Aluminium Sulfide (Al₂S₃): The Complete Guide

Aluminium sulfide is a highly moisture‑sensitive inorganic compound. It appears as a dull grey to yellowish solid and is infamous for its foul odor resulting from reaction with ambient moisture. It serves as a specialized laboratory reagent to generate hydrogen sulfide gas on demand.

1. Basic Identification

• Chemical Formula: Al₂S₃
• Alternative Names: Aluminium sulfide, dialuminum trisulfide
• Molecular Weight: 150.16 g/mol
• CAS Number: 1302-81-4
• Appearance: Dull grey to yellowish‑brown crystalline solid or powder

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2. Physical Properties

Property	Aluminum Sulfide
Melting Point	1100 °C (2010 °F)
Boiling Point	Sublimes/decomposes ~1500 °C
Density	2.02 g/cm³
Crystal Structure	Hexagonal (α), Trigonal (β)
Solubility	Insoluble in organic solvents; reacts violently with water

Pure Al₂S₃ forms hard greyish clumps. Older samples often have a white crust of aluminium hydroxide and emit a noxious stench due to moisture exposure.

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3. Chemical Behavior & Synthesis

3.1 Reaction with Water – H₂S Generation

Al₂S₃ + 6 H₂O → 2 Al(OH)₃ + 3 H₂S ↑

Even atmospheric humidity triggers localized hydrolysis, releasing hydrogen sulfide (H₂S) gas. Dropped into water, it bubbles violently, producing a white sludge and toxic H₂S clouds.

3.2 Synthesis

2 Al + 3 S → Al₂S₃

Fine aluminum powder and sulfur are mixed and ignited. The reaction is violently exothermic, fusing the elements into pure aluminum sulfide.

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4. Applications

4.1 Hydrogen Sulfide (H₂S) Generation

In analytical chemistry, Al₂S₃ provides a controlled source of H₂S for testing heavy metal ions (metal sulfide precipitation) without storing pressurized toxic gas tanks.

4.2 Inorganic Synthesis Precursor

Used as a reactive sulfur precursor to generate thio‑compounds without introducing water or handling gaseous H₂S.

4.3 Battery Research

Aluminum‑sulfur battery lattices based on Al₂S₃ are studied for lightweight, high‑capacity energy storage as an alternative to lithium‑ion.

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5. Safety & Hazard Management

GHS Symbol	Meaning
☠️ GHS06	Toxic gas releaser

Critical Warning: The reaction product – hydrogen sulfide (H₂S) – is extremely toxic, attacks the nervous system, and can be fatal.

5.1 Health Effects

Route of Exposure	Effect
Inhalation (H₂S gas)	H₂S paralyzes olfactory nerves (rotten‑egg smell disappears). Continued exposure causes pulmonary edema, coma, death.
Skin Contact	Solid may irritate; moisture on skin causes localized gas burns.
Eye Contact	Severe irritation due to hydrolysis on eye moisture.
Ingestion	Stomach acid accelerates H₂S release → internal poisoning, shock, potentially fatal.

5.2 PPE

• Respiratory: Fume hood mandatory. For large spills, SCBA (H₂S is not filtered by particulate masks).
• Hands: Thick nitrile or butyl rubber gloves
• Eyes: Tight chemical splash goggles

5.3 First Aid

• Inhalation: Evacuate to fresh air immediately. Administer oxygen. Seek emergency medical help.
• Skin: Brush off dry chemical; flush with water 15+ minutes.
• Eyes: Flush 15‑20 minutes; see ophthalmologist.

5.4 Firefighting

• H₂S is highly flammable and forms explosive mixtures with air.
• Use: Dry chemical powder or dry sand.
• Do NOT use: Water or foam – will increase toxic/flammable gas release.

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6. Storage & Handling

• Container: Tightly sealed glass or heavy plastic
• Atmosphere: Under dry inert gas (nitrogen/argon)
• Location: Cool, ventilated hazardous storage, away from humidity
• Incompatibles: Water, moisture, strong acids, strong oxidizers

6.1 Disposal

Do not discard in trash or drain. In a fume hood, suspend in non‑polar solvent, then slowly drip into dilute bleach (sodium hypochlorite) to oxidize sulfide to non‑toxic sulfate.

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7. Environmental Impact

Exposed Al₂S₃ breaks down in rainwater. Aluminum becomes inert mud, but sudden H₂S release can devastate wildlife and acidify streams. Spills must be kept away from groundwater/sewers.

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8. Comparison with Other Reactive Aluminum Compounds

Compound	Formula	Byproduct in Water	Primary Hazard
Aluminium Sulfide	Al₂S₃	Hydrogen sulfide (H₂S)	Toxic, flammable “rotten egg” gas
Aluminium Phosphide	AlP	Phosphine (PH₃)	Highly lethal fumigant
Aluminium Carbide	Al₄C₃	Methane (CH₄)	Flammable gas explosion risk
Aluminium Chloride	AlCl₃	Hydrogen chloride (HCl)	Corrosive acid clouds

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9. Frequently Asked Questions

Q: I found a bottle that smells strongly of rotten eggs. Is it leaking?
A: Yes. Even brief exposure to humid air generates H₂S, which escapes through threads.

Q: Why do chemistry teachers demonstrate this reaction?
A: Mixing aluminum and sulfur powders creates a dramatic flash of heat and light, illustrating exothermic reactions and activation energy.

Q: Are aluminum‑sulfur batteries available to buy?
A: No. Research is ongoing; engineering challenges remain for long‑cycle stability.

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10. Summary Data Sheet

Property	Value
Chemical Name	Aluminum Sulfide
Formula	Al₂S₃
Appearance	Grey to yellowish solid clumps
Melting Point	1100 °C
Defining Trait	Emits H₂S (rotten egg) in humid air
Primary Utility	Laboratory H₂S generator
Storage	Watertight, inert gas blanket