Determination of Dissolved Oxygen in Water

Aim

To determine the Dissolved Oxygen content present in the given wastewater sample.

Apparatus

BOD bottles, burette, pipette, conical flask, measuring jar.

Reagents

1. Manganous sulphate solution
2. Alkali Iodide azide
3. Concentrated Sulphuric acid
4. Starch solution
5. 1N Stock Sodium-thio-sulphate solution
6. 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.

Procedure

1. Fill up the BOD bottle with the given water sample up to the neck.
2. 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.
3. 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.
4. Take 203 ml of this solution in a conical flask and titrate with 0.025 N Na₂S₂O₃ to a pale straw color.
5. 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.
6. Hence the endpoint changes colour from Yellow to pale straw to blue to colorless.

Note

1 ml of 0.025 N Na₂S₂0₃ is equivalent to 0.2 mg of oxygen.

Observation

 DO in mg/L = (V x 0.2 x 1000 x N) / 200