Chemical Oxygen Demand Calculator

Glucose sets the reference at 1.07 g of oxygen per gram; hydrocarbons demand far more.

Theoretical oxygen demand1.06568 g O2 per gcomplete oxidation to CO2 and water

Oxygen demand of the sample266.42 mg/Lat 250 mg/L of this substance

Oxygen needed per mole6 mol O2

Molar mass180.156 g/mol

A typical dichromate test would recover253.1 to 266.42 mg/L95 to 100 % for ordinary organics, less for aromatics

Glucose is the usual reference at 1.07 g of oxygen per gram. A substance well above that is more reduced — hydrocarbons demand more oxygen per gram than carbohydrates, because they start further from CO2.

The formula

ThOD = (C + H/4 - O/2) * 32 / molar mass

Oxygen demand as a measure of pollution

Chemical oxygen demand asks how much oxygen it would take to oxidise everything in a water sample completely. It is the standard proxy for organic load in an effluent, because it can be measured in a couple of hours where a biochemical oxygen demand test takes five days.

Theoretical against measured

What this page computes is the theoretical oxygen demand: the exact figure the stoichiometry requires. A laboratory COD test using dichromate usually recovers 95 to 100 % of it for ordinary organics, and much less for compounds the reagent struggles with — benzene and pyridine among them. The gap between the two numbers is information, not error.

Why it matters

Discharge organic matter into a river and bacteria consume it, taking dissolved oxygen with them. Enough of it and the water goes anoxic and the fish die. The oxygen demand of an effluent is therefore a direct measure of the damage it can do, which is why consents are written in those terms.

Empirical is not molecular

Every method on these pages returns a ratio, because that is all a composition can determine. Glucose, formaldehyde and acetic acid all reduce to CH₂O, and no amount of elemental analysis will separate them. A molar mass from mass spectrometry or a freezing-point depression is what turns the ratio into a formula, and it has to come from a second, independent measurement.