Chemical Oxygen Chemical Oxygen Demand (COD)Demand (COD)

CE 370 - LabCE 370 - Lab

Basic PrinciplesBasic PrinciplesThe Chemical Oxygen Demand (COD) The Chemical Oxygen Demand (COD) test measures the oxygen required to test measures the oxygen required to oxidize organic matter in water and oxidize organic matter in water and wastewater samples by the action of wastewater samples by the action of strong oxidizing agents under acid strong oxidizing agents under acid conditions. conditions.

3222 272 NHOHCOONOHCOCr

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COD is often used as a quality COD is often used as a quality parameter to assess the extent of parameter to assess the extent of organic pollutants in municipal and organic pollutants in municipal and industrial wastewaters. industrial wastewaters.

This analysis is of interest since This analysis is of interest since carbon compounds serve as a food carbon compounds serve as a food source for microorganisms.source for microorganisms.

Org + Micro + OOrg + Micro + O22 More Micro + CO More Micro + CO22 + H + H22O + O + OthersOthers

If discharged untreated, organic If discharged untreated, organic compounds will result in microorganism compounds will result in microorganism blooms, resulting in oxygen depletion blooms, resulting in oxygen depletion and fish kills.and fish kills.

COD measurement provides a rapid COD measurement provides a rapid means of monitoring the organic means of monitoring the organic pollutants within the wastewater pollutants within the wastewater treatment plant, allowing for control of treatment plant, allowing for control of the treatment process to maintain the treatment process to maintain optimum plant operation. optimum plant operation.

COD MeasurementCOD Measurement

COD is measured by the oxidation of COD is measured by the oxidation of organic pollutants using a strong organic pollutants using a strong oxidizing agent (oxidizing agent (Potassium dichromatePotassium dichromate) under acidic ) under acidic conditions. The reaction of potassium conditions. The reaction of potassium dichromate with organic compounds is dichromate with organic compounds is given by:given by: 3

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72 O CrNHOHCOHCrNOHC cban

In the process of oxidation, potassium In the process of oxidation, potassium dichromate is reduced forming Crdichromate is reduced forming Cr3+3+. . The amount of CrThe amount of Cr3+3+ is determined after is determined after oxidization is complete, and is used as oxidization is complete, and is used as an indirect measure of the organic an indirect measure of the organic contents of the water sample.contents of the water sample.

For all organic matter to be completely For all organic matter to be completely oxidized, an excess amount of oxidized, an excess amount of potassium dichromate must be present. potassium dichromate must be present.

Once oxidation is complete, the amount Once oxidation is complete, the amount of excess potassium dichromate must be of excess potassium dichromate must be measured to ensure that the amount of measured to ensure that the amount of CrCr3+3+ can be determined with accuracy. can be determined with accuracy.

To do so, the excess potassium To do so, the excess potassium dichromate is dichromate is titratedtitrated with with ferrous ferrous ammonium sulfate (FAS)ammonium sulfate (FAS) until all of until all of the excess oxidizing agent has been the excess oxidizing agent has been reduced to Crreduced to Cr3+3+. .

The oxidation-reduction indicator The oxidation-reduction indicator FerroinFerroin is is added during this titration step. Once all the added during this titration step. Once all the excess dichromate has been reduced, the excess dichromate has been reduced, the Ferroin indicator changes from blue-green to Ferroin indicator changes from blue-green to reddish-brown. reddish-brown.

The amount of ferrous ammonium sulfate The amount of ferrous ammonium sulfate added is equivalent to the amount of excess added is equivalent to the amount of excess potassium dichromate added to the original potassium dichromate added to the original sample.sample.

A blank sample is created by adding all A blank sample is created by adding all reagents (e.g. acid and oxidizing agent) to a reagents (e.g. acid and oxidizing agent) to a volume of distilled water. COD is measured for volume of distilled water. COD is measured for both the sample and the blank, and the two are both the sample and the blank, and the two are compared. The oxygen demand in the blank compared. The oxygen demand in the blank sample is subtracted from the COD for the sample is subtracted from the COD for the original sample to ensure a true measurementoriginal sample to ensure a true measurement

Inorganic interferenceInorganic interferenceSome samples of water contain high levels Some samples of water contain high levels of oxidizable inorganic materials which may of oxidizable inorganic materials which may interfere with the determination of COD. interfere with the determination of COD. Chloride is often the most serious source of Chloride is often the most serious source of interference. Its reaction with potassium interference. Its reaction with potassium dichromate follows the equation:dichromate follows the equation:

Prior to the addition of other reagents, Prior to the addition of other reagents, mercuric sulfate can be added to the sample mercuric sulfate can be added to the sample to eliminate chloride interference.to eliminate chloride interference.