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Determination of Available Chlorine in Bleaching Powder – Environmental Engineering Lab Experiment

Determination of Available Chlorine in Bleaching Powder – Lab Experiment

Bleaching powder is widely used as a disinfectant in water treatment plants for killing harmful microorganisms. The effectiveness of bleaching powder depends on the amount of available chlorine present in it. Therefore, determining the available chlorine content is important for ensuring proper disinfection.

The available chlorine in bleaching powder is usually expressed as a percentage (%).

Aim

To determine the percentage of available chlorine in the given bleaching powder sample using the iodometric titration method.

Apparatus Required

  • Burette
  • Pipette
  • Conical flask
  • Beakers
  • Measuring cylinder
  • Glass rod

Chemicals Required

  • Bleaching powder solution
  • Potassium iodide (KI) solution
  • Standard sodium thiosulphate (Na₂S₂O₃) solution
  • Starch indicator
  • Acetic acid
  • Distilled water

Theory

Available chlorine in bleaching powder is determined by the iodometric titration method. When bleaching powder reacts with potassium iodide in acidic conditions, iodine is released.

The liberated iodine is then titrated with standard sodium thiosulphate solution using starch indicator. The amount of sodium thiosulphate used in the titration helps calculate the percentage of available chlorine in the sample.

Procedure

  1. Prepare a bleaching powder solution by dissolving a known quantity of bleaching powder in distilled water.
  2. Take a measured volume of the prepared solution in a conical flask.
  3. Add potassium iodide solution and a small amount of acetic acid.
  4. Iodine will be liberated, giving a brown color.
  5. Titrate the solution with standard sodium thiosulphate solution.
  6. When the color becomes pale yellow, add a few drops of starch indicator.
  7. Continue titration until the blue color disappears.
  8. Record the burette reading.

Observation Table

Sample Volume (mL) Initial Burette Reading (mL) Final Burette Reading (mL) Na₂S₂O₃ Used (mL)
______ ______ ______ ______

Calculation

Percentage of Available Chlorine =

(A × N × 35.45 × 100) / W

Where:

  • A = Volume of sodium thiosulphate used (mL)
  • N = Normality of sodium thiosulphate
  • W = Weight of bleaching powder sample (g)

Result

The percentage of available chlorine in the given bleaching powder sample was determined by iodometric titration.

Available chlorine = ______ %

Precautions

  • Prepare the bleaching powder solution freshly.
  • Use clean and dry glassware.
  • Add starch indicator only near the end point.
  • Perform titration slowly near the end point.

Conclusion

The experiment helps determine the available chlorine content in bleaching powder. This information is useful for ensuring effective disinfection in water treatment processes.

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