COD Calculator: Chemical Oxygen Demand from Titration

What is chemical oxygen demand (COD)?

Chemical oxygen demand (COD) is a measure of how much oxygen is needed to chemically break down the organic matter in a water sample. It is reported in milligrams of oxygen per liter of water (mg/L). A higher COD number means the water contains more material that can be oxidized, which usually points to more organic pollution. Labs measure COD with a titration test, and this calculator turns the titration numbers into a COD result.

COD is one of the most common tests in water quality work. Wastewater treatment plants run it to check incoming and outgoing water. Environmental scientists use it to check rivers and lakes. Factories use it to prove their discharge meets legal limits.

How the dichromate titration test works

The standard COD test uses a chemical called potassium dichromate to oxidize the organic matter in a water sample. A lab technician heats the sample with dichromate and strong acid for two hours at 150°C (302°F). The dichromate that does not react with the sample is then measured by titration, a process of slowly adding a second chemical (ferrous ammonium sulfate) until a color change signals the reaction is complete.

The test is run twice: once on the real water sample, and once on a "blank," which is the same mixture of chemicals but with no water sample added. The blank shows the maximum amount of titrant possible. Comparing the blank titration to the sample titration shows how much dichromate the organic matter in the sample actually used up.

COD formula

The COD formula is:

$C = \frac{(B - S) \times N \times 8000}{V}$

Where:

• C = the chemical oxygen demand, in mg/L
• B = titrant volume used for the blank test, in mL
• S = titrant volume used for the water sample, in mL
• N = normality of the titrant, in equivalents per liter (eq/L)
• V = volume of the water sample that was digested, in mL
• 8000 = a fixed conversion number (explained below)

The result can never be negative, because negative oxygen demand is not physically possible. The calculator checks the inputs before it applies the formula. The blank volume, the normality and the sample volume must all be greater than zero. The sample titrant volume must be zero or more, and it must not be larger than the blank volume.

Why the formula uses 8000

Oxygen has a molecular weight of 32 grams per mole. Each mole of oxygen molecules accepts 4 electrons when it is reduced to water, so one equivalent of oxygen weighs 32 ÷ 4 = 8 grams. That is 8 milligrams per milliequivalent. The titrant volumes are in millilitres, so the result also has to be scaled from millilitres to litres, which is another factor of 1000. Multiplying the two gives 8 × 1000 = 8000.

Worked example

Suppose a lab technician runs a COD test with these results:

• Blank titrant volume: 10.0 mL
• Sample titrant volume: 7.5 mL
• Titrant normality: 0.1 N
• Sample volume: 20 mL

Applying the formula:

COD = ((10.0 − 7.5) × 0.1 × 8000) / 20 COD = (2.5 × 0.1 × 8000) / 20 COD = 2000 / 20 = 100 mg/L

A result of 100 mg/L falls in the moderate range, typical of treated wastewater.

How to read a COD result

The calculator sorts the result into three bands:

• Under 50 mg/L: low COD. Typical of clean water, such as drinking water or well-treated effluent.
• 50 mg/L up to but not including 200 mg/L: moderate COD. Typical of treated wastewater leaving a treatment plant.
• 200 mg/L and above: high COD. Signals heavy organic pollution, such as raw sewage or untreated industrial wastewater.

Untreated domestic wastewater usually falls between about 250 and 1,000 mg/L. Industrial wastewater, such as from breweries or food plants, can reach several thousand mg/L before treatment.

Common problems in a COD test

Sample titrant volume is higher than the blank. This should not happen, because the sample always consumes some of the dichromate. It usually means the two readings were swapped, or the blank was contaminated. The calculator rejects this input and shows an error instead of a result.

Chloride interference. Salt water and brines contain chloride ions, which dichromate oxidizes the same way it oxidizes organic matter. This makes the COD result read too high. Standard Methods 5220 adds mercuric sulfate to the digestion mixture to tie up the chloride. The method still does not cover samples above about 2,000 mg/L of chloride, which need a different procedure.

Very high results. If a sample's COD is too high for the test to measure accurately, labs dilute the sample with clean water, run the test again, and multiply the result by the dilution factor.

Frequently asked questions

What is the difference between COD and BOD? COD measures everything in a sample that can be broken down by a strong chemical oxidant. Biochemical oxygen demand (BOD) only measures what bacteria can digest over 5 days. COD takes about 2 hours and gives more consistent results between labs; BOD better predicts the effect on a natural river or lake, because rivers break down waste biologically, not chemically.

Why can COD not be negative? Oxygen demand cannot go below zero. A negative result in the formula means the sample titration used more titrant than the blank, which points to a measurement error or contamination, not a real negative demand. The calculator refuses that input and asks for corrected readings. When the two titrations match exactly, the demand is 0 mg/L.

What sample volume should be used? It depends on the method. The open-reflux version of Standard Methods 5220 digests 50 mL. The closed-reflux version uses much smaller vessels and digests only a few millilitres. The calculator accepts any volume above zero, so enter the volume the lab actually digested.

Is this calculator accurate? The calculator applies the formula exactly, so its arithmetic is correct. The accuracy of the final COD value depends on the accuracy of the lab measurements entered, including the titration volumes and the normality of the titrant.

Can COD be used for industrial wastewater? Yes. The same formula applies to any water sample. Industrial samples often have much higher COD than domestic wastewater and may need to be diluted before titration so the result falls within the test's measurable range.

References

1. American Public Health Association, American Water Works Association, & Water Environment Federation. (2017). Standard Methods for the Examination of Water and Wastewater (23rd ed.).
2. U.S. Environmental Protection Agency. (1993). Method 410.4: The Determination of Chemical Oxygen Demand by Semi-Automated Colorimetry.
3. ISO 6060:1989. Water quality — Determination of the chemical oxygen demand. International Organization for Standardization.
4. Tchobanoglous, G., Burton, F. L., & Stensel, H. D. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). McGraw-Hill Education.