3. Water & Environmental / Dissolved Oxygen

COD or Chemical Oxygen Demand definition

A number of customers have asked us for a definition of COD or Chemical Oxygen Demand which is detailed below.

Chemical Oxygen Demand (COD)

Chemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals such as Ammonia and nitrite. COD measurements are commonly made on samples of waste waters or of natural waters contaminated by domestic or industrial wastes. Chemical oxygen demand is measured as a standardized laboratory assay. A closed water sample is incubated, with a strong chemical oxidant under specific conditions of temperature and for a particular period of time. A commonly used oxidant in COD assays is potassium dichromate (K2Cr2O7). This is used in combination with boiling sulfuric acid (H2SO4). Because this chemical oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.

Biochemical Oxygen Demand (BOD)

Chemical oxygen demand is related to biochemical oxygen demand (BOD). This is another standard test for assaying the oxygen-demanding strength of waste waters. However, biochemical oxygen demand only measures the amount of oxygen consumed by microbial oxidation. It is most relevant to waters rich in organic matter. It is important to understand that COD and BOD do not necessarily measure the same types of oxygen consumption. For example, COD does not measure the oxygen-consuming potential associated with certain dissolved organic compounds such as acetate. However, acetate can be metabolized by microorganisms and would therefore be detected in an assay of BOD. In contrast, the oxygen-consuming potential of cellulose is not measured during a short-term BOD assay, but it is measured during a COD test.

You can find more about Camlab, Hach and Hanna COD – Chemical Oxygen Demand test vials and colorimeters here. They make measurement simple and do the calculations for you. If you are interested in purchasing a starter kit for performing COD measurements, you can check out our specially compiled bundle. This includes everything you need to get started with COD measurements.

You can read more about the definition of COD here–> COD

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