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[Chemical Knowledge]：How to Measure COD with Potassium Dichromate -- Understand the Principle and Master the Skills

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COD (chemical oxygen demand) is a common indicator of pollutant concentration, which is widely used in water quality monitoring, water treatment, environmental monitoring and other fields. Potassium dichromate method is a commonly used method for COD measurement. Its basic principle is to oxidize the organic matter in the water sample into carbon dioxide and water, and the oxidant potassium dichromate is also reduced to trivalent chromium ions. The value of COD can be calculated by measuring chromium ions after reduction.

specific operation process is as follows:

1. prepare potassium dichromate solution and sulfuric acid solution. Potassium dichromate solution is a kind of toxic oxidant, need to pay attention to safety. The concentration of sulfuric acid solution should be kept stable to avoid fluctuations affecting the measurement results.

2. take a certain amount of water into the COD digester, add the right amount of potassium dichromate solution and sulfuric acid solution, digestion reaction. The digestion process is heated at the same time, and common tools such as heating plates, beakers and alcohol lamps are available.

3. After digestion is complete, place the sample in a sink to cool to room temperature. The potassium dichromate is then reduced to trivalent chromium ions by the addition of 37% hydrochloric acid.

4. Ammonium thiocyanate solution was used as an indicator to react chromium ions with ammonium thiocyanate to form a red complex. The concentration of the complex is proportional to the COD content in the water sample after digestion.

5. with a photometer or colorimeter to determine the absorbance of red complexes, and then calculate the value of COD. The accuracy and reliability of the

COD measurement method depend on the operation and control of each link. The main notes are as follows:

1. The selection and sampling of water samples should ensure representativeness and follow the relevant specifications and methods. The preservation and treatment of water samples should also avoid pollution and loss of COD.

2. The quality and stability of reagents should be guaranteed. Especially the purity and concentration of potassium dichromate will directly affect the accuracy of COD measurement.

3. When COD digestion, heating should be uniform, and the substances in the digestion liquid should be fully dissolved. Avoid COD drop or high due to improper operation or instrument failure.

4. The concentration of hydrochloric acid and ammonium thiocyanate solution should be controlled within a certain range, too strong or too weak will affect the accuracy of COD measurement results.

understand the principle and process of potassium dichromate COD measurement, master the operation skills and precautions, can improve the accuracy and reliability of COD measurement. At the same time, COD measurement is also an important work of water quality monitoring and environmental protection monitoring, which is of great significance to the protection of water environment and people's health.