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The key to wastewater treatment: Do you know how to test chemical oxygen demand?
In environmental chemistry, chemical oxygen demand (COD) is an indicative value used to indicate the amount of oxygen that can be consumed in a measurement solution. It is usually expressed as the mass of oxygen consumed and the volume of the solution, in milligrams per liter (mg/L). The COD test can quickly quantify the amount of organic matter in water. The most common application is in determining the mass of oxidizable pollutants in surface water or wastewater. COD is very useful for assessing water quality, providing an indication of the impact that discharges will have on receiving waters, similar to biochemical oxygen demand (BOD).
The testing principle of COD is that almost all organic compounds can be completely oxidized into carbon dioxide by strong oxidants under acidic conditions.
The basic principle of COD testing is that almost all organic compounds can be completely oxidized into carbon dioxide, ammonia and water by strong oxidants under acidic conditions. The amount of oxygen required for this oxidation reaction can be found in the chemical formula. It is important to note that standard COD tests do not include the oxygen demand due to the oxidation of ammonia to nitrate (the canonical reaction).
The most commonly used oxidizing agent is potassium dichromate. It is a strong oxidizing agent in acidic environments, and acidity is usually achieved by adding sulfuric acid. The reaction equation using potassium dichromate shows its reaction process with organic compounds, where the organic matter is oxidized and the potassium dichromate is reduced to trivalent chromium (Cr3+).
The organic matter content in water samples is indirectly assessed by measuring the residual Cr3+.
To ensure that all organic matter is completely oxidized, an excess of potassium dichromate must be added. After the oxidation process is complete, the excess potassium dichromate needs to be measured to ensure the correct amount of Cr3+ is obtained. This can be achieved by titrating with ferrous ammonium sulfate (FAS) until the excess oxidant is completely reduced. Ferroin, a redox indicator, is often added during the titration process, and the color change can help determine the endpoint of the titration.
Methods for measuring excess
When measuring excess, excess potassium dichromate reacts with FAS. During this process, the ferroin indicator changes from blue-green to red-brown, indicating that the reaction has reached the endpoint.
Preparation of ferroin indicator
The preparation of ferroin indicator requires adding 1.485 g of 1,10-phenanthroline monohydrate to 695 mg of ferrous sulfate heptahydrate and diluting to 100 ml with distilled water.
COD calculation
The formula used to calculate COD is:
COD = 8000 × (b - s) / sample volume
Where b represents the volume of FAS in the blank sample, s represents the volume of FAS in the original sample, and n is the concentration of FAS.
Inorganic interference
High concentrations of oxidizable inorganic substances in some water samples may interfere with the determination of COD. Chloride is a common source of interference. To eliminate chloride interference, mercury sulfate can be added before adding other reagents.
Government Regulations
Many governments set strict standards for the maximum chemical oxygen demand allowed in wastewater. For example, in Switzerland, wastewater or industrial water must meet a maximum oxygen demand of between 200 and 1000 mg/L before it can be returned to the environment.
Historical BackgroundFor many years, the strong oxidant potassium permanganate has been used to measure chemical oxygen demand, but its ability to oxidize organic compounds varies greatly, so in many cases the measured BOD is often greater than the COD. Measurement results. As the research deepened, it was found that different oxidants such as potassium dichromate were more effective and economical than potassium permanganate, and therefore were widely used in the determination of COD.
Faced with the increasingly serious water pollution, how can chemical oxygen demand testing tools help us improve water quality and safeguard the future of the ecological environment?
