Science & Lab Tools
Chemical Oxygen Demand (COD) Calculator
Calculate the Chemical Oxygen Demand (COD) in water samples to determine organic pollution levels and assess water quality.
Reviewed by the calculator.uk.com Team · Last reviewed 30 June 2026 · Our methodology
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Related to Chemical Oxygen Demand (COD) Calculator
The Chemical Oxygen Demand (COD) Calculator is a specialized tool that helps determine the amount of oxygen required to chemically oxidize organic and inorganic matter in water samples. This measurement is crucial for assessing water quality and pollution levels in environmental and wastewater analysis.
Calculation Method
COD is calculated using the following formula:
COD (mg/L) = ((Vb - Vs) × M × 8000) / mL sample
where:
- Vb = Volume of FAS used for blank (mL)
- Vs = Volume of FAS used for sample (mL)
- M = Molarity of FAS
- 8000 = Milliequivalent weight of oxygen × 1000 mL/L
The calculator takes into account the standard dichromate method, where a known amount of potassium dichromate is used to oxidize the organic matter in the sample. The remaining unreacted dichromate is then determined by titration with ferrous ammonium sulfate (FAS). The difference in FAS volume between the blank and sample, along with the FAS molarity, is used to calculate the COD value.
The COD value is expressed in milligrams of oxygen consumed per liter of sample (mg/L). Higher COD values indicate higher levels of organic pollution in the water sample. This measurement is particularly useful for:
Typical COD Ranges
- Clean Water: < 20 mg/L
- Treated Wastewater: 20-200 mg/L
- Raw Wastewater: 200-1000+ mg/L
- Industrial Effluent: Can exceed 1000 mg/L
COD results are used in conjunction with other parameters like BOD (Biochemical Oxygen Demand) to provide a comprehensive assessment of water quality. The COD test is preferred in many cases because it can be completed in a few hours, compared to the 5-day period required for BOD testing.
1. Why is COD important?
COD is a vital parameter in water quality assessment because it provides a quick and reliable measure of organic pollution. It helps in monitoring treatment plant efficiency, compliance with discharge regulations, and overall environmental impact assessment.
2. How accurate is the COD test?
The COD test is highly accurate when performed correctly. The dichromate method used in this calculator is a standard method approved by environmental protection agencies worldwide. However, accuracy depends on proper sampling, careful measurement of reagents, and following correct analytical procedures.
3. What can affect COD results?
Several factors can skew COD results, including improper sample preservation and storage, interfering substances such as chlorides that also consume dichromate, inconsistent digestion temperature, inaccurate reagent concentrations, and a poorly prepared blank. Chloride interference is one of the most common sources of error and is usually controlled by adding mercuric sulfate to the digestion mixture.
4. What is the difference between COD and BOD?
COD measures the total amount of oxygen needed to chemically oxidize both biodegradable and non-biodegradable organic matter, giving a result within a few hours. BOD (Biochemical Oxygen Demand) measures only the oxygen consumed by microorganisms breaking down biodegradable matter, typically over a 5-day incubation period. Because COD captures more of the total pollutant load and is much faster to run, it is often used as a quick screening tool alongside periodic BOD testing.
5. What is the scientific source for this calculator?
This calculator follows the closed reflux titrimetric dichromate procedure set out in Standard Methods for the Examination of Water and Wastewater, Method 5220C, jointly maintained by the American Public Health Association, American Water Works Association, and Water Environment Federation. It's consistent with the analogous EPA Method 410.4 used in US wastewater compliance testing. Both are the standard reference procedures for COD determination in environmental laboratories worldwide.