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Determination of Dissolved Oxygen in Water – Environmental Engineering Lab Experiment

Determination of Dissolved Oxygen (DO) in Water – Lab Experiment

Dissolved Oxygen (DO) refers to the amount of oxygen present in water. It is an important parameter used to assess the quality of water and the health of aquatic ecosystems. Adequate dissolved oxygen is necessary for the survival of fish and other aquatic organisms.

The dissolved oxygen content in water is usually expressed in milligrams per liter (mg/L).

Aim

To determine the dissolved oxygen (DO) present in the given water sample using the Winkler method.

Apparatus Required

  • Dissolved Oxygen (DO) bottle
  • Burette
  • Pipette
  • Conical flask
  • Measuring cylinder

Chemicals Required

  • Manganous sulfate solution
  • Alkaline iodide azide reagent
  • Concentrated sulfuric acid
  • Standard sodium thiosulphate solution
  • Starch indicator
  • Distilled water

Theory

Dissolved oxygen in water is determined by the Winkler method. In this method, dissolved oxygen oxidizes manganous ions to form manganese dioxide. When acid is added, iodine is released from potassium iodide equivalent to the oxygen present in the sample.

The liberated iodine is then titrated with standard sodium thiosulphate solution using starch indicator. The amount of sodium thiosulphate used helps calculate the dissolved oxygen concentration in the water sample.

Procedure

  1. Fill the DO bottle completely with the water sample without trapping air bubbles.
  2. Add manganous sulfate solution followed by alkaline iodide azide reagent.
  3. Stopper the bottle and mix gently.
  4. A brown precipitate will form.
  5. Add concentrated sulfuric acid to dissolve the precipitate.
  6. Take a measured volume of this solution in a conical flask.
  7. Titrate with standard sodium thiosulphate solution until the color becomes pale yellow.
  8. Add a few drops of starch indicator.
  9. Continue titration until the blue color disappears.
  10. Record the burette reading.

Observation Table

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

Calculation

Dissolved Oxygen (mg/L) =

(A × N × 8000) / V

Where:

  • A = Volume of sodium thiosulphate used (mL)
  • N = Normality of sodium thiosulphate
  • V = Volume of water sample (mL)

Result

The dissolved oxygen content of the given water sample was determined using the Winkler method.

Dissolved Oxygen (DO) = ______ mg/L

Precautions

  • Avoid trapping air bubbles in the DO bottle.
  • Use clean glassware for accurate results.
  • Add reagents carefully and mix properly.
  • Perform titration immediately after fixing the sample.

Conclusion

The experiment helps determine the dissolved oxygen level in water. This parameter is essential for evaluating water quality and the suitability of water for aquatic life.

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