To detect the content of organic pollutants in the water sample, first add strong oxidant to the water sample, then carry out oxidation digestion by heating reflux, and finally use chemical reagent titration to calculate the chemical oxygen demand of the water sample by measuring the chemical reagent consumed.
(1) Dichromate reflux method
Determination principle: in sulfuric acid medium, potassium dichromate is used as oxidant, silver sulfate as catalyst, mercury sulfate as masking agent of chloride ion, sulfuric acid acidity of digestion reaction liquid is 9mol / L, heating makes digestion reaction liquid boil, boiling point temperature of 148 ℃± 2 ℃ is digestion temperature.
The reaction was heated by water cooling and refluxing for 2 h. After the digestion solution cooled naturally, the residual potassium dichromate was titrated with ammonium ferrous sulfate solution, and the COD value of the water sample was calculated according to the consumption of ferrous sulfate solution.
(2) Potassium permanganate method
Determination principle: potassium permanganate is used as oxidant to determine COD. The COD measured is called permanganate index (CODMn).
After sulfuric acid is added to the water sample, a certain amount of potassium permanganate solution is added, and the reaction is heated in a boiling water bath for 30 minutes.
The residual potassium permanganate is reduced by adding excess sodium oxalate solution, and then the excess sodium oxalate is dropped back by potassium permanganate solution, and the permanganate index is calculated.