12/10/2024
COD test method examples, which one is suitable for you?
COD test method is an indispensable method in water quality monitoring. Its meaning refers to the amount of oxidant consumed when treating water samples with strong oxidants under certain conditions, expressed in mg/L of oxygen. Chemical oxygen demand reflects the degree of pollution of water by reducing substances. In the detection and analysis process, Cl- in water samples is easily oxidized by oxidants, and a large amount of Cl- makes the measurement result high. The measurement of high-chlorine and low-COD wastewater is a difficult problem now. In actual monitoring, it is found that many types of wastewater such as chemical wastewater, monosodium glutamate wastewater, and seafood processing wastewater have high Cl- contents. The COD determination requires shielding Cl- before determination. The vast majority of environmental monitoring workers have done a lot of work on the interference of Cl- on COD determination. The following is an explanation from the test method.
1. The dichromate reflux method is a classic standard method with good reproducibility, accuracy and reliability.
Determination principle: In sulfuric acid acid medium, potassium dichromate is used as oxidant, silver sulfate is used as catalyst, mercuric sulfate is used as masking agent for chloride ions, the sulfuric acid acidity of the digestion reaction solution is 9mol/L, and the digestion reaction solution is heated to boil, and the boiling point temperature of 148℃±2℃ is the digestion temperature. The reaction is refluxed and heated with water for 2h. After the digestion solution is cooled naturally, ferrochlore is used as indicator, and the remaining potassium dichromate is titrated with ammonium ferrous sulfate solution. The COD value of the water sample is calculated according to the consumption of ammonium ferrous sulfate solution.
2.The advantage of potassium permanganate method is that the pollution generated during the experiment is less than that of the national standard method. Potassium permanganate is used as an oxidant to determine COD. The COD measured by this COD test method is called permanganate index (CODMn). After the water sample is acidified by adding sulfuric acid, a certain amount of potassium permanganate solution is added, and the reaction is heated in a boiling water bath for 30min. The remaining potassium permanganate is reduced by adding excess sodium oxalate solution, and then the excess sodium oxalate is back-titrated with potassium permanganate solution, and the permanganate index is calculated.
