Shen ChangHong The operation steps and precautions of COD analyzer are the knowledge that many friends want to know before using. With the progress of the times, COD measuring instrument is also slowly emerging. COD measuring instrument is a kind of chemical oxygen demand measuring instrument, which can quickly calculate oxygen demand. Here is how to use and operate it
Precautions for use of COD tester
1. Before the standard cod digester is electrified, add as much distilled water as possible from the water injection port of the return pipe to ensure the cooling effect.
2. For water samples with serious pollution, especially water samples from industrial pollution sources, 1 / 10 of the required volume of test sample and 1 / 10 of the reagent can be selected and put into a 10 × 150 mm hard glass tube. Heat * * with an alcohol lamp for several minutes to observe whether the solution turns blue-green. If it turns blue-green, take less test sample and repeat the above experiment. The * * solution no longer turns blue-green to Stop. So as to determine the reasonable dilution ratio of the water sample to be tested. When diluting, the amount of waste water sample shall not be less than 5ml. If the chemical oxygen demand is very high (such as factory workshop waste water), the waste water sample shall be diluted several times.
3. The oxidation reflux of water sample shall be carried out in the fume hood to prevent harmful gases such as chlorine from interfering with the health of operators.
4. Put the evenly mixed water sample into a 250ml grinded reflux conical flask, accurately add 10.00ml potassium dichromate standard solution and several small glass beads or zeolites, connect the grinded reflux burr condensing tube, slowly add 30ml sulfuric acid silver sulfate solution from the upper mouth of the condensing tube, and gently shake the conical flask to make the solution evenly mixed. This step is to ensure that the exothermic reaction of concentrated sulfuric acid in water causes the temperature of the system to rise (about 90-95 ℃), and does not cause the organic matter with low boiling point to escape from the upper pipe orifice. (for example, the boiling point of methanol is 64.5 ℃, the boiling point of ethanol is 78.3 ℃, the boiling point of formaldehyde is - 19.5 ℃, and the boiling point of acetaldehyde is 20.8 ℃). If concentrated sulfuric acid is added directly in the open condition, the organic matter with low boiling point will escape without oxidation, which will result in the low result data of determination. For example, if the water contains one thousandth of ethanol (100ppm), its contribution to cod is 189mg / L (the theoretical COD of ethanol is 1.99g/g, and the oxidation rate of CODCr is 95.2%). If concentrated sulfuric acid is directly added to the open triangular flask, the loss of COD in this part is considerable.
5. The waste liquid produced in the COD determination process contains concentrated sulfuric acid, potassium dichromate and mercuric sulfate, which belongs to hazardous waste and should be treated as hazardous waste and not directly discharged into the sewer.
6. Since the detection limit of the method is 10mg / L, the determination of COD in the range of 10-30mg / L must use 0.025mol/l potassium dichromate solution to oxidize, and then use 0.01mol/l ammonium ferrous sulfate to titrate. In order to reduce the relative standard deviation of the determination, it is suggested to increase the sampling amount of the sample, take 50.0ml of * *, and the parallel determination should be more than three times. In order to reduce the titration error, 50.0ml of sample quantity can be used.
7. When determining the water sample with low COD concentration, some possible influencing factors should be considered, such as distilled water or deionized water in polyethylene drum. With the increase of placing time, the COD value will gradually increase, sometimes even more than 10 mg / L. Other experimenters use purified water such as Wahaha (this is the essential difference between the so-called "purified water" in drinks and the "purified water" in analytical sense), instead of distilled water or deionized water as blank, which will also lead to the phenomenon that the blank value increases.
8. In each experiment, the standard titration solution of ferrous ammonium sulfate should be calibrated, especially when the room temperature is high.
9. After the Reflux Digestion of water sample, add distilled water or deionized water slowly from the top of the condenser tube, so as to flush the VOCs attached to the inner wall of the tube into the test solution.
10. During titration, the conical flask shall not be shaken violently, and the test solution in the flask shall not splash, otherwise the determination result will be affected.
Determination steps of COD tester:
1. Add 2ml pure water and 2ml solution to be tested into different cod reagent tubes, tighten the cap, slowly shake the solution in the reagent tube, put the reagent tube into the digester, and digest for 20 minutes at 165 ℃. (danger: the reagent tube contains concentrated sulfuric acid. Please wear gloves during the operation. If you accidentally touch the skin, please wash it with plenty of water immediately.
2. After digestion, take out the reagent tube and put it on the tube rack to cool it to 80 ℃. (warning: the temperature of digester and reagent tube is high, please do not burn during operation.
3. Shake the liquid in the reagent tube again.
4. Place the reagent tube on the test tube rack and continue to cool to room temperature. At this time, it is forbidden to shake the reagent tube.
5. Press the "switch" key to start the machine, and "idle" will be displayed in the middle of the screen to indicate that the machine is finished and the instrument is in standby state.
6. Open the cover of the detection chamber to ensure that the surface of the zero adjustment water sample test tube is clean, put it into the detection chamber, cover the cover, press the "zero adjustment" key, when "0" is displayed in the middle of the screen, it means that the zero adjustment is completed.
7. Open the cover, take out the water sample for zero adjustment, put in the water sample to be measured, and cover the cover. 8. Press the "reading" key, and the number displayed in the middle of the screen indicates the test value of water sample. (unit: mg / L)
8. After the test, short press the "switch" key to turn off the instrument.
The above is the precautions and operation steps of the analyzer. The COD (chemical oxygen demand) micro reflux analysis system provided by shenchanghong company includes the spectrophotometer, digester and matched cod prefabricated reagent.
