Editing Chemical oxygen demand (section)

==History==
For many years, the strong [[oxidizing agent]] [[potassium permanganate]] ([[potassium|K]][[manganese|Mn]][[oxygen|O]]<sub>4</sub>) was used for measuring chemical oxygen demand. Measurements were called ''oxygen consumed'' from permanganate rather than organic substances' ''oxygen demand''. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and in many cases, [[biochemical oxygen demand]] (BOD) measurements were often much greater than results from COD measurements. This indicated that potassium permanganate could not effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing agent for determining COD.

Since then, other oxidizing agents such as [[ceric sulphate]], [[potassium iodate]], and [[potassium dichromate]] have been used to determine COD. Of these, potassium dichromate ([[Potassium dichromate|K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>]]) is the most effective: it is relatively cheap, easy to [[List of purification methods in chemistry|purify]], and can nearly completely oxidize almost all organic compounds.

In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed. After a refluxing digestion step, the initial concentration of organic substances in the sample is calculated from a titrimetric or spectrophotometric determination of the oxidant remaining in the sample. As with all colorimetric methods, [[blank (solution)|blanks]] are used to control for contamination by outside material.