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There are many ways to use a COD analyzer to do COD measurement tests, such as rapid digestion method, spectrophotometry, COD potassium dichromate standard method, potassium permanganate method, rapid digestion spectrophotometry, COD coulometric titration Method and COD UV meter measurement method, and so on. These COD measurement methods each have their own characteristics, the following are the advantages and disadvantages of these COD measurement methods.
1. Quick Digestion Method
The classic standard method is the 2h reflux method. In order to improve the analysis speed, various fast analysis methods have been proposed. 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, increase the co-catalyst, and other conditions to increase the reaction speed. The domestic method is based on GB/T14420-1993 'Boiler Water and Cooling Water Analysis Method-Determination of Chemical Oxygen Demand Rapid Method for Potassium Dichromate' and the unified method recommended by the State Environmental Protection Administration 'Coulomb Method' and 'Rapid Sealed Catalytic Digestion Method (including Photometric method)' is the representative of this method. Foreign countries are represented by the German standard method DIN38049T.43 'Quick Method for the Determination of Chemical Oxygen Demand of Water'.
Compared with the classic standard method, the acidity of the digestion system is increased from 9.0mg/l to 10.2mg/l, the reaction temperature is increased from 150℃ to 165℃, and the digestion time is reduced from 2h to 10min～15min.
The second is to change the traditional method of heating digestion by heat conduction radiation and use 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 the sodium dichloro isocyanate regulated by the international standard to replace the commonly used sodium hypochlorite, which greatly enhances the chlorine stability and effectiveness of the reagent solution (B Type) Due to the wide variety of microwave ovens and different powers, it is difficult to test a uniform power and time in order to achieve a good digestion effect. The price of microwave ovens is also very high, making it difficult to formulate a unified standard method.
Based on the classic standard method, potassium dichromate oxidizes organic matter and hexavalent chromium generates trivalent chromium. The water sample is determined by the relationship established between the absorbance value of hexavalent chromium or trivalent chromium and the COD value of the water sample COD value. Using the above principles, the main representative method of foreign countries is the US Environmental Protection Agency EPA.Method0410.4 'Automatic manual colorimetry'. American Society for Testing and Materials ASTM: D1252-2000 'Water COD Determination Method B-Seal Digestion Spectrophotometric Method' and the international standard ISO15705-2002 'Water Quality Determination of Chemical Oxygen Demand (COD) Small Sealed Tube Method'. Our country is a unified method of the State Environmental Protection Administration 'Quick Airtight Catalytic Digestion Method (Including Spectrophotometry)'.
3. COD potassium dichromate standard method
COD is measured by the potassium dichromate standard method to obtain the most accurate COD value, but the COD potassium dichromate standard method has many shortcomings. The COD potassium dichromate standard method is complicated in operation, requires a long time of heating and refluxing, and the equipment used for heating and refluxing is bulky, which makes it difficult to realize batch measurement. In addition, the COD potassium dichromate standard method requires the use of silver salt, which makes the cost higher.
4. Potassium Permanganate Method
Same as the potassium dichromate method, the potassium permanganate method is: uses potassium permanganate as an oxidant to measure COD, and the measured value is called the potassium permanganate index. Ammonia nitrogen analyzer has A.B type according to different test standards. Type A adopts Nessler's reagent colorimetric method GB7479-87; Type B adopts the national standard GB7148-81 and the international ISO7150/1-1948 standard salicylic acid photometric method as the basic test method, supplemented by sample colorimetric reaction pretreatment, To achieve rapid and accurate colorimetric determination of samples, it has the advantages of stability, sensitivity, reliability, and no mercury. (The potassium permanganate method is suitable for detection under the condition of low COD content.)
5. Fast Digestion Spectrophotometry
The chemical oxygen demand determination method, whether it is the reflux volumetric method, the rapid method, or the photometric method, uses potassium dichromate as the oxidant, silver sulfate as the catalyst, and mercury sulfate as the masking agent for chloride ions. It is measured under the acidic conditions of sulfuric acid. COD digestion system-based measurement method.
6. COD analyzer coulometric titration
COD coulometric titration method is suitable for the measurement of drinking water, surface water, industrial sewage, domestic wastewater, and other water quality, and has good measurement accuracy, and its measurement results are consistent with those of potassium dichromate standard method. . The operation process of COD coulometric titration is relatively simple. The determination speed is fast, and it is a commonly used COD measurement method.
7. COD measuring instrument UV meter measuring method
COD's UV meter measurement method is also a relatively late measurement method. It is suitable for colorless and transparent water quality measurement with a stable composition, accurate measurement. Investment (meaning: the accumulation of future profits) has the advantage of low cost. The cold light source display technology is applied, the life of the light source lamp is as long as 100,000 hours, and the switching of the measurement parameter optical path is changed from manual to automatic switching, which eliminates the error factor of artificial rotation. However, the use of UV meters for COD measurement is restricted by ethanol, organic acids, and other non-ultraviolet absorbing substances, so it is difficult to be widely promoted.
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