Coagulation

Coagulation

Coagulation is the process by which colloidal particles lose their stability and come together to form larger aggregates, ultimately leading to their separation from the dispersion medium. This phenomenon is also known as precipitation or flocculation of a colloid.

Causes of Coagulation

Colloidal particles carry a specific charge that keeps them apart due to mutual repulsion. When this charge is neutralized, the repulsive forces are reduced or eliminated, causing the particles to aggregate and settle.

Methods of Coagulation

• By Electrophoresis: When an electric field is applied, charged colloidal particles move toward the electrode of opposite charge and may get discharged, resulting in coagulation.
• By Mixing Oppositely Charged Sols: A negatively charged sol (e.g., arsenious sulfide sol) and a positively charged sol (e.g., ferric hydroxide sol) can neutralize each other's charge, causing coagulation.
• By Boiling: Boiling increases particle collisions and may remove adsorbed water or gases, leading to coagulation.
• By Adding Electrolytes: The addition of electrolytes neutralizes the charge on colloidal particles. Ions of higher valency are more effective in causing coagulation, as explained by Hardy-Schulze rule.

Hardy-Schulze Rule

According to this rule, ions with higher valency are more efficient at inducing coagulation of colloids compared to those with lower valency. For example, Al³⁺ is more effective than Ca²⁺, which in turn is more effective than Na⁺.

Applications of Coagulation

• Purification of drinking water by adding alum to remove suspended impurities.
• Treatment of industrial wastewater to remove colloidal pollutants.
• In the production of rubber from latex.
• Used in wastewater treatment to aggregate and eliminate organic impurities.