Determination of Chemical Oxygen Demand of Wastewater : Pharmaguideline
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  • Jun 16, 2013

    Determination of Chemical Oxygen Demand of Wastewater

    Learn how to determine the COD in waste water to know the pollution quantity in water by the titration method using Ferrous ammonium sulfate and Ferroin solution as indicator.

    Chemical Oxygen Demand (COD)

    Chemical oxygen demand (COD) is used to determine the quantity of pollution in water after wastewater treatment. The higher value of chemical oxygen demand indicates the higher organic pollution in the water sample. Only chemically digest able matter can be determined by the COD test. 

    COD determination takes less time than the Biological Oxygen Demand test. COD is recommended where the polluted water has toxicity and organic matter can’t be determined by biological oxygen demand and useful in water effluent treatment plants.


    The organic matter, present in the water sample is oxidized by potassium dichromate in the presence of sulfuric acid, silver sulfate and mercury sulfate to produce carbon dioxide (CO2) and water (H2O). The quantity of potassium dichromate used is calculated by the difference in volumes of ferrous ammonium sulfate consumed in blank and sample titrations. The quantity of potassium dichromate used in the reaction is equivalent to the oxygen (O2) used to oxidize the organic matter of wastewater.

    Preparation of Potassium dichromate (K2Cr2O7) Solution:
    Add 6.13 gm Potassium dichromate (previously dried at 105 °C for at least two hours) into 800 ml distilled water. Shake the flask well to dissolve the content and make up the solution to 1000 ml and mix well.

    Preparation of Silver sulfate-Sulfuric acid Solution:
    Dissolve 10 gm Silver sulfate (Ag2SO4) in 500 ml concentrated sulfuric acid and make up the solution to 1000 ml swirl the flask to mix well. Allow standing the solution for 24 hours before use.

    Preparation of Mercury sulfate Solution:
    Dissolve carefully 0.1 gm of HgSO4 in 5 ml of concentrated Sulfuric acid.

    Preparation of Ferrous ammonium sulfate Solution (0.025 M):
    Dissolve 9.8 g ferrous ammonium sulfate in a solution of 100 ml of distilled water and 20 ml concentrated Sulfuric acid. Cool the solution and make up the solution to 1000 ml of distilled water. Standardize the solution to determine the actual concentration to calculate the chemical oxygen demand.

    Preparation of Ferroin Indicator:
    Add 3.5 gm of Iron Sulfate heptahydrate and 7.5 gm of Phenanthroline monohydrate to 400 ml of distilled water. Mix well to dissolve and make up to 500 ml of distilled water.

    Test for Chemical Oxygen Demand:

    1. Take 10 ml of sample into a round bottom reflex flask.
    How to digest the sample in COD
    2. Add some glass beads to prevent the solution from bumping into the flask while heating.
    3. Add 1 ml of Mercury sulfate (HgSO4) solution to the flask and mix by swirling the flask.
    4. Add 5 ml of Potassium dichromate (K2Cr2O7) solution.
    5. Now add slowly and carefully 15 ml Silver sulfate- Sulfuric acid solution.
    6. Connect the reflex condenser and digest the content using a hot plate for 2 hours.
    7. After digestion cools the flask and rinses the condenser with 25 ml of distilled water collecting in the same flask.
    8. Add 2-4 drops of ferroin indicator to the flask and titrate with 0.025 M ferrous ammonium sulfate solution to the endpoint.
    9. Make the blank preparation in the same manner as sample using distilled water instead of the sample.

    Various stages of sample during COD testing

    Calculate the chemical oxygen demand by following formula:
    COD = 8x1000xDFxMx(VB - VS
                 Volume of sample (in ml)
    DF – Dilution Factor (if applicable)
    M – Molarity of standardized Ferrous Ammonium Sulfate solution
    VB – Volume consumed in titration with blank preparation
    VS – Volume consumed in titration with sample preparation

    Example Calculation:

    Volume of ferrous ammonium sulfate for Sample (VS) = 23.8 ml
    Volume of ferrous ammonium sulfate for Blank VB) = 25.6 ml
    Dilution Factor (DF) = 1 (sample used as it is)
     COD = 8x1000x1x0.025x(25.6-23.8)
              = 8000x0.025x1.8
              = 360  =  36 mg/lit or ppm

    18 comments: Post Yours! Read Comment Policy ▼

    1. All parameters are correct

    2. What is value of dilution factor if I taken 1ml sample diluted in 50ml of distilled water

      1. Dilution factor will be 50 because sample is diluted 50 times.

    3. What is standard value of volume OF FAS required for distilled water

      1. Titrate the distilled water with FAS as blank and note down the reading.

    4. Any standard value of volume of titrated blank solution

    5. Is thr any Standard value of blank solution

      1. No standard value for blank test.
        Blank test value shall be deducted from sample test.

    6. what happen if volume of sample is the same as the blank. Can COD become zero

      1. Sir, if after treating the sample, the sample turns darkish red and after the 2hrs heating, the colour remains the same, how do we determine the COD?

    7. What is the highest CoD that can be measured with this process without any dilution?

    8. which method from the two ways: (i) Open Reflux method; (ii) Closed reflux (titrimetric and colorimetric) method using COD digester - do you use for industries? Is there any particular method used for a particular industry?

    9. How can I remove the influence of chloride from the sample while determining the COD? I have ocean water with salinity as high as 17,000mg/l and require to determine its COD. Thank you.


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