Chemical oxygen demand (COD) is defined as the amount of a specified oxidant that reacts with the sample under controlled conditions. The quantity of oxidant consumed is expressed in terms of its oxygen equivalence. Because of its unique chemical properties, the dichromate ion (Cr2O72-) is the specified oxidant in Methods 5220 B, C, and D; it is reduced to the chromic ion (Cr3+) in these tests. Both organic and inorganic components of a sample are subject to oxidation, but in most cases the organic component predominates and is of the greater interest. To measure either organic or inorganic COD alone, additional steps not described here must be taken to distinguish one from the other. COD is a defined test; the extent of sample oxidation can be affected by digestion time, reagent strength, and sample COD concentration.
COD often is used as a measurement of pollutants in wastewater and natural waters. Other related analytical parameters are biochemical oxygen demand (BOD), total organic carbon (TOC), and total oxygen demand (TOD). In many cases it is possible to correlate two or more of these values for a given sample. BOD is a measure of oxygen consumed by microorganisms under specific conditions, TOC is a measure of organic carbon in a sample, and TOD is a measure of the amount of oxygen consumed by all elements in a sample when complete (total) oxidation is achieved.
In a COD analysis, hazardous wastes of mercury, hexavalent chromium, sulfuric acid, silver, and acids are typically generated. Methods 5220 C and D reduce these waste problems, but may be less accurate and less representative.