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Learn about TOC, COD two indicators and detection methods

by:BOQU     2023-03-09
The full name of TOC is 'total organic carbon', which is mainly used as an index to evaluate the organic pollution of water quality. It reflects the total amount of organic carbon species in water. TOC includes natural organic matter (NOMs) and synthetic organic matter in raw water. They can come from piped water networks, agricultural by-products and industrial wastewater. During standard drinking water treatment (coagulation, sedimentation, filtration, disinfection), a portion of organic carbon becomes disinfection by-products (DBPs) and is carcinogenic. Moreover, these organic carbon substances directly or indirectly affect human health to varying degrees. TOC can fully reflect the degree of organic pollution in drinking water. The higher the TOC value of water, the higher the content of organic pollutants in water. Therefore, TOC is also called PM2.5 in water, which can better reflect the comprehensive level of water quality. Supplement: The development of TОC monitoring in domestic drinking water. At present, foreign countries, especially developed countries in Europe and America, have included TOC in the routine monitoring items of drinking water. Law' (GB13193-91), and in 2006, TOC was included in the 'Drinking Water Hygienic Standards' (GB/T5749-2006) testing items, but so far TOC testing has only been carried out in some large and medium-sized developed cities. Compared with the country, both the scope of monitoring and the frequency of monitoring are far behind. There are many methods for measuring TOC, including more than ten methods such as combustion oxidation-non-dispersive infrared absorption method, wet oxidation-non-dispersive infrared absorption method, and ultraviolet method. Combustion oxidation-non-dispersive infrared absorption method is divided into subtraction method and direct method. Under the action of catalyst and oxygen, the water sample in the tube converts the organic compound into carbon dioxide; the water sample in the low temperature reaction tube is acidified to decompose the inorganic carbonate into carbon dioxide. Carbon dioxide generated from inorganic carbonates and organic compounds enters a non-dispersive infrared detector sequentially. Since infrared rays of a certain wavelength are selectively absorbed by carbon dioxide, and within a certain concentration range, the intensity of carbon dioxide’s absorption of infrared rays is proportional to the concentration of carbon dioxide, so the values ​​of total carbon TC and inorganic carbon IC in water samples can be measured, and then used The difference between carbon and inorganic carbon is the value of total organic carbon (TOC). The test principle of the direct method is to add acid to the water sample, acidify it to a pH value of less than 2, and aerate it with nitrogen, so that the inorganic carbonate is converted into carbon dioxide and removed. Then the water sample is injected into the high-temperature combustion tube, and the total organic carbon can be directly measured. The principle of the wet oxidation-non-dispersive infrared absorption method is to treat the test water sample with phosphoric acid before oxidation to remove inorganic carbon, and then measure the concentration of TOC. The principle of the ultraviolet method is that the absorbance of some organic matter in water under ultraviolet light with a wavelength of 254nm is linearly related to the amount of organic carbon in the water, so the concentration of total organic carbon TOC can be measured through the absorbance of the ultraviolet spectrum. With the advantages of contact measurement and good repeatability, this measurement method has developed rapidly in recent decades. The value of TOC can reflect the value of bacteria, viruses, antibacterial drug residues, chemical pesticide residues and other organic substances in the water. The tap water standard in our country requires TOC to be less than or equal to 5mg/L. The full name of COD is 'Chemical Oxygen Demand', which refers to the amount of oxidant consumed when the reducing substances in water are oxidized and decomposed under the action of an external strong oxidant. The amount of reducing substances in the system generally reflects the indicators of organic matter, nitrite and other substances. COD measurement methods mainly include: dichromate method, potassium permanganate method, spectrophotometric method, rapid digestion method, rapid digestion spectrophotometric method, etc. The principle of the dichromate method is to use potassium dichromate as an oxidant, silver sulfate as a catalyst, and mercury sulfate as a masking agent for chloride ions in an acidic medium of sulfuric acid. Potassium chromate, calculate the COD value of the water sample according to the consumption of ferrous ammonium sulfate solution. Because the oxidant used in this measurement method is potassium dichromate, it is called the dichromate method. However, the dichromate method occupies a large experimental space and consumes a large amount of chemical reagents, making it difficult to quickly test large quantities. The potassium permanganate method uses potassium permanganate as an oxidant to measure the COD value, that is, the amount of oxidant consumed when it is oxidized and decomposed. The spectrophotometric method is based on the dichromate method, and the COD value is measured by the absorbance value of hexavalent chromium or trivalent chromium of the oxide. The rapid digestion method refers to the method of increasing the concentration of oxidant in the digestion reaction system on the basis of the dichromate method, or increasing the acidity of sulfuric acid, increasing the reaction temperature, increasing the co-catalyst and other conditions to increase the reaction speed. Rapid digestion spectrophotometry combines the advantages of the above-mentioned various methods. It refers to using a sealed tube as a digestion tube, taking a small amount of water samples and reagents in the sealed tube, putting them into a small constant temperature heating dish, heating and digesting at a constant temperature, and then using COD values ​​were measured by spectrophotometry. When we test the COD value, the higher the COD value, the more serious the organic pollution of the water sample. The source of these organic pollution may be organic substances such as pesticides, environmental hormones, chemical plants, organic fertilizers, etc. Our national tap water standards It is required that the COD measured by the potassium permanganate method be less than or equal to 3mg/L, and not exceed 5mg/L under special circumstances.
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