Aluminium Sulphate — Salt Analysis, Formula and Properties

Chemical formula: Al₂(SO₄)₃ Chemical name: Aluminium sulphate Common name: Alum of sulphate, papermakers' alum, filter alum

Formation: 2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O

Composition: • Cation: Al³⁺ (aluminium, tripositive) • Anion: SO₄²⁻ (sulphate, dinegative) • Ratio: 2 Al³⁺ : 3 SO₄²⁻ (to balance charges: 2×(+3) = 6 = 3×(−2) ✓)

Molar mass: 342.15 g/mol Solubility: Soluble in water

Nature of solution: • Formed from weak base Al(OH)₃ + strong acid H₂SO₄ • Aqueous solution is acidic (pH < 7) due to hydrolysis of Al³⁺: Al³⁺ + 3H₂O ⇌ Al(OH)₃ + 3H⁺

Test for Al³⁺ (aluminium ion):

1. NaOH test: Al³⁺ + 3OH⁻ → Al(OH)₃↓ (white gelatinous precipitate) With excess NaOH: Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (soluble — dissolves) Observation: White precipitate dissolves in excess NaOH (amphoteric)

2. Ammonia test: Al³⁺ + 3NH₃ + 3H₂O → Al(OH)₃↓ + 3NH₄⁺ Observation: White precipitate, insoluble in excess NH₃

3. Litmus/pH: Acidic solution (due to hydrolysis)

Test for SO₄²⁻ (sulphate ion):

1. BaCl₂ test: Ba²⁺ + SO₄²⁻ → BaSO₄↓ (white precipitate, insoluble in HCl) Add dilute HCl first (to exclude carbonate, sulphite) then BaCl₂ Observation: Persistent white precipitate insoluble in HCl → confirms SO₄²⁻

Physical properties: • White crystalline solid • Soluble in water • Hydrated form: Al₂(SO₄)₃·18H₂O • Aqueous solution is acidic

Chemical properties: • Amphoteric nature (Al³⁺ forms Al(OH)₄⁻ with excess base) • Hydrolysis: Al₂(SO₄)₃ + 6H₂O ⇌ 2Al(OH)₃ + 3H₂SO₄

Uses:

1. Water treatment — coagulant that removes suspended particles Al³⁺ + 3H₂O → Al(OH)₃ (gelatinous floc) + 3H⁺ Al(OH)₃ floc traps suspended dirt → settles out

2. Paper making — as a sizing agent (aluminium rosin soap)

3. Fire retardants — decomposes to Al₂O₃ and SO₃ at high temperature

4. Food additive (E520) — pickling agent

5. Dyeing — mordant to fix dyes to fabric

6. Sewage treatment — precipitation of phosphates