It is commonly expressed in mass of oxygen consumed over volume of solution, which in SI units is milligrams per liter (mg/L).
The most common application of COD is in quantifying the amount of oxidizable pollutants found in surface water (e.g. lakes and rivers) or wastewater.
COD is useful in terms of water quality by providing a metric to determine the effect an effluent will have on the receiving body, much like biochemical oxygen demand (BOD).
The amount of Cr3+ is determined after oxidization is complete and is used as an indirect measure of the organic contents of the water sample.
Note: Ferroin indicator is bright red from commercially prepared sources, but it exhibits a green hue when added to a digested sample containing potassium dichromate.
Its reaction with potassium dichromate follows the equation: To eliminate chloride interference, mercuric sulfate can be added to the sample prior to the addition of other reagents.
Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in wastewater before it can be returned to the environment.
For example, in Switzerland, a maximum oxygen demand between 200 and 1000 mg/L must be reached before wastewater or industrial water can be returned to the environment [2].
For many years, the strong oxidizing agent potassium permanganate (KMnO4) was used for measuring chemical oxygen demand.
Of these, potassium dichromate (K2Cr2O7) is the most effective: it is relatively cheap, easy to purify, and can nearly completely oxidize almost all organic compounds.
In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed.