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The Magical Oxygen Consumption: How to Quickly Assess Organic Matter in Water?
In environmental chemistry, chemical oxygen demand (COD) is an indicator that reveals the amount of oxygen that can be consumed in a measured solution. This measurement, usually expressed in milligrams of oxygen consumed per liter of solution, is an important indicator for water quality assessment. Through COD testing, environmental scientists can quickly quantify the content of organic matter in water, which is indispensable when controlling water pollution or treating wastewater.
COD provides an indicator of the impact of wastewater discharges on water bodies, equivalent to the role of biochemical oxygen demand (BOD).
The principle of the COD test is that almost all organic compounds can be completely oxidized to carbon dioxide by strong oxidants in an acidic environment. The amount of oxygen required for this process determines the amount of organic matter in the water. In common environmental assessments, the most commonly used oxidant is potassium chromate (K2Cr2O7). Its oxidation effect is extremely effective in acidic environments and can oxidize almost all organic compounds.
How to conduct COD testing
When conducting COD testing, potassium chromate is usually mixed with sulfuric acid to create an acidic environment, thereby activating its strong oxidizing properties. In this reaction, potassium chromate oxidizes organic matter in the water sample and subsequently converts it into Cr3+. This Cr3+ content will be measured after the reaction is completed as an indirect indicator of the organic content of the water sample.
During the 650054% COD test, the excess of chromium needs to be measured after the oxidation reaction is completed to ensure that the Cr3+ content can be accurately calculated.
Once the oxidation reaction is complete, sulfur dioxide is used to titrate excess potassium chromate. A redox indicator is used in this process to indicate the end point of the reaction, and you will usually see a change in color from blue-green to reddish-brown, which is an indicator of the success of the test.
Interference from organic and inorganic substances
High concentrations of inorganic materials in water samples may interfere with the accuracy of COD testing, especially chloride ions. In some cases, its reaction with chromate can affect test results. Therefore, in order to eliminate the interference of chlorine, mercury sulfate is often added during the detection process.
The government has also formulated many regulations on environmental protection, setting limits on the maximum allowable value of COD in wastewater. For example, in Switzerland, the concentration of COD must be controlled between 200 and 1000 mg/L before treatment can be carried out. The resulting water body is then discharged into the natural environment.
History and current situation
Previously, many tests were conducted using potassium permanganate, but due to differences in its ability to oxidize different organic compounds, the accuracy of the COD test was not high. With the continuous advancement of technology, many laboratories now choose potassium chromate for measurement because it is cost-effective and can almost completely oxidize most organic matter.
Future Outlook
As the main water quality testing method, COD still has a lot of room for improvement. For example, how to improve the efficiency of testing and ensure that there is no interference from inorganic substances in complex water samples is one of the challenges for scientists in the future. Advances in water quality monitoring technology will not only improve ecosystem protection, but may also revolutionize the way we manage water resources. In this scientific exploration, how can we more effectively achieve a balance between environmental protection and development?
