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. The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by:
This expression does not include the oxygen demand caused by nitrification, the oxidation of ammonia into nitrate:
Dichromate, the oxidizing agent for COD determination, does not oxidize ammonia into nitrate, so nitrification is not included in the standard COD test.
The International Organization for Standardization describes a standard method for measuring chemical oxygen demand in ISO 6060 [http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=12260&ICS1=13&ICS2=60&ICS3=50].