Chemical Oxygen Demand (COD) is a vital water quality parameter that measures the amount of oxygen required to chemically oxidize organic and inorganic matter present in water. It indicates the level of pollution and helps assess the strength of wastewater or industrial effluents. A higher COD value means greater organic pollution, which can harm aquatic life and degrade water quality.

Here we take the Waste Water Sample to test COD.

The COD test is based on the oxidation of organic and inorganic matter by a strong oxidizing agent (potassium dichromate) in acidic medium. The sample is digested at 150°C in the presence of silver sulphate catalyst. After oxidation, the excess dichromate is titrated with ferrous ammonium sulphate using ferroin indicator. The amount of dichromate consumed is directly proportional to the oxygen required to oxidize the organic matter — giving the COD value.

Equipments, Glassware, and Chemicals Used in Chemical Oxygen Demand (COD) Testing in Water:

Equipments & Glassware:

COD Digester	Heats the samples at 150°C for 2 hours to promote oxidation of organic matter.
COD Digester Flask(250ml)	To hold the sample during digestion.
Air Condenser	Prevents the loss of volatile substances during digestion.
Pipettes & Burettes(10ml&50ml)	Used for accurate measurement of samples,reagents and titration.
Conical Flask(500ml)	Used for sample titration.
Measuring Cylinder (50ml)	For measuring and mixing reagents.
Analytical Balance(4-digit)	Used to weigh chemicals for standard solution & reagent preparation.

Chemicals Used:

Potassium Dichromate (K₂Cr₂O₇)	Acts as a strong oxidizing agent in acidic conditions.Here we have used all the following chemicals/reagents of Nice company. You can use the same company or any other companies.
Sulphuric Acid (H₂SO₄)	Provides the acidic medium required for oxidation and helps maintain reaction temperature.
Mercuric Sulphate (HgSO₄)	Removes interference caused by chloride ions.
Silver Sulphate (Ag₂SO₄)	Used as a catalyst to speed up oxidation of organic compounds.
0.1 (N) Ferrous Ammonium Sulphate (FAS)	Used as a standardized titrant to determine the remaining dichromate after digestion.Dissolve 39 g Fe(NH4)(S04)2. 6H20 in distilled water. Add 20 ml concentrated H2SO4,cool, and dilute to 1 Litre.Standardize this solution daily against standard K2Cr207.
Ferroin Indicator	Indicates the endpoint of titration by color change.Dissolve 1.485 g 1,10 Phenanthroline monohydrate and 695 mg FeSO4.7H20 in water and dilute to 100 ml.
Distilled Water	To prepare Standard Solution & reagent.The conductivity of distilled water should be <5 μS/cm to prevent contamination.

Testing Process of Chemical Oxygen Demand (COD) in Water followed in our lab:

Take 20 ml of water Sample in COD digester tube,mix with 20 ml distilled water
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Add 0.4 gm Mercuric Sulphate (HgSO₄) to remove chlorides
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Add 10 ml K₂Cr₂O₇ (oxidizing agent) to sample
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Add 30 ml H₂SO₄ & 0.4 gm Silver Sulphate (Ag₂SO₄) into it
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Mix and seal reflux tube with air condenser
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Digest at 150°C for 2 hours in COD reactor
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Cool the sample
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Add few drops of ferroin indicator
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Titrate with Ferrous Ammonium Sulphate (FAS) until color changes
from Blue-green to Reddish-brown
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Perform blank under same steps
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Calculate COD using formula:
COD (mg/L) = (A − B) × S_FAS × 8000 /sample taken, [A= Titre value of Blank, B=Titre value of Sample]

What is the purpose of Chemical Oxygen Demand (COD) Testing in Water Sample:

The main purpose of COD testing is to determine the amount of organic and inorganic pollutants present in water that can be chemically oxidized. It helps to:

1. Assess water pollution levels in industrial effluents, sewage, and natural water bodies.
2. Evaluate wastewater treatment efficiency by comparing influent and effluent COD values.
3. Estimate the oxygen demand that pollutants place on aquatic ecosystems.
4. Monitor environmental compliance with pollution control standards.
5. Predict the potential impact of waste discharge on water quality and aquatic life.
6. To ensure the standards with compliance of Pollution Control Board i.e.<250 mg/L.
7. To ensure the further treatments required for the polluted water before discharge based on high COD values.

Handling Mistakes to Be Kept in Mind During COD Testing of WasteWater:

1. Use clean and dry glassware — moisture or residue affects reagent strength.Avoid contamination of glassware and samples, as even small impurities can alter COD results. Proper cleaning Process described previously.
2. Handle concentrated sulphuric acid carefully to prevent burns and ensure correct reagent mixing.
3. Maintain accurate digestion temperature (150°C) and time (2 hours) for complete oxidation.
4. Prevent chloride interference by adding sufficient mercuric sulphate.
5. Ensure tight condenser fitting to prevent loss of volatile organics.
6. Avoid air bubbles in titration burette for accurate endpoint detection.
7. Use freshly prepared reagents for reliable results.

Conclusion for testing procedure of COD in Water Sample :

The Chemical Oxygen Demand (COD) test is an essential method for assessing the level of organic and inorganic pollution in waste water/STP/ETP/Effluent water. It provides a quick and reliable measure of the amount of oxygen required to chemically oxidize contaminants. By determining COD, we can evaluate the efficiency of wastewater treatment plants, monitor environmental pollution, and ensure water quality standards are maintained for the protection of aquatic life and ecosystems.By following these method, you can easily test the Chemical Oxygen Demand (COD) content at environmental science laboratory or any laboratory & also manufacturing industry with availability of the equipments & chemicals. If you can’t understand the procedure you can check our real time photo attached with this writing or also you can reach to Pro Research & Testing Laboratory for the testing purposes.

FAQ on Chemical Oxygen Demand (COD) Test in Water Sample :

Q1. What is COD?
A: COD (Chemical Oxygen Demand) measures the amount of oxygen required to chemically oxidize organic and inorganic matter in wastewater.

Q2. Why is COD important?
A: It indicates the level of pollution and helps evaluate the efficiency of wastewater treatment processes.

Q3. What causes high COD in water?
A: High COD is caused by the presence of organic waste, industrial effluents, and chemical pollutants.

Q4. How is COD different from BOD?
A: COD measures total chemically oxidizable matter (fast test), while BOD measures biologically degradable matter (slow test).

Q5. What are the common oxidizing agents used in COD tests?
A: Potassium dichromate (K₂Cr₂O₇) is commonly used as the oxidizing agent.

Q6. Why is mercury sulphate used in COD testing?
A: It eliminates chloride interference that could otherwise increase COD values falsely.

Q7. How long does the COD test take?
A: The digestion process usually takes 2 hours at 150°C.

Q8. What safety precautions should be followed?
A: Handle acids, mercury compounds, and dichromate carefully using gloves, goggles, and a fume hood.

How We Verified This Testing/Research Procedure :

This testing is done under qualified analysts in our lab.Continually monitored by expertise.Repeatedly testing is always done to get accurate result.

Written by
Anwesha Das (M.Sc Microbiology,BU)
Designation – Microbiologist

Reviewed by
Riya Ghosh (M.Sc. Food Technology, MAKAUT)
Designation – Chemist

Verified By
Dr. Jyotirmoy Kumar Dey (Phd,Chemistry)
Designation – Senior/Chief Chemist
Experience – 25 Years +