Coagulation is one of the most important properties of colloids. Here’s a clear explanation:
Coagulation of Colloids
- Definition: Coagulation (also called precipitation or flocculation) is the process of settling or aggregation of colloidal particles into larger particles, which then separate from the dispersion medium.
- Cause: Colloids are normally stable because their particles carry the same type of electric charge, creating repulsion. When this stability is disturbed (by adding an electrolyte, heating, or mixing two oppositely charged sols), particles lose their charge and aggregate.
Methods of Coagulation
- By Electrophoresis
- On applying an electric field, charged colloidal particles migrate to the oppositely charged electrode and get discharged, causing coagulation.
- By Mixing Oppositely Charged Sols
- Example: Mixing arsenic sulfide (As₂S₃) sol (negative sol) with ferric hydroxide (Fe(OH)₃) sol (positive sol) leads to mutual coagulation.
- By Boiling/Heating
- Heating increases particle collisions and reduces charge, leading to coagulation.
- By Addition of Electrolytes
- The most common method.
- Oppositely charged ions from the electrolyte neutralize the charge on colloidal particles.
- Once neutralized, particles clump together and settle.
- The minimum concentration of electrolyte required to cause coagulation is called the coagulation value.
Examples
- Curdling of milk (lactic acid causes coagulation of casein proteins).
- Formation of delta at river mouths (salts in seawater coagulate colloidal clay particles).
- Purification of drinking water (alum is used as a coagulant).
In short: Coagulation of colloids is the process of destabilizing and aggregating colloidal particles into larger ones, usually by electrolytes, heating, or mixing with an oppositely charged sol.