EXPERIMENT VI - Chlorination for Disinfection

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Chlorine is used in, both drinking water and wastewater treatment, as a disinfectant. Its germicidal action is due to the hypochlorous acid (HOCl) that forms when chlorine is added to water. Exactly how this acid exerts its killing power is not yet fully known. It possibly releases a reactive form of oxygen that combines with the protoplasm of the microbial cell and, therefore, destroys it. The hypochlorite ion OCl- also has some killing potential.

This experiment will help you understand what is the most effective concentration of chlorine for “killing” organisms in water. This experiment also explains why it is important to keep a residual concentration of chlorine in drinking water.

Material Needed:

  • Four 2-L bottles
  • Pond water
  • Bleach (NaOCl)
  • Microscopes
  • Slides
  • Stain (crystal violet)
  • Eye droppers
  • Stirring rods


  1. Use the first gallon bottle of collected water as a control.
  2. To the second bottle add 1 drop of NaOCl (bleach).
  3. To the third bottle add 2 drops of NaOCl.
  4. To the fourth bottle add 4 drops. Let them sit for 30 minutes.
  5. Make up some microscope slides with water from each bottle and observe.

Source: Enviromental Protection Agency

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