Q1(h). Best coagulant for the Sb₂S₃ sol

Answer in brief

Al₂(SO₄)₃. The Sb₂S₃ sol is negative, so the cation coagulates it, and Al³⁺ carries the highest charge (Hardy–Schulze rule).

Key points

• The Sb₂S₃ sol is made with H₂S and adsorbs HS⁻/S²⁻ ions: it is negatively charged (like the As₂S₃ sol).
• Coagulation is caused by the ion of opposite charge, the cation; the anions do not matter here.
• Hardy–Schulze rule: coagulating power rises steeply with the charge of the active ion; flocculation value ∝ 1/z⁶.
• Cations: Na⁺, NH₄⁺ (1), Ca²⁺ (2), Al³⁺ (3) ⇒ Al₂(SO₄)₃ > CaCl₂ > Na₂SO₄ ≈ NH₄Cl.
• Al³⁺ compresses the diffuse layer, the ζ potential falls and van der Waals attraction wins.