COD

In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers), making COD a useful measure of water quality. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older references may express the units as parts per million (ppm).

"The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions".

All organic matter to be completely oxidized, an excess amount of potassium dichromate (or any oxidizing agent) must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr3+ can be determined, by titration with ferrous ammonium sulphate (FAS) until the entire excess oxidizing agent has been reduced to Cr3+.

Typically, the oxidation-reduction indicator Ferroin is added during this titration step as well. Once all the excess dichromate has been reduced, the Ferroin indicator changes from blue-green to reddish-brown.

The amount of ferrous ammonium sulphate added is equivalent to the amount of excess potassium dichromate added to the original sample.

Interference:

Some samples of water contain high levels of oxidisable inorganic materials which may interfere with the determination of COD. Because of its high concentration in most wastewater, chloride is often the most serious source of interference. Its reaction with potassium dichromate follows the equation:

6Cl + Cr2O7 + 14 H ---> 3 Cl2 + 2 Cr + 7H2O

Prior to the addition of other reagents, mercuric sulphate can be added to the sample to eliminate chloride interference.