Testing for Chemical Oxygen Demand - COD

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The Chemical Oxygen Demand (COD) test measures the oxygen equivalent consumed by organic matter in a sample during strong chemical oxidation. The strong chemical oxidation conditions are provided by the reagents used in the analysis. Potassium dichromate is used as the oxygen source with concentrated sulfuric acid added to yield a strong acid medium. Several reagents are added during the set up of the analysis to drive the oxidation reaction to completion and also to remove any possible interferences. Specifically, these reagents are mercuric sulfate, silver sulfate and sulfamic acid. Mercuric sulfate is added to remove complex chloride ions present in the sample. Without the mercuric sulfate the chloride ions would form chlorine compounds in the strong acid media used in the procedure. These chlorine compounds would oxidize the organic matter in the sample, resulting in a COD value lower than the actual value. Silver sulfate is added as a catalyst for the oxidation of short, straight chain organics and alcohols. Again, without the silver sulfate the COD of the sample would be lower than the actual value. Sulfamic acid is added to remove interferences caused by nitrite ions. Without sulfamic acid the COD of the sample would measure higher than the actual value.

Even with the use of these additional reagents the oxidation of the organic matter is not always 100% complete. Volatile organics, ammonia and aromatic hydrocarbon are not oxidized to any great degree during the procedure.

The advantages of the COD test as compared to the BOD test are:

1. COD results are available much sooner.
2. The COD test requires fewer manipulations of the sample.
3. The COD test oxidizes a wider range of chemical compounds.
4. It can be standardized more easily.

The major disadvantage of the COD test is that the results are not directly applicable to the 5-day BOD results without correlation studies over a long period of time. The samples used for the COD analysis may be grab or composite. Preservation of the sample can be accomplished by adding sulfuric acid to depress the pH to 2 and the holding time with preservation is 7 days.

The amount of organic matter in water is estimated based on their oxidisability by chemical oxidants, such as potassium permanganate or potassium dichromate. For many years, the potassium permanganate was used as oxidizing agent for measuring chemical oxygen demand. But the oxidizing capacity of potassium permanganate varied widely. Nowadays, Potassium dichromate is used instead of potassium permanganate because it is more effective, relatively cheap, easy to purify and is able to oxidize almost all organic compounds.
In this method, a fixed volume of oxidant (here potassium dichromate) is added to the water sample. The organic matter present in the water sample is first oxidized with known volume of potassium dichromate and then excess of oxygen is allowed to react with potassium iodide to liberate iodine in amounts equal to the excess oxygen, which is estimated titrimetrically with sodium thiosulphate as an indicator.
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