Talking about the comparison of COD determination methods in chemical wastewater with high concentration chloride ion

In dye chemical industry, organic chlorination industry and hydrochloric acid industry, due to the large amount of chlorine-containing compounds used, the discharged wastewater contains high chloride, which not only affects the biochemical treatment of wastewater, but also directly affects the accuracy of COD determination. At present, the determination of COD generally adopts the national standard (GB11914-1989) potassium dichromate. There are large errors in the determination of chlorine ion-containing wastewater by this method. The main reason is that chloride ions or chlorides are easily oxidized to chlorine gas by potassium dichromate in strong acidic medium, which leads to deviations in the determination of COD. Although mercuric sulfate was used as a masking agent to eliminate the influence of chloride ions in the actual measurement, when the concentration of chloride ions in wastewater exceeds 2000 mg/L, errors in the determination of COD will still occur. The following is a comparison and discussion of several commonly used methods to eliminate the interference of chloride ions for reference in the actual determination of COD. The standard method of potassium dichromate (GB11914-1989, hereinafter referred to as the potassium dichromate method) is commonly used in the determination of COD by the dilution method. In this method, if the concentration of chloride ions in the water sample is higher than 2000mg/L, it should be diluted quantitatively. , to reduce its mass concentration to below 2000mg/L before measuring, in order to reduce the impact of chloride ions on COD determination. However, this method is not suitable for water samples with very low COD and high chloride ion concentration, because the COD of diluted water samples may be lower (<50mg/L), and then use 0.25mol/L potassium dichromate to measure COD, the repeatability is very poor. Although the addition of mercuric sulfate in the COD determination can form a stable complex with chloride ions to eliminate the influence of chloride ions, but there is a problem of reaction balance. The actual operation shows that the error of COD determination increases with the increase of chloride ion concentration. Silver nitrate precipitation method The principle of this method is to use Ag+ and Cl- to react to form AgCl precipitation, and the solubility product constant of AgCl is 1.8×10-10. The method is to add a certain amount of solid silver nitrate to the water sample to be tested, stir it thoroughly, make it react completely, filter the precipitate by centrifugation, and then take the supernatant to measure the COD value by the potassium dichromate method. The result of this determination method is relatively stable, but before adding silver nitrate, because the Cl- concentration is not known in advance, the added silver nitrate cannot completely form AgCl precipitation with some non-ionic organic chlorides, so the actual amount of silver nitrate added It must be in excess, and silver nitrate is expensive, which increases the cost of analysis. In addition, when there are suspended solids in the water sample, the organic matter and colloids in the water will form co-precipitation with AgCl, and some suspended solids in the wastewater will be taken out while centrifuging to remove the precipitate, which will also cause a certain negative deviation in the measurement results. . The silver nitrate and chromium potassium sulfate method is expensive because the silver nitrate precipitation method is expensive. In actual operation, an appropriate amount of silver nitrate can be added to eliminate the interference of most chloride ions, and the remaining chloride ions can be treated with an appropriate amount of chromium potassium sulfate to make the chloride ions interact with it. The reaction generates complex [CrCl2]+, and the generated complex is adsorbed on the surface of silver chloride precipitation, which can prevent the dissociation of silver chloride and achieve the purpose of reducing interference. This method has better accuracy, but the addition of potassium chromium sulfate may hinder the oxidation of some organic compounds. For the determination of COD with low chloride ion content in wastewater samples by the excess mercury sulfate method, the ratio of m (mercuric sulfate) : m (chloride ion) = 10: 1 stipulated by the potassium dichromate method can be based on the previously measured chloride ion concentration. Dosing mercury sulfate for determination; when the mass concentration of chloride ion is higher than 1000mg/L and COD is lower than 250mg/L, the method of adding excess mercury sulfate can be used to ensure the accuracy of the measurement results. However, the consumption of mercury sulfate is large, and the determination cost is relatively high. The chlorine gas correction method adopts the same digestion and measurement conditions as the potassium dichromate method, so that the degree of pollutant digestion is exactly the same as that of the potassium dichromate method, and potassium iodide is used to absorb the chlorine gas produced by oxidation during the digestion process, and then deducted from the total amount. Because the volatility of iodine solution is greatly affected by temperature, the measured results are unstable. In order to eliminate the influence of iodine volatilization, sodium hydroxide solution was used instead to absorb the chlorine gas produced. But the law applies to Cl-<20000mg/L, COD>Determination of 30mg/L water sample. Because the chlorine gas calibration method has the same digestion conditions as the potassium dichromate method, the degree of digestion is also consistent, thus ensuring the accuracy of the measured value. However, if it is used on site, a set of chlorine gas collection, absorption and measurement devices is required, and the operation is more complicated. COD-chloride standard curve method The potassium dichromate method (without mercury sulfate) was used to measure the COD value of different concentrations of pure sodium chloride solution, and the measured results were plotted into a standard curve corresponding to the concentration of COD and Cl-. The COD measured by the standard method of potassium dichromate (without adding mercury sulfate) to the water sample containing chloride ions is written as COD (I). Use the silver nitrate titration method to measure the chloride ion content in the water sample, calculate the COD value corresponding to the chloride ion content according to the standard curve and write it as COD (Ⅱ), then the real COD value of the water sample is: COD=COD(Ⅰ)-COD (II). This method is simple to operate, the result is reliable, and it also avoids the pollution of the harmful substance mercury sulfate to the environment. But there is a problem that the correlation coefficient in the regression equation of the COD mass concentration-Cl-mass concentration standard curve needs to be corrected in order to make the repeatability of the results better.
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