To determine the Dissolved Oxygen content present in the given wastewater sample.
BOD bottles, burette, pipette, conical flask, measuring jar.
- Manganous sulphate solution
- Alkali Iodide azide
- Concentrated Sulphuric acid
- Starch solution
- 1N Stock Sodium-thio-sulphate solution
- 025 N Sodium-thio-sulphate (Na2S203)
Principle (The Winkler method with Azide modification)
When MnS04 and alkali iodide azide are added to water containing DO, the oxygen oxidizes Mn2+ to give a precipitate of Mn(OH)2. By the addition of H2SO4, free iodine is liberated, which is converted to blue colour by the addition of starch.
- Fill up the BOD bottle with the given water sample up to the neck.
- Add 2ml of MnSO4 and 2ml of alkali iodide azide solution to the BOD bottle. Exclude the air bubbles and mix the contents of the bottle by inverting the bottle a few times.
- After the precipitate has settled to about 1/3rd the volume of the BOD bottle from the bottom, add 2 ml of concentrated sulphuric acid. Once again mix the contents gently till the suspension is completely dissolved and a uniform yellow colored solution is obtained.
- Take 203 ml of this solution in a conical flask and titrate with 0.025 N Na2S2O3 to a pale straw color.
- Add 1 ml of starch, which turns the solution blue in color. Continue the titration till the blue color disappears. Note down the volume of Na2S203 consumed, which gives the DO directly.
- Hence the endpoint changes colour from Yellow to pale straw to blue to colorless.
1 ml of 0.025 N Na2S203 is equivalent to 0.2 mg of oxygen.
|Trial No||Initial Reading||Final reading||Difference|
DO in mg/L = (V x 0.2 x 1000 x N) / 200