Coagulation = destabilisation of colloid by neutralising the charge on particles. Methods: (1) Adding electrolyte (most common). (2) Mixing oppositely charged colloids. (3) Boiling (reduces adsorbed ions). (4) Electrophoresis (particles deposited at electrode). Hardy-Schulze rule: coagulating power of an ion increases with its valence (charge). The effective ion is the one with charge OPPOSITE to the colloid. For As2S3 (negative): $Al^{3+}$ > $Ba^{2+}$ > Na+. Coagulation values $\frac{mmol}{L}$: $Al^{3+}$ ≈ 0.1, $Ba^{2+}$ ≈ 1, Na+ ≈ 50 (approximate). Lower coagulation value = higher coagulating power. Flocculation value = minimum concentration of electrolyte needed to cause coagulation.