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The Mystery of Chemical Oxygen Demand: Why Measuring Water Quality is So Important?
In today's field of environmental chemistry, chemical oxygen demand (COD) is an important indicator that shows the consumption of oxygen in an aqueous solution. This indicator is commonly expressed in the form of milligrams per liter (mg/L) and can quickly quantify the content of organic matter in water. COD testing is most commonly used to quantify oxidizable contaminants in surface waters (such as lakes and rivers) or wastewater. In water quality testing, COD provides an indicator to help analyze the impact of sewage discharge on the receiving system, similar to biochemical oxygen demand (BOD).
Principle of COD test
The basis of the COD test is that almost all organic compounds can be completely oxidized to carbon dioxide under the action of strong oxidants. This reaction usually takes place in an acidic environment, which is why sulfuric acid is often added to the test. In an acidic environment, potassium chromate is used as the oxidizing agent for testing. The result of this reaction is:
CnHaObNc + (n + a/4 - b/2 - 3/4c) O2 → n CO2 + (a/2 - 3/2c) H2O + c NH3
The results of this chemical reaction show that by measuring the amount of oxygen required, the amount of organic matter in the water sample can be indirectly deduced.
Measurement process using potassium chromate
Potassium chromate is a compound that exhibits strong oxidizing properties in acidic environments. When conducting COD testing, sulfuric acid is often added to ensure that the reaction proceeds under acidic conditions. During the entire reaction process, potassium chromate will be reduced to form Cr3+, and the amount of Cr3+ generated is an indirect indicator for measuring organic matter in water samples.
CnHaObNc + d Cr2 O72- + (8d + c) H+ → n CO2 + (a + 8d - 3c)/2 H2O + c NH4+ + 2d Cr3+
Measurement process and calculation method
During the measurement process, an excess of potassium chromate must be maintained to ensure that all organic matter can be completely oxidized. After the oxidation reaction is completed, ferrous ammonium sulfate (FAS) is used for titration to determine the reduced Cr3+, so that the content of organic matter can be accurately calculated. The formula for calculating COD is as follows:
COD = 8000 × (b - s) / sample volume
Where b is the volume of FAS in the blank sample, s is the volume of FAS in the original sample, and the calculated COD value will be expressed in mg/L.
Inorganic interference substances and environmental regulations
In water samples, high concentrations of oxidizable inorganic substances may also interfere with COD measurements. For example, chloride is ubiquitous in wastewater and reacts with potassium chromate, potentially causing measurement errors. In order to reduce this effect, chemicals such as mercury sulfate can be added to the sample to eliminate the interference of chlorine. Many countries have also established strict specifications for the amount of COD allowed in wastewater to protect the environment. For example, Switzerland requires that the COD of wastewater must reach between 200 and 1000 mg/L before being discharged.
After these analyses, we can’t help but think: in the face of increasingly severe water resources problems, how to ensure the accuracy and reliability of water quality testing to protect our living environment?
