Analyze the COD measurement principle/method of COD analyzer and the significance of COD
COD is a commonly used comprehensive indicator for assessing the degree of water pollution. It is the abbreviation of English chemical oxygen demand. The Chinese name is "chemical oxygen demand" or "chemical oxygen demand". It refers to the oxidative decomposition of reducing substances (such as organic matter) in water by chemical oxidants (such as potassium dichromate). The amount of oxygen consumed. It reflects the extent to which water bodies are contaminated with reducing substances. Since organic matter is the most common reducing substance in water, COD reflects to some extent the extent to which water is contaminated by organic matter. The higher the COD, the more serious the pollution. China's "Surface Water Environmental Quality Standards" stipulates that the COD concentration of drinking water sources should be less than 15 mg / liter, and the general landscape water COD concentration should be less than 40 mg / liter.
Principle/method of COD measurement
(1) Potassium dichromate standard method 1. Principle: Add a certain amount of potassium dichromate and catalyst silver sulfate to the water sample, heat and reflux in a strong acidic medium for a certain period of time, and some potassium dichromate is oxidized by the water sample. After reduction, the remaining potassium dichromate was titrated with ammonium ferrous sulfate, and the value of COD was calculated based on the amount of potassium dichromate consumed.
Second, the instrument 1 1.250mL all glass reflux device 2. Heating device (electric furnace) 3.25mL or 50mL acid burette, conical flask, pipette, volumetric flask and so on.
3. Reagent 1. Potassium dichromate standard solution (c1/6K2Cr2O7=0.2500mol/L) 2. Test yatting indicator solution 3. Ammonium sulphate standard solution [c(NH4)2Fe(SO4)2·6H2O≈ 0.1mol/L] (calibrated before use) 4. Sulfuric acid-silver sulfate solution
IV. Determination step Ammonium ferrous sulfate calibration: Accurately absorb 10.00mL potassium dichromate standard solution in 500mL conical flask, dilute to 110mL with water, slowly add 30mL concentrated sulfuric acid, shake well, after cooling, add 3 drops The ferrous solution (about 0.15 mL) was titrated with a solution of ammonium ferrous sulfate. The color of the solution was yellow from blue-green to reddish brown.
V. Determination: Take 20mL water sample (if necessary, take less water to 20 or dilute and then take it), add 10mL potassium dichromate, insert the reflux device, add 30mL silver sulfate sulfate, heat reflux for 2h, cool down, Rinse the condensation tube wall with 90.00mL water and remove the conical flask. After the solution is cooled again, add 3 drops of ferrous iron indicator solution and titrate with ammonium ferrous sulfate standard solution. The color of the solution is from yellow to green to reddish brown. That is the end point, record the amount of ammonium ferrous sulfate standard solution. While measuring the water sample, take 20.00mL of double distilled water, and follow the same procedure as a blank experiment. Record the amount of ammonium ferrous sulfate standard solution when titrating the blank.
Sixth, calculate CODCr (O2, mg / L) = [8 × 1000 (V0-V1) · C] / V VII, Note 1, the use of 0.4g of mercury sulfate complex chloride ion up to 40mg, such as A water sample of 20.00 mL of water, which is the highest concentration of 2000 mg/L chloride ion, was used. If the concentration of chloride ions is low, it is also possible to add less mercury sulphate to maintain mercury sulphate: chloride ion = 10:1 (W/W). If a small amount of mercury chloride precipitates, it does not affect the determination. 2. The method measures COD in the range of 50-500 mg/L. For water samples with a chemical oxygen demand of less than 50 mg/L, a standard solution of 0.0250 mol/L potassium dichromate should be used instead. A 0.01 mol/L ferrous ammonium sulfate standard solution was used for the back drop. For water samples with a COD greater than 500 mg/L, they should be diluted before measurement. 3. After the water sample is heated and refluxed, the remaining amount of potassium dichromate in the solution should be 1/5-4/5 of the added amount. 4. When checking the quality and operation technology of the reagent with potassium hydrogen phthalate standard solution, since the theoretical CODCr per gram of potassium hydrogen phthalate is 1.176g, 0.4251g of potassium hydrogen phthalate (HOOCC6H4COOK) is dissolved. In the double distilled water, transfer to a 1000 mL volumetric flask and dilute to the mark with heavy distilled water to make it a 500 mg/L CODcr standard solution. New with time. 5. The CODCr measurement results should retain three significant figures. 6. For each experiment, the standard titration solution of ammonium ferrous sulfate should be calibrated. Pay attention to the change of concentration when the room temperature is high. (You can also add 10.0ml of potassium dichromate standard solution to the blank after titration and titrate with ammonium ferrous sulfate to the end point.)
(2) Ultraviolet absorption conversion method
The absorption of ultraviolet light by conventional organic matter conforms to the principle of Beer-Lambert law. The total absorption (organic matter + turbidity) is measured by one ultraviolet light (UV), and the turbidity absorption is measured by another visible light (VIS). After automatic processing by computer, the influence of turbidity is deducted, and finally the accurate absorption of pure organic matter is obtained, and the content of organic matter is calculated, and the COD number is determined by a fixed coefficient.
1: COD instrument components
Heating module
A: Heating module: two methods, 150 degrees Celsius, heating for 2 hours, heating at 165 degrees Celsius for 10 minutes
Different heating modules can be called: traditional national standard method microcirculation heating method, rapid digestion method
B: Measurement module: The absorbance is determined by different wavelengths, so a visible spectrophotometer is required.
C: reagent, main component potassium dichromate (potassium potassium permanganate), sulfuric acid, mercury sulfate, silver sulfate,
COD application range
The reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts and the like. But the main thing is organic matter. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the amount of organic matter in water. The greater the chemical oxygen demand, the more serious the water is contaminated by organic matter. The measurement of chemical oxygen demand (COD) varies depending on the amount of reducing substance in the water sample and the measurement method. The most common application at present is the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method. The potassium permanganate (K2MnO4) method has a low oxidation rate, but is relatively simple. It can be used when determining the relative comparison of the organic matter content in the water sample. Potassium dichromate (K2Cr2O7) method has high oxidation rate and good reproducibility and is suitable for determining 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 contaminates the ion exchange resin when passing through the desalination system, and is particularly liable to contaminate the anion exchange resin, resulting in a decrease in resin exchange capacity. When the organic matter is pretreated (coagulation, clarification and filtration), it can be reduced by about 50%, but it cannot be removed in the desalination system, so it is often brought into the boiler by the make-up water to lower the pH of the furnace water. Sometimes organic matter may also be carried into the steam system and condensate to lower the pH and cause system corrosion. High levels of organic matter in the circulating water system promote microbial growth. Therefore, regardless of the desalination, furnace water or circulating water system, the COD is as low as possible, but there is no uniform limit. When the COD (DmnO4 method) > 5 mg/L in the circulating cooling water system, the water quality has begun to deteriorate.
Precautions
1. The maximum amount of chloride ion complexed with 0.4 g of mercury sulphate can reach 40 mg. For example, 20.00 mL of water sample is used, that is, a water sample with a maximum concentration of 2000 mg/L chloride ion. If the concentration of chloride ions is low, it is also possible to add less mercury sulphate to maintain mercury sulphate: chloride ion = 10:1 (W/W). If a small amount of mercury chloride precipitates, it does not affect the determination. 2. The method measures COD in the range of 50-500 mg/L. For water samples with a chemical oxygen demand of less than 50 mg/L, a standard solution of 0.0250 mol/L potassium dichromate should be used instead. A 0.01 mol/L ferrous ammonium sulfate standard solution was used for the back drop. For water samples with a COD greater than 500 mg/L, they should be diluted before measurement. 3. After the water sample is heated and refluxed, the remaining amount of potassium dichromate in the solution should be 1/5-4/5 of the added amount. 4. When checking the quality and operation technology of the reagent with potassium hydrogen phthalate standard solution, since the theoretical CODCr per gram of potassium hydrogen phthalate is 1.176g, 0.4251g of potassium hydrogen phthalate (HOOCC6H4COOK) is dissolved. In the double distilled water, transfer to a 1000 mL volumetric flask and dilute to the mark with heavy distilled water to make it a 500 mg/L CODcr standard solution. New with time. 5. The CODCr measurement results should retain three significant figures.
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