Chemical Oxygen Demand (COD) Calculator

COD (mg/L) = [(A - B) x N x 8000] / V, where A is the mL of FAS titrant used for the blank, B is the mL of FAS titrant used for the sample, N is the normality of the FAS titrant, V is the mL of sample used, and 8000 is the milliequivalent weight of oxygen (8 mg/meq) multiplied by 1000 to give a result in mg/L.

In the dichromate reflux method, a fixed excess of potassium dichromate is added to both the blank and the sample. The sample's organic matter consumes some of that dichromate during digestion, leaving less unreacted dichromate to titrate afterward, so the sample needs less FAS titrant. The blank has no organic matter to consume dichromate, so it always needs at least as much (and usually more) titrant than the sample.

8000 comes from the milliequivalent weight of oxygen, 8 mg per milliequivalent, multiplied by 1000 to convert from mg per mL of sample to mg per litre. Oxygen's equivalent weight is 8 because each O2 molecule (32 g/mol) accepts 4 electrons in the reduction half-reaction, giving 32/4 = 8 g per equivalent.

Raw (untreated) domestic sewage typically has a COD between roughly 250 and 1000 mg/L, though it varies by region and dilution. Well-treated effluent discharged to surface water is usually well under 50 to 100 mg/L, depending on local regulatory limits. Industrial wastewater can range far higher, sometimes into the thousands of mg/L.

COD (Chemical Oxygen Demand) measures the oxygen equivalent of all oxidizable material in a sample using a strong chemical oxidant (potassium dichromate), typically taking a few hours. BOD (Biochemical Oxygen Demand) measures only the oxygen consumed by living microorganisms breaking down biodegradable material, typically over 5 days. COD is almost always higher than BOD for the same sample because it also oxidizes non-biodegradable organics that BOD testing cannot capture.

The standard open-reflux dichromate method uses ferrous ammonium sulfate (FAS), also called Mohr's salt, as the titrant to measure unreacted potassium dichromate after digestion. A ferroin indicator signals the endpoint with a sharp color change from blue-green to reddish-brown as excess dichromate is fully reduced.

FAS solutions slowly oxidize in air, so their exact normality drifts over time even if the label says a round number like 0.1 N. Standardizing against a known-strength potassium dichromate solution before each titration session gives the true, current N value, without which the COD calculation would be systematically wrong.

COD should never be negative in a correctly run test; a negative result (B greater than A) signals a titration or dilution error, most often insufficient blank titrant or sample contamination. A COD of exactly zero is possible for a genuinely oxidant-free sample like pure distilled water, but is unusual for any real environmental sample.

The sample volume depends on the standard method being followed, commonly 20 mL for open reflux and as little as 2 to 3 mL for closed-vial micro methods. Whatever volume is used during digestion and titration must be entered as V; the formula automatically scales the milliequivalents to a per-litre result.

COD is a fast, reproducible proxy for the total organic pollution load of water, used to size treatment plants, monitor treatment efficiency (comparing influent and effluent COD), and check compliance with discharge permits. Because it takes hours rather than the 5 days BOD requires, COD gives operators same-day feedback to adjust a treatment process.

Yes. Chloride ions are oxidized by dichromate and can inflate the apparent COD if not corrected for, which is why standard methods add mercuric sulfate to complex chloride before digestion. High-chloride samples (like seawater or brackish water) need this correction; otherwise the reported COD overstates the true organic pollution load.