Chemical Oxygen Demand (COD) is defined as the amount of a specific oxidant that reacts with the sample under controlled conditions. The quantities of oxidant consumed is expressed in terms of its oxygen equivalence. 2- Because of Dichromate ions specific oxidant properties, (CrO ) unique chemical 7 3+ properties it gets reduced to the chromic ion (Cr ) in COD test. COD is a defined test, the extent of sample oxidation can be affected by digestion time, reagent strength & sample COD concentration. COD is often used on a measurement of pollutants in waste water & natural water. To determine COD two methods are useful, the open reflux method is suitable for a wide range of waste water & large sample size is preferred. Another is the closed reflux method is suitable for smaller quantities. It requires homogenisation of sample. Accurate measurement of sample values as well as reagent volumes and concentration are critical.