Sterilization of Water Using Bleaching Powder

Concept — Chemical disinfection with bleaching powder

Bleaching powder (calcium hypochlorite, Ca(ClO)₂) releases active chlorine when dissolved in water. The key disinfectant is hypochlorous acid (HClO), which penetrates microbial cells and oxidizes their internal structures. The amount of HClO depends on pH: when pH is below the pKa (7.53), more HClO forms, making disinfection more effective.

What the animation shows

Phase 1 – Dissolution: Solid Ca(ClO)₂ dissolves into Ca²⁺ and ClO⁻ ions. Watch the solid shrink as ions move into the water.

Phase 2 – Equilibrium: ClO⁻ reacts with water in a reversible reaction: ClO⁻ + H₂O ⇌ HClO + OH⁻. The arrow shows which direction dominates based on pH.

Phase 3 – Inactivation: HClO molecules attack microorganisms, turning them from red (active) to gray (inactive).

Phase 4 – Contact Time: The timer shows how long HClO needs to act to ensure all microbes are inactivated.

Quantities

• % Available Cl₂ — mass fraction of active chlorine in bleaching powder (33 % w/w).
• pKa (HClO) — acid dissociation constant (7.53 at 25 °C).
• [Free Cl₂] residual — remaining free chlorine after reaction (0.2 mg/L).
• pH — water acidity; determines HClO/ClO⁻ ratio.
• Microbes left — number of active microorganisms remaining.

Sources

Hammer, M. J., & Hammer Jr, M. J. (2001). Water Treatment Principles and Design (3rd ed.). John Wiley & Sons.

Faust, S. D., & Aly, O. M. (1998). Chemistry of Water Treatment (2nd ed.). CRC Press.

World Health Organization. (2011). Guidelines for Drinking-water Quality (4th ed.). WHO Press.