What is Chemical Oxygen Demand?
Chemical oxygen demand, or COD, measures the capacity of water to consume oxygen during the decomposition of organic matter. In other words, it is the amount of oxygen needed to oxidize all organic compounds present in a given volume of water through chemical means.
The unit of COD is milligrams per liter (mg/L), indicating how much oxygen is consumed per liter of solution. A low COD indicates cleaner water with fewer pollutants, while a higher COD signals higher levels of organic contamination.
Whether you are a scientist, environmental engineer, or water quality professional, knowing the COD helps you understand just how clean — or polluted — a body of water really is.
How to Use the Chemical Oxygen Demand Calculator
Our COD calculator is based on the standard dichromate titrimetric method (APHA Standard Methods). Simply enter the four measured values:
- Volume of FAS for blank run (A) — the amount of ferrous ammonium sulfate (FAS) titrant consumed in the blank (reagent) run, in mL or fl oz.
- Volume of FAS for sample (B) — the amount of FAS titrant consumed when titrating the sample, in mL or fl oz.
- Normality of FAS (N) — the normality of the ferrous ammonium sulfate solution (typically 0.10 N or 0.25 N).
- Volume of original sample (V) — the volume of the water sample taken for the test, in mL or fl oz.
Select your preferred unit system (Metric or American) and click Calculate. The result is displayed instantly in mg/L.
Tip: Make sure A is always greater than or equal to B. If the sample consumes more FAS than the blank, the result would be negative, indicating a measurement error.
Chemical Oxygen Demand Formula
The COD formula used in this calculator is:
COD (mg/L) = [(A − B) × N × 8000] / V
Where:
- A = volume of FAS used in blank titration (mL)
- B = volume of FAS used in sample titration (mL)
- N = normality of FAS solution (eq/L)
- V = volume of original water sample (mL)
- 8000 = milliequivalent weight of oxygen (8 g/eq) multiplied by 1000 mL/L
If you use American units (fl oz), the calculator automatically converts them to mL before applying the formula (1 fl oz = 29.5735 mL).
Water Quality Interpretation
| COD (mg/L) | Water Quality | Typical Source |
|---|---|---|
| < 20 | Clean / Excellent | Surface water, drinking water |
| 20 – 200 | Moderate pollution | Urban runoff, treated effluents |
| 200 – 1000 | Polluted | Industrial wastewater, domestic sewage |
| > 1000 | Highly polluted | Industrial effluents, untreated sewage |
Difference Between BOD and COD
Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are both measures of organic pollution in water, but they differ in method and scope:
- BOD measures the amount of oxygen that microorganisms require to biologically decompose organic matter over a set period (typically 5 days at 20 °C).
- COD measures the amount of oxygen required to chemically oxidize all organic matter (both biodegradable and non-biodegradable) using a strong chemical oxidant like potassium dichromate.
As a rule of thumb, COD > BOD because COD accounts for all oxidizable organics, including those that microorganisms cannot break down. The COD/BOD ratio gives insight into the biodegradability of the wastewater.
Significance of Determining Chemical Oxygen Demand
COD testing is a cornerstone of water quality analysis for several reasons:
- Pollution monitoring: Regulatory agencies use COD limits for effluent standards and industrial discharge permits.
- Wastewater treatment design: Engineers use COD data to size treatment facilities and assess treatment efficiency.
- Environmental assessment: COD helps identify sources of organic contamination in rivers, lakes, and groundwater.
- Process control: In industrial processes, COD monitoring helps optimize production and minimize waste generation.
- Faster than BOD: COD tests can be completed in a few hours, versus 5 days for BOD, making them preferred for rapid assessments.
FAQs
What does a negative COD value mean?
A negative COD value occurs when the volume of FAS for the sample (B) exceeds the volume for the blank (A). This is physically impossible and indicates a measurement or calculation error. Re-run the test and verify your measurements.
What is the normal COD of drinking water?
For drinking water, COD is typically below 10–15 mg/L. WHO guidelines recommend COD below 10 mg/L for potable water. Higher values suggest contamination and treatment is required.
Why is FAS used in the COD test?
Ferrous ammonium sulfate (FAS) is used as a back-titrant in the dichromate method. After the sample is digested with excess potassium dichromate (K₂Cr₂O₇), the remaining (unreacted) dichromate is titrated with FAS to determine how much dichromate was consumed, which corresponds to the oxygen demand.
What is the difference between metric and American units in this calculator?
In the metric system, volumes are measured in milliliters (mL). In the American system, volumes are measured in fluid ounces (fl oz). The calculator automatically converts fl oz to mL (1 fl oz = 29.5735 mL) before computing COD. The result is always expressed in mg/L regardless of the unit system selected.