COD from the blank and sample titrations
You measure COD by back-titration, not directly. Digest the sample with a known excess of potassium dichromate, then titrate the leftover dichromate with ferrous ammonium sulfate (FAS, Mohr's salt). Run an identical blank with distilled water in place of the sample. The difference in FAS volume between blank and sample is what the organics consumed.
What each term is
- A — mL of FAS used to titrate the blank (no sample, all dichromate intact).
- B — mL of FAS used to titrate the digested sample.
- N — normality of the FAS titrant, standardised against the dichromate before each run because FAS slowly oxidises in air.
- Vsample — volume of the water sample in mL, typically 10 to 20 mL.
- 8000 — 8 mg O per milli-equivalent × 1000 mL/L, the fixed conversion to mg/L.
Typical COD ranges
| Water | COD (mg/L) |
|---|---|
| Treated drinking water | < 5 |
| Clean river | 5–15 |
| Polluted river | 30–100 |
| Raw domestic sewage | 250–400 |
| Organic industrial effluent | 500–5000 |
| Treatment-plant outfall | < 100 |
The dichromate method (ISO 6060, EPA 410.3) oxidises roughly 90% of organic matter, far more than the older permanganate test. Add mercuric sulfate before digestion when the sample is salty: chloride is oxidised by dichromate too and inflates the reading by 20 to 50% if it is not masked.
Common questions
What is the formula for COD by titration?
COD in mg/L equals (A minus B) times N times 8000, divided by the sample volume in mL. A is the FAS titrant used on the blank, B is the FAS used on the sample, and N is the normality of the FAS solution.
Why is the factor 8000 in the equation?
It combines the equivalent weight of oxygen (8 grams per equivalent) with the 1000 that converts mL and litres. One equivalent of dichromate consumed corresponds to 8 mg of oxygen, and scaling to a full litre gives the 8000.
Why does the blank use more titrant than the sample?
The blank has no organic matter, so all of its dichromate survives the digestion and needs the most FAS to titrate. The sample consumes some dichromate oxidising its organics, so less is left and it needs less FAS. The gap between the two is what the sample used up.
