Chemical oxygen demand
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 Chemical oxygen 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.
Most applications 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 waters or 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 strong chemical 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 with
boilingsulfuric 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 COD assay.
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 oxidize ammonia 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 measuring chemical oxygen demand in ISO 6060 [1].
Chemical oxygen demand is related to biochemical oxygen demand (BOD), another
standard test for assaying the oxygen-demanding strength of waste waters.
However, biochemical oxygen demand only measures the amount of oxygen consumed
by microbial oxidation and is most relevant to waters rich in organic matter.
It is important to understand that COD and BOD do not necessarily measure the
same types of oxygen consumption.
Read more:
https://science.jrank.org/pages/1388/Chemical-Oxygen-Demand.html#ixzz0kBGsxIhr
https://en.wikipedia.org/wiki/Chemical_oxygen_demand
