1.2 Disinfection of the Biofilter

1.2.2 Detection and Quantification of Sodium Hypochlorite

1.2.2.2 Titration of Hypochlorite

1.2.2.2.3 Determination of the Activity

Active chlorine liberates free iodine from a KI solution at pH 8 or less. The liberated iodine is titrated with a standard solution of Na₂S₂O₃, with starch as indicator.
The reaction that takes place is:

2Na₂S₂O₃ + NaOCl + H₂O <-> Na₂S₄O₆ + NaCl + 2NaOH

For the titration of 1 ml of chlorine we need the following reagents:

• acetic acid (glacial, concentrated)
  
• KI crystals
  
• Na₂S₂O₃, 0.1 N standard solution
  
• starch indicator solution: mix 5 g starch with a little cold water and grind in a mortar. Pour into 1 l of boiling distilled water, stir, and let settle overnight. Use the clear supernatans. Preserve with 1.25 g salicylic acid.

With these attributes we can perform the actual titration.

DO THE EXERCISES...

HOME                           PREVIOUS STEP | EXERCISES| NEXT STEP                            INDEX