There are many ways to use COD
analyzers to measure COD, such as rapid digestion, spectrophotometry, COD potassium dichromate standard method, potassium permanganate method, rapid digestion spectrophotometry, COD coulometric titration and COD The UV meter measurement method and so on. Each of these COD measurement methods has its own characteristics. The following are the advantages and disadvantages of these COD measurement methods. 1. The classic standard method of rapid digestion method is the reflux 2h method. In order to improve the analysis speed, various rapid analysis methods have been proposed. There are mainly two methods: one is to increase the concentration of oxidant in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, and increase the co-catalyst and other conditions to increase the reaction rate. Domestic methods are based on GB/T14420-1993 'Analytical Methods for Boiler Water and Cooling Water - Determination of Chemical Oxygen Demand Quick Method for Potassium Dichromate' and the unified method 'Coulometric Method' and 'Rapid Closed Catalytic Digestion Method' recommended by the State Environmental Protection Administration (including Photometry)' is the representative of this method. Foreign countries are represented by the German standard method DIN38049 T.43 'Quick Method for Determination of Chemical Oxygen Demand of Water'. Compared with the classic standard method, the above method increases the acidity of sulfuric acid in the digestion system from 9.0mg/l to 10.2mg/l, the reaction temperature increases from 150°C to 165°C, and the digestion time decreases from 2h to 10min~15min. The second is to change the traditional The method of heating and digesting by heat conduction radiation, and the method of using microwave digestion technology to increase the speed of digestion reaction. The ammonia nitrogen analyzer adopts the national standard: salicylic acid colorimetric method to complete the water quality ammonia nitrogen measurement, and adopts dichloroisotricyanide regulated by international standards Sodium hypochlorite replaces the commonly used sodium hypochlorite, which greatly enhances the stability and effectiveness of the chlorine in the reagent solution (Type B). Due to the wide variety of microwave ovens and different powers, it is difficult to test out a uniform power and time in order to achieve a good digestion effect. Microwave ovens The price is also very high, and it is difficult to formulate a unified standard method. 2. The spectrophotometric method is based on the classic standard method. Potassium dichromate oxidizes organic matter, and hexavalent chromium generates trivalent chromium. Through hexavalent chromium or trivalent chromium The relationship between the absorbance value of the water sample and the COD value of the water sample is used to determine the COD value of the water sample. Using the above principle, the main representative method abroad is the US Environmental Protection Agency EPA.Method 0410.4 'Automatic Manual Colorimetry', the American Society for Testing and Materials ASTM: D1252-2000 'Method for Determination of Chemical Oxygen Demand of Water B - Sealed Digestion Spectrophotometry' and International Standard ISO15705-2002 'Small Sealed Tube Method for Determination of Water Quality Chemical Oxygen Demand (COD)'. China is a national environmental protection The unified method of the General Administration 'rapid closed catalytic digestion method (including spectrophotometry)'. 3. COD potassium dichromate standard method COD can obtain the most accurate COD value by using potassium dichromate standard method, but COD dichromate There are many shortcomings in the potassium standard method. The operation of the COD potassium dichromate standard method is complicated, and it needs long-term heating and reflux, and the equipment used for heating and reflux is large in volume, which is difficult to realize batch determination. In addition, the COD potassium dichromate standard method needs to use Silver salt makes the cost higher. 4. Potassium permanganate method is the same as potassium dichromate method. Potassium permanganate is sent out: use potassium permanganate as oxidant to measure COD, and what is measured is called potassium permanganate Index. According to the different test standards, the ammonia nitrogen analyzer has types A and B. Among them, type A adopts Nessler's reagent colorimetric method GB7479-87; Acid photometry is the basic test method, supplemented by sample colorimetric reaction pretreatment, to achieve fast and accurate colorimetric determination of samples, with the advantages of stability, sensitivity, reliability and mercury-free. (Potassium permanganate method is suitable for COD content 5. Rapid digestion spectrophotometric chemical oxygen demand determination method, whether it is reflux capacity method, fast method or photometric method, uses potassium dichromate as oxidant, silver sulfate as catalyst, mercury sulfate It is a masking agent for chloride ions, and it is a method based on the determination of COD digestion system in acidic conditions of sulfuric acid. 6. Coulometric titration method of COD measuring instrument The coulometric titration method of COD is suitable for drinking water, surface water, industrial sewage, biological It is suitable for the measurement of various water quality such as living wastewater, and has good measurement accuracy, and its measurement results are consistent with the measurement results of potassium dichromate standard method. The coulometric titration method of COD has a relatively simple operation process and a fast determination speed, and is a commonly used COD measurement method. 7. UV meter measurement method of COD tester The UV meter measurement method of COD is also a measurement method that rises late. It is suitable for colorless, transparent, and stable water quality measurement. Cumulative) advantage of low cost. With the application of cold light source display technology, the service life of the light source lamp is as long as 100,000 hours, and the switching of the optical path of the measurement parameters is changed from manual to automatic switching, eliminating the error factor of human rotation. However, the measurement of COD by UV meter is limited by ethanol, organic acid and other non-ultraviolet absorbing substances, so it is difficult to be popularized on a large scale.
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