Reducing substances in water are all kinds of organic matter, nitrite, sulfide, ferrous salt and so on. But the main thing is organic matter. Therefore, chemical oxygen demand (COD) is often used as a measure of the amount of organic matter in water. The greater chemical oxygen demand, indicating that the water pollution by organic matter more serious. Determination of chemical oxygen demand (COD), with the determination of reducing substances in water samples and determination of different methods, the measured values are also different. At present, the most commonly used is the acid potassium permanganate oxidation and potassium dichromate oxidation. Potassium permanganate (K2MnO4) method, the oxidation rate is lower, but more convenient, in determining the relative value of the organic matter in water samples can be used. Potassium dichromate (K2Cr2O7) method, high oxidation rate, good reproducibility, suitable for determination of the total amount of organic matter in water samples. Organic matter is very harmful to industrial water systems. Water containing a large amount of organic matter can contaminate the ion exchange resin when passing through the desalination system, and can contaminate the anion exchange resin particularly easily, thereby deteriorating the resin exchange ability. Organic matter in the pretreatment (coagulation, clarification and filtration), about 50% reduction, but in the desalination system can not be removed, it is often brought to the boiler by the make-up water, so that the boiler water pH lower. Sometimes organic matter may also be introduced into steam and condensate to lower the pH and cause system corrosion. High levels of organic matter in circulating water systems promote microbial growth. Therefore, regardless of salt, water or circulating water system, COD is as low as possible, but there is no uniform limit. In the circulating cooling water system COD (DmnO4 method)> 5mg / L, the water quality has started to deteriorate.
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