Three versions of COD on-line analyzers are available:
- dichromate method
- permanganate index method
- UV direct reading method.
The dichromate method follows the international standard where in the sample, after filtration, Potassium Dichromate, Mercury (II) sulfate and Sulfuric acid with a proper catalyst are added; Mercury (II) sulfate concentration is adjusted to mask the expected Chloride amount.
The sample is then heated at 170° for 20 minutes or at 150° up to 2 hours (digestion time and digestion temperature can be adjusted depending on the matrix to digest completely all the substances that give a contribute to COD). After digestion the sample is cooled and the absorbance measured at 592 nm is proportional to a COD concentration.
The permanganate index is defined as the quantity of oxygen, expressed as mg/l, equivalent to the consumed permanganate when a water sample is treated with as solution of potassium permanganate in acid environment and at a temperature of 100°C for 10 minutes. The sample is mixed with diluted sulphuric acid, it is then heated at about 100°C, when the first addition of KMnO4 is performed: this first addition may be followed by others, depending on the measuring range. The digestion time in the standard configuration is set to 10 minutes but it can be selected depending on the matrix.
After digestion the sample is cooled and the decrease of the absorbance measured at 525 nm corresponds to the permanganate consumed by the organic substance.
The UV direct reading method is a more simple method where the COD is correlated to the direct light absorption of the water sample measured at 254 nm.