Why measure COD?

In the cycle of nature, reducing substances in water, especially organic compounds, consume dissolved oxygen in the process of biooxidative degradation, resulting in the lack of oxygen in the water body. Even the dissolved oxygen is exhausted and anaerobic bacteria breed, causing the water body to turn black and smelly.

This requires monitoring of organic matter in the water. Since there are millions of organic compounds, it is difficult to qualitatively and quantitatively monitor them separately. On the basis of practice, environmental analysts have sought another way to determine a comprehensive index, using the reducing properties of organic compounds, and taking the amount of oxygen consumption as a new index, so that chemical oxygen demand and biochemical demand Oxygen come into being.

Because biological oxidation is a slow process, it can only be oxidized to about 70% in a month, which is lagging behind the actual operation of pollution control. Volume (BOD5), although the time is shortened to some extent, it still seems long. In this case, chemical oxygen demand occurs.

Chemical oxygen demand (Chemical Oxygen Demand, COD for short) refers to the amount of oxidant consumed by reducing substances in water that are easily oxidized by strong oxidants (usually potassium dichromate), and the result is converted into the amount of oxygen (in mg / L meter).

It is a comprehensive indicator to characterize reducing substances in water bodies. Except for special water samples, the reducing substances are mainly organic compounds, and elements such as carbon, nitrogen, sulfur, and phosphorus that make up organic compounds are often in a lower oxidation state.

In order to distinguish it from the measurement using potassium permanganate as oxidant, the result is called chemical oxygen demand or chromium method COD, recorded as CODcr, and the result measured using potassium permanganate as oxidant is called permanganic acid. Salt index or manganese method COD was recorded as common.