What this calculator does
Chemical oxygen demand measures how much oxygen would be needed to chemically oxidise all the organic and inorganic matter in a water sample. It is one of the standard indicators of water quality, used for everything from municipal wastewater treatment to monitoring an industrial discharge.
The dichromate reflux method oxidises the sample with a known excess of potassium dichromate, then back-titrates the unreacted dichromate with ferrous ammonium sulfate (FAS). A blank sample run through the same procedure shows how much FAS is needed with no oxidisable matter present, and the difference between the blank and the sample titration is what the formula converts into a COD figure.
The formula
Subtract the FAS volume used for the sample from the FAS volume used for the blank, multiply by the normality of the FAS titrant and by 8,000 (the milliequivalent weight of oxygen, 8 mg/meq, expressed per litre), then divide by the volume of sample used in the test.
| Term | Meaning |
|---|---|
| COD | Chemical oxygen demand, in mg/L of oxygen equivalent: (blank − sample) × N × 8,000 / sample volume. |
| Blank titration | The FAS volume needed to titrate a sample containing no oxidisable matter, run through the identical procedure. |
| Normality (N) | The equivalent concentration of the FAS titrant, standardised before use. |
The inputs explained
| Field | What to enter |
|---|---|
| FAS used, blank titration (mL) | The volume of FAS titrant used to reach the endpoint on the blank. |
| FAS used, sample titration (mL) | The volume of FAS titrant used to reach the endpoint on the actual sample. |
| Normality of FAS titrant (N) | The standardised normality of the FAS titrant solution. |
| Sample volume tested (mL) | The volume of sample that was digested and titrated, not the total sample collected. |
When to use it
Monitoring wastewater treatment
COD is measured on influent and effluent streams to track how much organic load a treatment process is removing, and to check compliance with discharge limits.
Comparing COD with biochemical oxygen demand (BOD)
COD oxidises essentially everything chemically, including material that microbes cannot break down, so it is normally higher than BOD on the same sample; the ratio between the two is itself a useful indicator of how biodegradable the waste is.
Auditing a lab titration
Recomputing COD from the raw blank and sample titration volumes checks that a reported result matches the underlying titration data, independent of any spreadsheet used in the lab.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How COD changes with the sample volume used in the test
The same titration result, applied to a range of sample volumes.
| Sample volume tested | Chemical oxygen demand |
|---|---|
| 4 mL | 800.0 mg/L O₂ |
| 6 mL | 533.3 mg/L O₂ |
| 8 mL | 400.0 mg/L O₂ |
| 10 mL | 320.0 mg/L O₂ |
| 15 mL | 213.3 mg/L O₂ |
| 20 mL | 160.0 mg/L O₂ |
Questions
Why does the blank need more FAS than the sample?
More unreacted dichromate is left over in the blank because nothing in it consumed dichromate during digestion, so more FAS is needed to titrate that excess. A sample with real oxygen demand leaves less dichromate unreacted, and therefore needs less FAS at the endpoint.
What does a negative COD result mean?
It means the sample titration used more FAS than the blank, which is not physically meaningful for this method and usually points to a titration, standardisation or dilution error rather than a genuine result.
Is COD the same as BOD?
No. COD is a chemical oxidation measured in a few hours; BOD measures oxygen consumed by living microorganisms over five days and only captures biodegradable material. COD is quicker to run and is normally the higher of the two figures.
Why is the constant in the formula 8,000?
Oxygen has an equivalent weight of 8 g per equivalent (32 g/mol divided by the 4 electrons transferred per O2 molecule in this redox context), and the result is expressed in mg per litre, so the milliequivalent weight of 8 mg is scaled up by 1,000 to convert the small-volume titration into a per-litre figure.
For a related water-quality figure derived from ion concentrations rather than a titration, see the water hardness calculator.