The Meaning and Testing Method of the Chemical Oxygen Demand (COD) Indicator in Water

Chemical Oxygen Demand (COD) is one of the important indicators in water quality testing. It refers to the amount of oxygen consumed by oxidized organic matter and reducing inorganic matter when water samples are treated with strong oxidizing agents under certain conditions. It is an important indicator for measuring the degree of pollution of organic matter and reducing inorganic matter in water bodies, usually expressed in mg/L of oxygen. There are various methods for determining COD, but they are mainly divided into two categories: chemical titration and redox titration.

1. The meaning of COD

COD is one of the important indicators for expressing the degree of pollution of organic matter and reducible inorganic matter in water bodies. The higher the COD value, the higher the content of organic matter and reducible inorganic matter in water bodies, and the more serious the degree of water pollution. The determination methods of COD can be divided into chemical titration and redox titration, among which redox titration is the most widely used method. This method utilizes strong oxidants (such as potassium dichromate) to undergo redox reactions with organic matter and reducible inorganic matter in water samples, and the amount of potassium dichromate consumed is the COD value.

2. COD detection methods

Chemical titration method

Chemical titration method is a method for determining the COD value through chemical reactions. This method uses potassium dichromate as an oxidant to oxidize organic matter and reducible inorganic matter in water samples, and the Cr3+ formed reacts with ferrous sulfate ammonium to form Fe2O3 precipitate. The COD value is calculated by measuring the mass of the precipitate. This method is simple to operate, but requires a large amount of potassium dichromate and has certain requirements for the experimental environment.

Redox titration method

Redox titration method utilizes strong oxidants (such as potassium dichromate) to undergo redox reactions with organic matter and reducible inorganic matter in water samples, and the amount of potassium dichromate consumed is the COD value. This method is simple to operate, does not require a large amount of potassium dichromate, and has low requirements for the experimental environment. However, the determination results of redox titration methods may be affected by certain inorganic reducing substances (such as nitrites, ferrous salts, etc.) in the water samples, so pre-treatment of the water samples is required before determination to remove these interfering substances.

On-site rapid detection method

With the development of science and technology, on-site rapid detection methods are also gradually applied to the determination of COD. This method uses portable COD measurement instruments to determine the COD value through the reaction of water samples with oxidants, and has the characteristics of rapidity, simplicity, and accuracy. However, the determination results of on-site rapid detection methods may be affected by environmental factors, so strict control of experimental conditions should be ensured when used to ensure the accuracy of the determination results.

3. Applications of COD

The COD index is widely used in the assessment of water pollution degree in industrial wastewater, domestic wastewater, drinking water sources, and other water bodies. By measuring the COD value in water samples, the degree of pollution of organic matter and reducible inorganic matter in water bodies can be understood, providing a scientific basis for water body pollution control. In addition, the COD index is also used to evaluate the self-purification capacity of water bodies and predict the biological toxicity of water bodies.

In summary, the COD index is one of the important indicators for measuring the degree of organic matter and reducible inorganic matter pollution in water bodies, and its determination methods mainly include chemical titration and redox titration. In practical applications, appropriate determination methods should be selected according to the characteristics of the water samples and experimental conditions to ensure the accuracy of the determination results. At the same time, with the development of science and technology, on-site rapid detection methods are also gradually applied to the determination of COD, providing a more convenient means for water body pollution control.