Chemical oxygen demand
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Description
The chemical oxygen demand is a measure of the total oxygen required to decompose all organic matter in waste water. This is in distinction to biological oxygen demand, which measure the oxygen required for micro-organisms to decompose the organic pollutants.
Detailed description
The following information is an edited excerpt from Wikipedia [1]
Introduction
The basis for the COD test is that nearly all organic compounds can be fully oxidised to carbon dioxide with a strong oxidising agent under acidic conditions. The amount of oxygen required to oxidise an organic compound to carbon dioxide, ammonia and water may be calculated from the end products of the reaction.
This does not include the oxygen demand caused by the oxidation of ammonia into nitrate. The process of ammonia being converted into nitrate is referred to as nitrification.
This is applied after the first one to include oxidation due to nitrification if the oxygen demand from nitrification must be known. Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test.
The International Organization for Standardisation describes a standard method for measuring chemical oxygen demand in ISO 6060 [3].
Using potassium dichromate
Potassium dichromate is a strong oxidising agent under acidic conditions. (Acidity is usually achieved by the addition of sulfuric acid.) Most commonly, a 0.25N solution of potassium dichromate is used for COD determination, although for samples with COD below 50 mg/l, a lower concentration of potassium dichromate is preferred.
In the process of oxidising the organic substances found in the water sample, potassium dichromate is reduced, forming Cr3+. The amount of Cr3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
Blanks
Because COD measures the oxygen demand of organic compounds in a sample of water, it is important that no outside organic material be accidentally added to the sample to be measured. To control for this, a so-called blank sample is required in the determination of COD (and BOD, for that matter). A blank sample is created by adding all reagents (e.g. acid and oxidizing agent) to a volume of deionised water. COD is measured for both the water and blank samples, and the two are compared. The oxygen demand in the blank sample is subtracted from the COD for the original sample to ensure a true measurement of organic matter.
Measurement of excess
For all organic matter to be completely oxidised, an excess amount of potassium dichromate (or any oxidising agent) must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr3+ can be determined with accuracy. To do so, the excess potassium dichromate is titrated with ferrous ammonium sulfate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+. Typically, the oxidation-reduction indicator Ferroin is added during this titration step as well. Once all the excess dichromate has been reduced, the Ferroin indicator changes from blue-green to reddish-brown. The amount of ferrous ammonium sulfate added is equivalent to the amount of excess potassium dichromate added to the original sample.
Calculations
A formula is used to calculate COD. If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/l.
Inorganic interference
Some samples of water contain high levels of oxidisable inorganic materials which may interfere with the determination of COD. Because of its high concentration in most waste water, chloride is often the most serious source of interference, reacting with potassium dichromate. Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference.
The following table lists a number of other inorganic substances that may cause interference. The table also lists chemicals that may be used to eliminate such interference, and the compounds formed when the inorganic molecule is eliminated.
Government regulation
Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in waste water before they can be returned to the environment. For example, in Switzerland, a maximum oxygen demand between 200 and 1000 mg/l must be reached before waste water or industrial water can be returned to the environment [2].
