Chemical Oxygen Demand (COD) Explained | Definition, Principle, Procedure, Calculation & Importance
Chemical Oxygen Demand (COD) is one of the most important parameters used in environmental science and water/wastewater quality analysis. It indicates the amount of oxygen required to chemically oxidize oxidizable substances present in a water sample.
What is Chemical Oxygen Demand (COD)?
COD is the amount of oxygen required to oxidize organic and certain inorganic substances present in water or wastewater by a strong chemical oxidizing agent.
It is generally expressed as:
mg/L of O₂ (milligrams of oxygen per litre)
A high COD value usually indicates a high concentration of oxidizable pollutants and therefore greater pollution potential.
Principle of COD Test
In the conventional dichromate method, a known quantity of water sample is heated under acidic conditions with a strong oxidizing agent, usually potassium dichromate (K₂Cr₂O₇).
The oxidation is carried out in the presence of sulfuric acid (H₂SO₄). Certain chemicals, such as mercuric sulfate (HgSO₄), are used to reduce interference from chloride ions.
After digestion, the excess potassium dichromate is determined by titration with ferrous ammonium sulfate (FAS) using ferroin indicator.
The quantity of dichromate consumed is proportional to the amount of oxidizable matter in the sample.
Simplified principle:
Organic/oxidizable matter + K₂Cr₂O₇ + H₂SO₄ → Oxidized products
The more oxidizable material present, the more potassium dichromate is consumed, resulting in a higher COD value.
Chemicals Used in COD Analysis
Common reagents include:
Potassium dichromate (K₂Cr₂O₇) – primary oxidizing agent
Sulfuric acid (H₂SO₄) – provides strongly acidic conditions
Mercuric sulfate (HgSO₄) – helps minimize chloride interference
Ferrous ammonium sulfate (FAS) – used for titration of excess dichromate
Ferroin indicator – indicates the titration endpoint
Safety: The conventional COD method uses hazardous chemicals, particularly dichromate, concentrated sulfuric acid and mercury compounds. Testing should be performed in a properly equipped laboratory following the applicable standard method and waste-disposal requirements.
COD Test Procedure
A simplified overview of the conventional titrimetric method is:
Step 1: Take the sample
A measured volume of water or wastewater is taken in a COD digestion flask.
Step 2: Add reagents
Potassium dichromate and sulfuric acid reagent are added under controlled laboratory conditions.
Step 3: Digestion
The mixture is heated for a specified period so that oxidizable substances react with dichromate.
Step 4: Cool the sample
After digestion, the solution is allowed to cool.
Step 5: Titrate
The remaining dichromate is titrated against standardized ferrous ammonium sulfate (FAS) using ferroin indicator.
Step 6: Calculate COD
The amount of dichromate consumed is converted into the oxygen equivalent and reported as mg/L COD.
COD Calculation
For the conventional titrimetric method, COD can be calculated using:
Where:
A = volume of FAS used for blank (mL)
B = volume of FAS used for sample (mL)
N = normality of FAS
V = volume of sample (mL)
8000 = oxygen equivalent factor
The final result is expressed as:
COD = mg/L as O₂
Importance of COD
COD is important because it helps to:
Assess the pollution strength of wastewater
Monitor industrial effluents
Evaluate wastewater-treatment performance
Study the organic/oxidizable load of water
Monitor changes in water quality
Support environmental compliance and wastewater management
COD vs BOD
COD and BOD are related but different parameters.
Parameter
COD
BOD
Full form
Chemical Oxygen Demand
Biochemical Oxygen Demand
Measurement
Chemical oxidation
Biological oxidation
Time
Relatively rapid
Traditionally about 5 days for BOD₅
Measures
Chemically oxidizable matter
Biodegradable organic matter
Oxidizing agent
Chemical oxidant
Microorganisms
Application
Rapid wastewater assessment
Biodegradable organic pollution assessment
Generally, COD is higher than BOD because COD can include substances that are chemically oxidizable but not readily biodegradable.
Factors Affecting COD Measurement
Several factors can influence COD results, including:
Chloride concentration
Type and concentration of organic matter
Oxidation conditions
Reagent concentration
Digestion time and temperature
Sample handling and preservation
Accuracy of titration
Proper blanks, standards and quality-control procedures are therefore important.
Advantages of COD
1. Faster than BOD:
COD can provide results much more quickly than the traditional BOD₅ test.
2. Useful for wastewater monitoring:
It is widely used to assess the strength of wastewater and industrial effluents.
3. Quantitative parameter:
The result provides a numerical estimate of the oxygen equivalent of oxidizable substances.
Limitations of COD
COD does not directly measure only biodegradable organic matter. Some substances that contribute to COD may not be biodegradable.
Therefore, COD should be interpreted along with other parameters such as:
BOD
TOC
DO
TSS
pH
Other site-specific pollutants
Conclusion
Chemical Oxygen Demand (COD) is a key water-quality parameter used to estimate the oxygen equivalent required to chemically oxidize oxidizable substances in water or wastewater. The potassium dichromate method is a conventional and widely used approach, involving acidic digestion followed by measurement of the remaining dichromate.
Understanding COD is especially important for Environmental Science, Environmental Engineering, wastewater treatment and competitive-exam preparation.
📌 Quick Revision
COD → Chemical Oxygen Demand
Unit → mg/L as O₂
Oxidizing agent → Potassium dichromate (K₂Cr₂O₇)
Acidic medium → H₂SO₄
Titrant → Ferrous ammonium sulfate (FAS)
Indicator → Ferroin
Main purpose → Measure chemically oxidizable pollution load
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