The pollution level caused by organic and inorganic substances of a waterway (wastewater or surface water) can be evaluated using a parameter called COD.
The abbreviation COD stands for the Chemical Oxygen
Demand for oxidation of substances dissolved and suspended in water. By determining the COD it is possible to proportion chemically oxidable substances with energy oxidants such as potassium bichromate in a strongly acidic solution. The acid environment is caused by the presence of
concentrated sulfuric acid.
Both inorganic compounds and organic substances can be oxidized. This determination allows us to also detect organic substances using the equivalent biological method (BOD: Biological Oxygen Demand) such as, for example, cellulose.
Since oxidation takes place in drastic conditions, it is more
complete than biological oxidation, and as a consequence
the COD value of a given sample is, in most cases, higher
than the corresponding BOD value.

Thermoreaction (digestion) of a sample
The sample digestion phase is one of the most important
steps in many chemical reactions. Digestion is conducted
in order to convert low-solubility compounds or substances present in the form of aggregates into soluble compounds, to degrade organic substances into inorganic molecules, or to eliminate interfering substances and solubilize metallic ions. Digestion takes place by adding decomposition reagents to the sample and then controlling the temperature of the mixture prepared this way.