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Lab Experiment: Determination of Break-Point Chlorination

Chlorination is one of the most common methods used for disinfection of drinking water. Chlorine is added to water to kill harmful microorganisms such as bacteria and viruses. However, the amount of chlorine added must be carefully controlled to ensure effective treatment.

The point at which the chlorine demand of water is fully satisfied and free residual chlorine begins to appear is known as the break-point chlorination. Determining this point helps water treatment plants apply the correct chlorine dosage.

Aim of the Experiment

To determine the break-point chlorination of a water sample by adding different doses of chlorine and measuring the residual chlorine.

Apparatus and Materials Required

  • Water samples
  • Beakers or conical flasks
  • Measuring cylinder
  • Pipette
  • Chlorine solution (bleaching powder solution)
  • Orthotolidine reagent or DPD reagent
  • Comparator or colorimeter
  • Glass stirring rod

Theory

When chlorine is added to water, it first reacts with impurities such as ammonia, organic matter, and other reducing substances. During this stage, no free chlorine is present because it is consumed in chemical reactions.

As more chlorine is added, combined chlorine compounds are formed. Eventually, these compounds are destroyed and the chlorine demand is satisfied. At this stage, free residual chlorine starts appearing in the water. This point is called the break-point.

Beyond the break-point, any additional chlorine remains in the water as free residual chlorine, which ensures proper disinfection.

Procedure

  1. Take several clean beakers and fill them with equal volumes of the water sample.
  2. Add increasing doses of chlorine solution to each beaker.
  3. Mix the samples properly using a glass rod.
  4. Allow the samples to stand for about 30 minutes for proper reaction.
  5. Add orthotolidine or DPD reagent to each sample.
  6. Measure the residual chlorine using a comparator or colorimeter.
  7. Record the residual chlorine values for each chlorine dose.
  8. Plot a graph between chlorine dose and residual chlorine.

Observation Table

Sample Number Chlorine Dose (mg/L) Residual Chlorine (mg/L) Observation
1 1 ____ ____
2 2 ____ ____
3 3 ____ ____
4 4 ____ ____
5 5 ____ ____

Result

The break-point chlorination is identified from the graph where the residual chlorine suddenly starts increasing after reaching the minimum point.

Break-point chlorine dose = ______ mg/L

Precautions

  • Use fresh chlorine solution for accurate results.
  • Measure chlorine doses carefully.
  • Allow sufficient reaction time before testing residual chlorine.
  • Use clean glassware to avoid contamination.

Conclusion

This experiment helps determine the break-point chlorination level required for effective water disinfection. Maintaining chlorine dosage beyond the break-point ensures safe drinking water by eliminating harmful microorganisms while maintaining adequate residual chlorine.

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