Biochemical oxygen demand – Oxygen needed to remove organics from water.As with all colorimetric methods blanks are used to control for contamination by outside material. After a refluxing digestion step, the initial concentration of organic substances in the sample is calculated from a titrimetric or spectrophotometric determination of the oxidant still remaining in the sample. In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed. Of these, potassium dichromate ( K 2Cr 2O 7) has been shown to be the most effective: it is relatively cheap, easy to purify, and is able to nearly completely oxidize almost all organic compounds. Since then, other oxidizing agents such as ceric sulphate, potassium iodate, and potassium dichromate have been used to determine COD. This indicated that potassium permanganate was not able to effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing agent for determining COD. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and in many cases biochemical oxygen demand (BOD) measurements were often much greater than results from COD measurements. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand of organic substances. įor many years, the strong oxidizing agent potassium permanganate ( K Mn O 4) was used for measuring chemical oxygen demand. For example, in Switzerland, a maximum oxygen demand between 2 mg/L must be reached before waste water or industrial water can be returned to the environment. Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in waste water before they can be returned to the environment. The table also lists chemicals that may be used to eliminate such interference, and the compounds formed when the inorganic molecule is eliminated. The following table lists a number of other inorganic substances that may cause interference. Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference. The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by:Ĭ n H a O b N c + ( n + a 4 − b 2 − 3 4 c ) O 2 → n CO 2 + ( a 2 − 3 2 c ) H 2 O + c NH 3 The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. COD is useful in terms of water quality by providing a metric to determine the effect an effluent will have on the receiving body, much like biochemical oxygen demand (BOD). The most common application of COD is in quantifying the amount of oxidizable pollutants found in surface water (e.g. A COD test can be used to easily quantify the amount of organics in water. It is commonly expressed in mass of oxygen consumed over volume of solution which in SI units is milligrams per litre ( mg/ L). In environmental chemistry, the chemical oxygen demand ( COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. Measure of the amount of oxygen that can be consumed by reactions in a solution
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