concept
Chemical Oxygen Demand (Chemical Oxygen Demand) refers to the amount of oxidant consumed in the treatment of water samples with potassium dichromate as the oxidant under strong acid and heating conditions, expressed as mg/L of oxygen. It reflects the degree of contamination of the water by reducing substances.
principle
The COD measurement uses a strong oxidant (potassium dichromate), strong acid, and organic carbon as carbon dioxide and water by heating; it is an indirect method of measuring organics and measures the consumption of organic oxidants for decomposition:Greater consumption = higher organic concentration Less consumption = lower organic concentration
Chemical principleSulfuric acid and potassium dichromate oxidation of organic matter is
   
2KC 8 H 3 O 4 + 10K 2 Cr 2 O 7 + 41H 2 SO 4 = 16CO2 + 46H2 O + 10Cr 2 (SO 4) 3 + 11K 2 SO 4
Instrument measurement method
Instrument Measurement Principle (HJ/T399-2007)
- Organics are oxidized, turning yellow hexavalent chromium into green trivalent chromium
- Cr6+ is yellow, Cr3+ is green, and the consumption of oxidant can be known by colorimetric method;
— Low range: 420nm, measuring the degree of yellow light intensity reduction High range: 610nm, measuring the degree of increase in green light intensity The degree of light intensity change is linear with the COD value;
COD analyzer analysis flow chart
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