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Determination of Residual Chlorine in Water – Environmental Engineering Lab Experiment

Determination of Residual Chlorine in Water – Lab Experiment

Residual chlorine is the amount of chlorine that remains in water after the disinfection process. It ensures that harmful microorganisms are continuously destroyed during storage and distribution of water.

Determining residual chlorine is important in water treatment plants to confirm that sufficient chlorine is present to maintain safe and disinfected drinking water.

Aim

To determine the residual chlorine in a given water sample using the Orthotolidine (OT) or DPD method.

Apparatus Required

  • Comparator or colorimeter
  • Test tubes
  • Measuring cylinder
  • Pipette
  • Water sample

Chemicals Required

  • Orthotolidine reagent or DPD reagent
  • Distilled water

Theory

When chlorine is added to water, part of it reacts with impurities while the remaining chlorine stays in the water as residual chlorine. This residual chlorine provides continued protection against microbial contamination.

In the Orthotolidine or DPD method, the reagent reacts with residual chlorine in water and produces a yellow or pink color. The intensity of the color is proportional to the amount of residual chlorine present and can be measured using a comparator or colorimeter.

Procedure

  1. Take a known volume of the water sample in a clean test tube.
  2. Add a few drops of orthotolidine or DPD reagent.
  3. Mix the solution gently.
  4. Allow the color to develop for a few seconds.
  5. Compare the color intensity with the standard color scale in the comparator.
  6. Record the residual chlorine value.

Observation Table

Sample Color Developed Residual Chlorine (mg/L)
Water Sample ______ ______

Result

The residual chlorine in the given water sample was determined using the comparator method.

Residual chlorine = ______ mg/L

Precautions

  • Use clean glassware to avoid contamination.
  • Add reagent in the correct quantity.
  • Take readings immediately after color development.
  • Avoid exposure of samples to direct sunlight.

Conclusion

The experiment helps determine the residual chlorine present in water. Maintaining an adequate residual chlorine level ensures effective disinfection and safe drinking water.

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