluminum sulfate is more acidic and would sting more.

Q: Does this mean I’m drinking aluminum?
A: No. The aluminum hydroxide floc settles out and is removed. The clarified water contains negligible aluminum.

Q: How fast does it turn hydrangeas blue?
A: Over a growing season. It lowers soil pH, freeing natural aluminum for root uptake, gradually shifting flowers from pink to blue.

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10. Summary Data Sheet

Property	Value
Chemical Name	Aluminum Sulfate
Common Name	Filter Alum
Formula	Al₂(SO₄)₃
Appearance	White granules/powder
Solubility	Highly soluble in water (acidic solution)
Primary Utility	Water purification coagulant / paper sizing
Storage	Keep dry to prevent caking