Chemical Oxygen Demand Calculator
Calculate chemical oxygen demand (COD) for water quality analysis. Enter FAS blank volume, FAS sample volume, normality, and sample volume to get COD in mg/L.
What is Chemical Oxygen Demand?
Chemical Oxygen Demand (COD) is a critical water quality measurement that determines the amount of oxygen required to chemically oxidize organic and inorganic matter in water. It is expressed in milligrams per liter (mg/L) and is widely used to assess the pollution level of wastewater, natural waters, and industrial effluents.
Unlike Biochemical Oxygen Demand (BOD), which measures oxygen consumed by biological processes over several days, COD can be measured in just a few hours using a chemical digestion method. This makes COD a faster indicator of water pollution for environmental monitoring and regulatory compliance.
How COD is Calculated
The COD test typically uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent in a strong acid medium. The sample is digested at 150°C for two hours, and the amount of oxidant consumed is measured by titration with ferrous ammonium sulfate (FAS). The COD value is calculated using the formula:
$$\text{COD (mg/L)} = \frac{(A - B) \times N \times 8000}{V}$$
where A is the FAS volume for the blank (mL), B is the FAS volume for the sample (mL), N is the normality of FAS (eq/L), and V is the sample volume (mL). The calculator performs this calculation instantly.
Applications of COD Testing
COD testing is essential in wastewater treatment plants, environmental monitoring, food processing, pharmaceutical manufacturing, and chemical industries. High COD levels indicate significant organic pollution that can deplete oxygen in receiving waters, harming aquatic life. Regulatory agencies set COD discharge limits to protect water quality.
Frequently Asked Questions
What is a normal COD value for clean water?
Clean natural water typically has a COD below 20 mg/L. Drinking water should have COD less than 10 mg/L. Untreated domestic wastewater may have COD between 250-800 mg/L, while industrial effluents can range from 1,000 to over 10,000 mg/L.
What is the difference between COD and BOD?
COD measures all chemically oxidizable matter and takes only 2-3 hours to complete. BOD measures biologically oxidizable matter over 5 days (BOD₅). COD values are typically higher than BOD values, and the COD:BOD ratio helps characterize wastewater treatability.
What does a high COD indicate?
High COD indicates a high concentration of organic pollutants in the water. This can result from industrial discharges, sewage overflows, agricultural runoff, or food processing waste. High COD levels can cause oxygen depletion in receiving waters, leading to fish kills and ecosystem damage.
How is the COD test performed?
The standard COD test uses closed reflux and colorimetric or titrimetric methods. A water sample is mixed with potassium dichromate, sulfuric acid, and silver sulfate catalyst, then digested at 150°C for 2 hours. The remaining dichromate is measured by titration with FAS.
What interferences affect COD measurements?
Chloride ions are the most common interference in COD testing. Mercury sulfate is added to complex chlorides and reduce their interference. Other interferences include nitrite, ferrous iron, sulfide, and hydrogen peroxide, which can cause falsely elevated COD results.
Is COD regulated by environmental agencies?
Yes. The EPA and other environmental agencies regulate COD discharges through wastewater discharge permits. Typical limits range from 50-200 mg/L depending on the industry and receiving water body. Regular COD monitoring is required for compliance with discharge standards.
For related water quality and analysis tools, see the Beer Lambert Law Calculator and the Log Reduction Calculator.
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