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Comparison of reflux titration method and rapid method for COD determination

by:BOQU     2022-01-03

Water quality detection COD detection standard: GB11914-89 'Water quality chemical oxygen demand determination dichromate method' HJ/T399-2007 'Water quality chemical oxygen demand determination rapid digestion spectrophotometric method' ISO6060 'Water Quality Chemistry Determination of oxygen demand' 1. Determination of chemical oxygen demand of water quality Dichromate method: Standard number: 'GB/T11914-89' Potassium dichromate method uses a strong acid solution to fully oxidize the water sample and reflux for 2 hours The pretreatment operation so that most of the organic matter in the water sample is completely oxidized. Features: It has the advantages of wide measurement range (5-700mg/L), good reproducibility, strong interference elimination, high accuracy and precision, but at the same time there are long digestion times and large secondary pollution. Samples are measured in large quantities The time efficiency is low and has certain limitations. Disadvantages:

1. It takes too much time, and it takes 2 hours to reflow for each sample;

2. The reflow equipment takes up a lot of space and it is difficult to measure in batches;

3. Higher analysis cost;

4. In the process of measurement, the waste of reflux water is amazing;

5. Toxic mercury salt is easy to cause secondary pollution;

6. The amount of reagents is large, and the cost of consumables is high;

7. The testing process is complicated and not suitable for promotion

Determination of Water Quality Chemical Oxygen Demand Rapid Digestion Spectrophotometric Method: Standard No.: HJ/T399-2007 COD rapid determination method is mainly used in emergency monitoring of pollution sources and used in mass determination of wastewater samples. This method is mainly prominent The advantage lies in the small number of sample reagents, energy-saving, time-saving, simple and fast, which makes up for the shortcomings of the classic analysis method. The principle is: in a strong acid medium, under the condition of the presence of a composite catalyst, the water sample is digested at a constant temperature at 165℃15 Minutes, the reducing substances in the water are oxidized by potassium dichromate, and the hexavalent chromium ions are reduced to trivalent chromium ions. The chemical oxygen demand in the water is proportional to the concentration of Cr3+ generated by the reduction. When the COD value in the sample is 100-1000mg /L, measure the absorbance of trivalent chromium produced by reduction of potassium dichromate at a wavelength of 600nm±20nm; when the COD value is 15-250mg/L, measure potassium dichromate at a wavelength of 440nm±20nm without reduction The total absorbance of the two chromium ions of hexavalent chromium and trivalent chromium produced by reduction. This method uses potassium hydrogen phthalate to draw a standard curve. According to Beer’s law, the absorbance of the solution is comparable to that of the water sample within a certain concentration range. The COD value has a linear relationship. According to the absorbance, it is converted into the chemical oxygen demand of the measured water sample according to the calibration curve. Features: This method has the advantages of simple operation, safety, stability, accuracy, and reliability; and it has the characteristics of fast analysis speed, suitable for large-scale determination; small space occupation, low energy consumption, small reagent consumption, waste liquid reduction to the smallest degree, and two reductions Secondary pollution, etc., are widely used in daily and emergency monitoring, make up for the shortcomings of the classic standard method, and can completely replace the old-fashioned electric furnace heating national standard reflow method.

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