How to detect COD in water?
According to different types of oxidants, it is divided into CODCr and CODMn.
(1.) CODMn refers to the potassium permanganate method, because the oxidation rate is relatively low, only about 45%, so it is usually used for a content of COD in surface water species, which is expressed by the potassium permanganate index.
(2.) CODCr refers to the potassium dichromate method, which requires 20 minutes of high temperature digestion at 165 degrees before measurement. The oxidation rate is above 95%. The detection accuracy is relatively high. Therefore, the national standard requires the determination of COD in wastewater by the potassium dichromate method.
The most commonly used potassium dichromate method is reflux titration and rapid digestion photometry.
Principle of reflux titration method: in acidic sulfuric acid medium, potassium dichromate is used as oxidant, silver sulfate is used as catalyst, mercury sulfate is used as masking agent of chloride ion, the reaction liquid is boiled by heating and digestion, and the boiling point temperature of 148℃±2℃ is the digestion temperature . Heat the reaction with water cooling and reflux to heat the reaction for 2H. After the digestion solution is naturally cooled, dilute with water and use ferriline as the indicator. Titrate the remaining potassium dichromate with the ferrous ammonium sulfate solution according to the consumption of the ferrous ammonium sulfate solution Calculate the COD of the water sample. The oxidant used is potassium dichromate, and the one with oxidation performance is hexavalent chromium, so it is called the dichromate method.
The principle of rapid digestion spectrophotometry: 1.) Add a certain amount of potassium dichromate to the sample, use potassium dichromate as the oxidant, sulfuric acid as the reaction medium, silver sulfate as the catalyst, mercury sulfate as the shielding agent, and heat at 165 degrees After digestion for 20 minutes, the content of hexavalent chromium or trivalent chromium was measured by spectrophotometry to determine the COD value. A
The advantages of rapid digestion photometry: time: from the original 120 minutes to 20 minutes. Operation: The cumbersome operation process is now only a few steps. Reagents: The traditional reflux method requires a large amount of potassium dichromate, silver sulfate solution, water sample, and a large amount of tap water for reflux. Now the rapid digestion method only needs to use a small amount of reagents and water samples, and does not require a large amount of clean water as reflux.
Operation process: Take water sample concentration of 0-150mg as an example.
1. When the COD of the water sample is 0-150mg/L. ①Aspirate 2ml of purified water with a pipette and add it to an L R reagent tube (for zero adjustment) ②Aspirate 2ml of water sample to be tested with a pipette and add it to another L R reagent tube.
2. Tighten the reagent tube cap, shake upside down. Place the reagent tube in the digestion apparatus and digest it at 165C for 20 minutes.
3. After the digestion is completed, remove the reagent tube and place it on the test tube rack to cool to about 80°C (hands can bear), and 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" button to start the computer. "idLE" is displayed in the middle of the screen to indicate that the boot is complete. Press the "Switch" button to select the desired test item.
6. Open the cover of the test chamber, make sure that the surface of the zero water sample tube is clean, put it into the test chamber, close the cover, and press the "zero" key. When the screen displays "0" in the middle, the zero adjustment is completed.
7. Open the cover, take out the zero-adjusting reagent tube, put the reagent tube of the water sample to be tested, ensure that the surface of the reagent tube is clean, and close the cover.
8. Press the "Reading" key, the number displayed in the middle of the screen indicates the detection value of the water sample. (Unit: mg/L)
Industry application: The CODCr method is widely used and applicable to various types of industrial wastewater. Printing and dyeing, electroplating, petrochemical industry, etc.