What is COD and how to measure the COD in water?

COD is the abbreviation of chemical oxygen demand, which is the amount of oxidant consumed when a certain strong oxidant is used to treat water samples under certain conditions. It is an indicator of the amount of reducing substances in water. Reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts, etc., but the main ones are organic substances. Therefore, chemical oxygen demand is often used as an indicator to measure the content of organic matter in water. Reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts, etc., but the main ones are organic substances. Therefore, chemical oxygen demand is often used as an indicator to measure the content of organic matter in water. The greater the chemical oxygen demand, the more serious the water body is polluted by organic matter. For the measurement of chemical oxygen demand, the measured values vary depending on the reducing substances in the water sample and the measurement methods.

Method 1: Dichromate method
Represented by Chinese standard GB11914 "Dichromate method for determination of chemical oxygen demand in water quality" and international standard ISO6060 "Determination of chemical oxygen demand in water quality", this method has high oxidation rate, good reproducibility, accuracy and reliability, and has become a common practice in the international community. Recognized classical standard method. However, the reflux device occupies a large experimental space, consumes large amounts of water and electricity, uses a large amount of reagents, and the operation remains unchanged, making it difficult to conduct rapid measurements in large quantities.
The determination principle of the dichromate method is as follows: in an acidic medium of sulfuric acid, potassium dichromate is used as the oxidant, silver sulfate is used as the catalyst, mercury sulfate is used as the masking agent for chloride ions, the acidity of the sulfuric acid in the digestion reaction solution is 9mol/L, and the digestion is achieved by heating The reaction liquid boils, and the boiling point temperature of 148℃±2℃ is the digestion temperature. Cool and reflux the reaction with water for 2 hours. After the digestion solution is naturally cooled, use ferrous ferrous sulfate as an indicator and titrate the remaining potassium dichromate with ammonium ferrous sulfate solution. Calculate the COD value of the water sample based on the consumption of ammonium ferrous sulfate solution.

Method 2: Potassium permanganate oxidation method
After adding sulfuric acid to the water sample to make it acidic, add a certain amount of potassium permanganate solution and heat the reaction in a boiling water bath for 30 minutes. The remaining potassium permanganate is reduced by adding excess sodium oxalate solution, and then the excess sodium oxalate is dripped back with potassium permanganate solution, and the permanganate index is calculated through calculation.
The oxidation rate is low, but it is relatively simple and can be used when measuring the relative comparative value of organic matter content in water samples. However, the disadvantage is that excessive sodium oxalate needs to be dripped back during the experiment, which takes a long time, and the acidic potassium permanganate has low oxidizing properties and incomplete oxidation, so the measured permanganate index is lower than the dichromate index, which is usually the same as the national standard method. The measurement results vary by 3-8 times. Therefore, CODMn mainly targets river water and surface water with relatively low pollutants, while CODcr mainly targets wastewater with relatively high content of reducing pollutants.

Method 3: Potassium dichromate method
Based on the classic standard method, potassium dichromate oxidizes organic matter, and hexavalent chromium generates trivalent chromium. The COD value of the water sample is measured through the relationship established between the absorbance value of hexavalent chromium or trivalent chromium and the COD value of the water sample.

Method 4: Rapid digestion method
The classic standard method is the reflux 2h method. In order to improve the analysis speed, people have developed various rapid analysis methods. There are two main methods: one is to increase the oxidant concentration in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, add cocatalysts and other conditions to increase the reaction speed.

Method 5: Spectrophotometry
In an acidic solution, the reducing substances in the test solution react with potassium dichromate to generate trivalent chromium ions. Trivalent chromium ions have a great ability to absorb light with a wavelength of 600 nm. Its absorbance is directly related to the concentration of trivalent chromium ions. The relationship obeys the Lambert-Beer law. Trivalent chromium ions are related to the amount of reducing substances in the test solution, so the COD value of the test solution can be briefly measured by measuring the absorbance of trivalent chromium. Compared with the traditional national standard method, this method effectively saves the time spent in preparing chemical reagents, does not require titration, and is easy to operate. However, the only drawback is that the digestion process in the experiment still takes 2 hours.

The above methods are basically laboratory operation methods. In industrial online monitoring, we usually use COD sensors and the measurement principle is UV spectrophotometry. The COD351 online COD Sensor developed and produced by Daruifuno adopts this principle.

The amount of organic matter in water represents the amount of COD in water. Although various organic matter has its own absorbance and the absorbance of light of each wavelength is different, we found that most organic matter in the ultraviolet spectrum region has a wavelength of There is strong absorption at 254nm, so it is very accurate to measure sewage COD in the ultraviolet light region of 254nm wavelength. However, most of the actual sewage samples we usually make are turbid and contain a large amount of suspended matter and some colloids. These will have a certain impact on the COD content during the measurement process and will affect the measurement results. It is too large, so we must correct the COD amount measured at this time. Therefore, the COD sensor can also measure the turbidity value. In the visible light region with a wavelength of 546nm, the turbidity of the water is measured, so that the difference between the two measurements is the COD content in the water.

The above is an introduction to the COD measurement method. Delfino focuses on the R&D and production of water quality analysis instruments. Our measurement parameters include pH, ORP, conductivity, TDS, salinity, dissolved oxygen, COD, ammonia nitrogen, turbidity, chlorophyll, blue-green algae, residual chlorine, and chlorine dioxide. wait. Our main products include pH ORP analyzer, conductivity analyzer, turbidity analyzer, COD analyzer, dissolved oxygen analyzer, ammonium analyzer, chlorine analyzer, etc.