Chemical oxygen demand (COD)
The chemical oxygen demand test is a test that indirectly measures the total amount of organic matter in water. COD is usually measured by a spectrophotometer.
In fact, this parameter is defined as the amount of oxidant that reacts with the sample under controlled conditions and determines the amount of oxygen required for the chemical oxidation of organic matter using a strong chemical oxidant such as potassium dichromate. This test is widely used to determine the following:
What is the application of chemical oxygen demand?
This test is widely used to determine:
- The degree of pollution in waters and their self-purification capacity
- The efficiency of treatment plants
- The pollution load
- It gives an approximate idea of the biochemical oxygen demand (BOD) which can be used to determine the sample size for estimating BOD.
- A measure of the quality of water and wastewater
Many organic substances are destroyed when treated with a mixture of potassium dichromate and sulfuric acid, producing carbon dioxide and water. The sample is returned with a specific amount of potassium dichromate in sulfuric acid medium and the excess amount of dichromate is titrated against ferrous ammonium sulfate. The amount of dichromate consumed is proportional to the oxygen required to oxidize the oxidizable organic substances.
Selection of COD determination methods
There are two methods for COD determination: open reflux and closed reflux.
Principles of open reflux:
- Suitable for a wide range of wastes with large sample volumes.
- Due to the higher oxidation capability, the dichromate reflux method is preferred over other methods using other oxidants (for example, potassium permanganate).
- Oxidation of most organic compounds is up to 100 - 95% of the theoretical value.
Principles of closed reflux:
- This method is carried out with culture tubes with measured reagents.
- Measurement of sample volume and reagent volume is very important.
- This method is economical in using metal salt reagents and produces a small amount of hazardous waste.
- Volatile organic compounds (VOCs) are completely oxidized in a closed system due to the longer contact time with the oxidants.
Chemical reactions:
CnHaObNc+ d Cr2O72- + (8d+c) H+ => nCO2 +[(a+8d-3c)/2]H2O+c NH4+ +2dCr3+
d= (2n/3) + (a/6)-(b/3)-(c/2)
During the experiment, the excess concentration of dichromate is determined by titrating it with ferric ammonium sulfate (FAS).
The reaction is given by:
6Fe2+ + Cr2O72- + 14 H+ => 6Fe3++ 2Cr3+ +7H2O
d= (2n/3) + (a/6)-(b/3)-(c/2)
What is the difference between BOD and COD?
COD is the amount of oxygen required for the chemical decomposition of pollutants while BOD is the amount of oxygen required to do this biologically through microorganisms. Specifically, it measures the equivalent amount of oxygen required to chemically oxidize organic compounds in water, thus eliminating pollution.
The laboratories of Abrizan Industrial Research Company, located in Fars Science and Technology Park, are capable of measuring a variety of water parameters, including COD testing, with an experienced staff and the use of advanced devices and equipment.
