How this instrument works
Chemical oxygen demand (COD) is a water-quality measurement used across environmental engineering and wastewater treatment: it estimates the total amount of oxygen that would be consumed if every oxidizable substance in a water sample — organic matter and many inorganic reducing agents alike — were fully chemically oxidized. A high COD means a sample is heavily loaded with pollutants capable of consuming dissolved oxygen once released into a river or lake, which is exactly the concern regulators and treatment plants track it for.
The standard method (APHA Standard Methods 5220, the dichromate reflux method) doesn't measure oxygen directly. Instead, a known excess of potassium dichromate — a strong oxidizer — is refluxed with the sample in concentrated sulfuric acid, oxidizing everything it can. Whatever dichromate is left unreacted afterward is then back-titrated with a ferrous ammonium sulfate (FAS) solution of known normality, using a ferroin indicator that snaps from blue-green to a wine-red endpoint the instant all the leftover dichromate is consumed. The less dichromate left over, the more of it the sample used up oxidizing pollutants — and the higher the sample's COD.
The titrant volume used on a blank (a control run with no polluted sample) sets the baseline for 'nothing was oxidized,' so the actual COD calculation depends on the difference between the blank titration volume and the sample titration volume, not either one alone. This titration involves concentrated sulfuric acid and hexavalent chromium (Cr⁶⁺), a recognized carcinogen and a hazardous waste stream requiring proper disposal — it is bench chemistry for a trained lab technician working in a fume hood with appropriate PPE, not a casual measurement to attempt without lab training and the right safety equipment.
- Enter the FAS titrant volume used for the blank titration, A, in mL.
- Enter the FAS titrant volume used for the sample titration, B, in mL — it should be less than or equal to the blank volume.
- Enter the normality, N, of the standardized FAS titrant solution.
- Enter the sample volume used in the digestion, in mL.
- Read the calculated chemical oxygen demand, COD, in mg/L.
Worked example — a wastewater dichromate titration
A blank titration consumes 300 mL of 3 N FAS titrant; the same reflux run on a 45,000 mL wastewater sample consumes only 129 mL of the same FAS titrant, because the sample's pollutants used up dichromate that the blank didn't need to. Plugging into the formula: COD = (300 − 129) × 3 × 8000 ⁄ 45,000 = 171 × 3 × 8000 ⁄ 45,000 = 91.2 mg/L.
That the sample used far less FAS than the blank (129 mL versus 300 mL) is the whole signal: the pollutants in the sample consumed dichromate during digestion, leaving less of it around afterward to react with the FAS titrant. A sample with essentially no oxidizable material would use almost the same amount of FAS as the blank, giving a COD close to zero — exactly the internal correctness check this instrument's own test vectors include.
Questions
What does chemical oxygen demand (COD) actually measure?
COD estimates the total amount of oxygen that would be needed to chemically oxidize all the oxidizable material in a water sample — mostly organic pollutants, but also some inorganic reducing substances. It's reported in mg/L (milligrams of oxygen equivalent per liter of sample) and is one of the standard indicators regulators and treatment plants use to gauge how polluted a water sample is and how much it would deplete dissolved oxygen if discharged untreated.
What's the difference between COD and BOD?
COD (chemical oxygen demand) uses a strong chemical oxidizer to oxidize essentially everything oxidizable in a sample within a few hours, including material that microorganisms can't break down. BOD (biochemical oxygen demand) instead measures oxygen consumed by living microorganisms actually digesting the sample's organic matter over a period of days, so it reflects only the biodegradable fraction. COD values are typically higher than BOD values for the same sample, and the two are often reported together because their ratio (COD:BOD) tells you how biodegradable a sample's pollutant load actually is.
Why does a smaller titrant volume for the sample mean higher COD?
Because the titration measures leftover, unreacted dichromate after digestion — and the sample's pollutants consumed some of that dichromate oxidizing themselves, leaving less of it around to react with the FAS titrant afterward. A smaller sample titration volume means more dichromate got used up during digestion, which means the sample had more oxidizable material in it, which is exactly what a higher COD represents.
What is the 8000 constant in the COD formula?
It's the standard conversion factor built from the equivalent weight of oxygen (8 mg per milliequivalent) multiplied by 1000 to convert the titration's volume-times-normality result (in milliequivalents) into milligrams of oxygen per liter of sample. It appears as a fixed constant in every standard dichromate-titration COD calculation, not a value you look up or adjust.
Is this titration safe to run without special training?
No — the standard dichromate reflux method uses concentrated sulfuric acid and potassium dichromate containing hexavalent chromium, a recognized carcinogen classified as hazardous waste. It should only be performed by trained personnel working in a fume hood with appropriate personal protective equipment and proper waste disposal procedures in place, following your laboratory's established safety protocol, not attempted as a casual or unsupervised measurement.