In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. In other words Chemicaloxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals such as ammonia and nitrite.
Mostapplications of COD is to 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).
COD measurements are commonly made on samples of waste watersor of natural waters contaminated by domestic or industrial wastes. Chemical oxygen demand is measured as a standardized laboratory assay in which a closed water sample is incubated with a strongchemical oxidant under specific conditions of temperature and for a particular period of time. A commonly used oxidant in COD assays is potassium dichromate (K2Cr2O7) which is used in combination withboiling sulfuric acid (H2SO4). Because this chemical oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a CODassay.
The process of ammonia being converted into nitrate is referred to as nitrification. The following is the equation for the oxidation of ammonia into nitrate.
Dichromate does not oxidizeammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test.
The International Organization for Standardization describes a standard method for measuringchemical oxygen demand in ISO 6060 .
Chemical oxygen demand is related to biochemical oxygen demand (BOD), another standard test for assaying the oxygen-demanding strength of waste waters....