Q.What is Chemical Oxygen Demand (COD)?

COD measures water pollution by quantifying how much oxygen a strong chemical oxidant needs to completely oxidise the organic/inorganic matter in a water sample.

Water bodies contain dissolved organic matter (sewage, industrial effluents, decaying plant/animal matter) and some oxidisable inorganic substances. To assess the extent of pollution, a known volume of the water sample is refluxed with a measured excess of a strong oxidising agent (commonly potassium dichromate, K₂Cr₂O₇, in the presence of sulphuric acid and a silver catalyst) for a fixed time.

The oxidant oxidises essentially all the oxidisable matter present (unlike Biochemical Oxygen Demand, BOD, which only measures what microorganisms can biodegrade in 5 days). The amount of oxidant consumed is then back-titrated and converted to an equivalent amount of oxygen.

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COD measures water pollution by the amount of oxygen equivalent needed to chemically oxidise ALL oxidisable matter in a water sample, biodegradable or not.

Chemical Oxygen Demand (COD) is a measure of water quality/pollution. It is defined as the amount of oxygen (usually expressed in mg O2 per litre) required to oxidise, by a strong chemical oxidising agent (commonly potassium dichromate, K2Cr2O7, in acidic medium), all the organic and inorganic substances present in a water sample.

Key points:

• A measured excess of oxidant (K2Cr2O7) is added to the water sample and refluxed under acidic conditions; the unreacted oxidant is then back-titrated (e.g. with ferrous ammonium sulphate) to find how much oxidant was actually consumed, which is converted to an equivalent mass of O2.
• Unlike Biochemical Oxygen Demand (BOD), which only measures oxygen consumed by micro-organisms breaking down biodegradable matter over 5 days, COD oxidises both biodegradable AND non-biodegradable matter, and the test is completed in a few hours rather than days. …

COD measures water pollution as the mass of oxygen (mg/L) needed to chemically oxidise all oxidisable matter in a sample using a strong oxidant like K2Cr2O7.

Chemical Oxygen Demand (COD) is defined as the amount of oxygen (expressed in milligrams per litre, mg/L, or parts per million, ppm) required to oxidise the organic and inorganic matter present in a given sample of water, when the matter is oxidised chemically using a strong oxidising agent (typically acidified potassium dichromate, K2Cr2O7, refluxed with the sample).

Unlike Biochemical Oxygen Demand (BOD), which measures only oxygen consumed by microorganisms decomposing biodegradable matter (and takes about 5 days), COD is measured quickly (a few hours) and oxidises essentially all organic matter, biodegradable or not — so COD is always numerically greater than or equal to BOD for the same sample.

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