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  Section: Plant Protocol » Environmental Science Methodology
 
 
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Determination of Available Chlorine in Bleaching Powder

 
     
 
Aim
To determine the available chlorine in the given sample of bleaching powder by the iodometric method.

Principle
Bleaching powder is commonly used as a disinfectant. The chlorine present in the bleaching powder gets reduced with time. So, to find the exact quantity of bleaching powder required, the amount of available chlorine in the sample must be found out.

Chlorine will liberate free iodine from potassium iodide solution when its pH is 8 or less. The iodine liberated, which is equivalent to the amount of active chlorine, is titrated with standard sodium thiosulphate solution using starch as indicator.
Apparatus
  1. Mortar and pestle
  2. Volumetric flask
  3. Burette
  4. Pipette
  5. Erlenmeyer flask.





Reagents (» click to check the preparation of reagents)
  1. Concentrated glacial acetic acid
  2. Standard sodium thiosulphate solution (0.025N)
  3. Potassium iodide
  4. Starch indicator
  5. Iodine solution (0.025 N).
Procedure
  1. Dissolve 1g bleaching powder in 1 litre of distilled water in a volumetric flask, and stopper the container.
    (This can be done by first making a paste of the bleaching powder with mortar and pestle.)
  2. Place 5 mL acetic acid in an Erlenmeyer flask and add about 1g potassium iodide crystals. Pour 25 mL of bleaching powder solution prepared above and mix with a stirring rod.
  3. Titrate with 0.025 N sodium thiosulphate solution until a pale yellow colour is obtained. (Deep yellow changes to pale yellow.)
  4. Add 1mL of starch solution and titrate until the blue colour disappears.
  5. Note down the volume of sodium thiosulphate solution added (V1).
  6. Take a volume of distilled water corresponding to the sample used.
  7. Add 5 mL acetic acid, 1g potassium iodide and 1 mL starch solution.
  8. If blue colour occurs, titrate with 0.025 N sodium thiosulphate solution until the blue colour disappears.
  9. Record the volume of sodium thiosulphate solution added (A1).
  10. If no blue colour occurs, titrate with 0.025 N iodine solution until a blue colour appears. Note down the volume of iodine (A2).
  11. Then, titrate with 0.025 N sodium thiosulphate solution till the blue colour disappears. Record the volume of sodium thiosulphate solution added (A3). Note down the difference between the volume of iodine solution and sodium thiosulphate as A4(A4=A2- A3).
    Note: Blank titration is necessary to take care of the oxidising or reducing reagents' impurities.



Observation

Bleaching powder solution x Standard sodium thiosulphate solution (0.025 N)
 
Distilled water × Standard sodium thiosulphate solution (0.025 N)
 
Distilled water x Standard iodine solution (0.025N)


Calculation
mg of Cl2/mL (B) = (V – A1) or (V + A4) x N x 35.46
mL of bleaching powder solution taken

1000 mL of bleaching powder solution contains 1000 x B mg of Cl2
i.e., 1000 mg bleaching powder contains 1000 B mg of Cl2

therefore, 100 mg of bleaching powder contains = 1000 x B
10

% of chlorine available = .........


Results
Available chlorine in the given bleaching powder is. ...%
 
     
 
 
     



     
 
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