COD is the chemical oxygen demand, that is, under certain conditions, the amount of oxidation consumed when treating water samples with a strong oxidant. It is an index indicating the quantity of reducing substances in water. Reducing substances in water include various organic substances, nitrite, sulfide, ferrous salt and so on. But mainly organic matter. Therefore, chemical oxygen demand (COD) is often used as an index to measure the organic matter content in water. The greater the chemical oxygen demand, the more serious the water body is polluted by organic matter. The determination of chemical oxygen demand varies with the determination and determination method of reducing substances in water samples. At present, the most commonly used methods are acid potassium permanganate oxidation and potassium dichromate oxidation.

When using potassium permanganate as oxidant to determine COD, the measured value is called permanganate index (namely CODmn).

Using potassium dichromate as oxidant to determine COD, the measured value is called potassium dichromate index (CODcr).