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COD Determination

Potassium dichromate is added to the sample for digestion of the organic matter present in the sample since potassium dichromate is capable of oxidizing a wide varieties of organic substances almost completely to carbon dioxide and water. For potassium dichromate to oxidize organic matter completely, the solution must be strongly acidic and heated to an elevated temperature. Reflux condensers are ordinarily used to prevent the escape of volatile materials during digestion. In the analysis for COD, an excess of oxidizing agent must be present to ensure that all organic matter is oxidized completely. It is, therefore, necessary to measure the excess of oxidizing agent by using ferrous ion as reducing agent with ferroin indicator. The COD analysis could be completed within two to three hours.

The ratio of COD to BOD does not change significantly and a constant for any particular wastewater. The ratio can be determined by analyzing the BOD and COD of the samples. COD/ BOD ratio for domestic wastewater varies from 1.5 to 1.9. By establishing COD / BOD, COD analysis of sample of wastewater could be used conveniently as a substitute for the BOD test in many instances; for interpreting the value of BOD to assess the efficiency of the treatment units and monitoring the effluent quality since the COD determination could be done within short period.

One of the chief limitations of the COD analysis is its inability to differentiate between biodegradable and non-biodegradable organic matter. In addition it does not provide any evidence of the rate at which the biodegradable organic material would be stabilized under the conditions that exist in nature.