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Determination of Chemical Oxygen Demand (COD) – Environmental Engineering Lab Experiment

Determination of Chemical Oxygen Demand (COD) – Lab Experiment

Chemical Oxygen Demand (COD) is an important parameter used to measure the amount of organic pollutants present in water or wastewater. It indicates the amount of oxygen required to chemically oxidize organic matter present in the sample.

COD is widely used to determine the pollution level of wastewater and is expressed in milligrams per liter (mg/L).

Aim

To determine the Chemical Oxygen Demand (COD) of the given water or wastewater sample using the dichromate reflux method.

Apparatus Required

  • COD digestion flask
  • Reflux condenser
  • Burette
  • Pipette
  • Conical flask
  • Heating mantle or hot plate

Chemicals Required

  • Potassium dichromate (K₂Cr₂O₇) solution
  • Concentrated sulfuric acid (H₂SO₄)
  • Ferrous ammonium sulfate (FAS) solution
  • Ferroin indicator
  • Silver sulfate catalyst
  • Mercuric sulfate
  • Distilled water

Theory

In the COD test, organic matter present in the water sample is oxidized by potassium dichromate in the presence of concentrated sulfuric acid. Silver sulfate acts as a catalyst to speed up the oxidation reaction.

After digestion, the remaining potassium dichromate is titrated with ferrous ammonium sulfate (FAS) using ferroin indicator. The amount of dichromate consumed during the reaction is used to calculate the COD of the sample.

Procedure

  1. Take a known volume of the water sample in a COD digestion flask.
  2. Add potassium dichromate solution to the flask.
  3. Add concentrated sulfuric acid containing silver sulfate catalyst.
  4. Attach the reflux condenser and heat the mixture for about 2 hours.
  5. Allow the solution to cool after digestion.
  6. Titrate the remaining dichromate with ferrous ammonium sulfate (FAS) solution.
  7. Add ferroin indicator near the end point.
  8. The color changes from blue-green to reddish-brown at the end point.
  9. Record the burette reading.

Observation Table

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

Calculation

Chemical Oxygen Demand (mg/L) =

(A − B) × N × 8000 / V

Where:

  • A = Volume of FAS used for blank (mL)
  • B = Volume of FAS used for sample (mL)
  • N = Normality of FAS solution
  • V = Volume of water sample (mL)

Result

The Chemical Oxygen Demand (COD) of the given water sample was determined using the dichromate reflux method.

COD of the sample = ______ mg/L

Precautions

  • Handle concentrated sulfuric acid carefully.
  • Ensure proper reflux during digestion.
  • Use clean glassware for accurate results.
  • Perform titration carefully near the end point.

Conclusion

The COD test helps determine the organic pollution level of water and wastewater. Higher COD values indicate higher levels of organic contaminants, which may require treatment before discharge or reuse.

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