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| In [[environmental chemistry]], the '''chemical oxygen demand''' ('''COD''') test is commonly used to indirectly measure the amount of [[organic compound]]s in [[water]]. Most applications of COD determine the amount of [[organic compound|organic]] [[pollutant]]s found in [[surface water]] (e.g. [[lake]]s and [[river]]s) or [[wastewater]], making COD a useful measure of [[water quality]]. It is expressed in milligrams per liter ([[milligram|mg]]/[[Litre|L]]) also referred to as ppm (parts per million), which indicates the [[mass]] of oxygen consumed per liter of [[solution]].
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| ==Overview==
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| 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 [[acid]]ic conditions. The amount of [[oxygen]] required to oxidize an organic compound to carbon dioxide, [[ammonia]], and water is given by:
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| :<math>\mbox{C}_n\mbox{H}_a\mbox{O}_b\mbox{N}_c + \left( n + \frac{a}{4} - \frac{b}{2} - \frac{3}{4}c \right)\mbox{O}_2 \rightarrow n\mbox{CO}_2 + \left( \frac{a}{2} - \frac{3}{2}c \right)\mbox{H}_2\mbox{O} + c\mbox{NH}_3</math>
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| This expression includes the oxygen demand caused by the oxidation of ammonia into nitrate. The process of ammonia being converted into nitrate is referred to as ''nitrification.'' The following is the correct equation for the oxidation of ammonia into nitrate.
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| :<math>\mbox{N}\mbox{H}_3 + 2\mbox{O}_2 \rightarrow \mbox{N}\mbox{O}_3^- + \mbox{H}_3\mbox{O}^+</math>
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| It is applied after the 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.
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| The [[International Organization for Standardization]] describes a standard method for measuring chemical oxygen demand in ISO 6060 [http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=12260&ICS1=13&ICS2=60&ICS3=50].
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| ==History==
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| For many years, the strong [[oxidizing agent]] [[potassium permanganate]] ([[potassium|K]][[manganese|Mn]][[oxygen|O]]<sub>4</sub>) was used for measuring chemical oxygen demand. Measurements were called ''oxygen consumed'' from permanganate, rather than the ''oxygen demand'' of organic substances. 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. 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.
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| Since then, other oxidizing agents such as [[ceric sulphate]], [[potassium iodate]], and [[potassium dichromate]] have been used to determine COD. Of these, potassium dichromate ([[potassium|K]]<sub>2</sub>[[chromium|Cr]]<sub>2</sub>[[oxygen|O]]<sub>7</sub>) has been shown to be the most effective: it is relatively cheap, easy to [[List of purification methods in chemistry|purify]], and is able to nearly completely oxidize almost all organic compounds.
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| In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed. 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.
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| == Using potassium dichromate==
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| [[Potassium dichromate]] is a strong oxidizing agent under acidic conditions. (Acidity is usually achieved by the addition of [[sulfuric acid]].) The reaction of potassium dichromate with organic compounds is given by:
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| :<math>\mathrm{C_nH_aO_bN_c\ +\ dCr_2O_7^{2-}\ +\ (8d\ +\ c)H^+ \rightarrow nCO_2\ +\ \frac {a + 8d - 3c}{2}H_2O\ +\ cNH_4^+\ + \ 2dCr^{3+}}</math>
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| where ''d = 2n/3 + a/6 - b/3 - c/2''. Most commonly, a 0.25 [[normality (chemistry)|N]] 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.
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| In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all [[redox]] reactions, one reagent is oxidized and the other is reduced), forming Cr<sup>3+</sup>. The amount of Cr<sup>3+</sup> is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
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| ===Blanks===
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| 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 [[biochemical oxygen demand|BOD]] -biochemical oxygen demand - for that matter). A blank sample is created by adding all reagents (e.g. acid and oxidizing agent) to a volume of [[distilled 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.
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| ===Measurement of excess===
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| For all organic matter to be completely oxidized, an excess amount of potassium dichromate (or any oxidizing agent) must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr<sup>3+</sup> can be determined with accuracy. To do so, the excess potassium dichromate is [[titration|titrated]] with [[ferrous ammonium sulfate]] (FAS) until all of the excess oxidizing agent has been reduced to Cr<sup>3+</sup>. 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 reddish-brown to a blue-green. The amount of [[ferrous ammonium sulfate]] added is equivalent to the amount of excess potassium dichromate added to the original sample.
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| ==Preparation Ferroin Indicator reagent==
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| A solution of 1.485 g 1,10-[[phenanthroline]] monohydrate is added to a solution of 695 mg FeSO<sub>4</sub>·7H<sub>2</sub>O in water, and the resulting red solution is diluted to 100 mL.
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| ===Calculations===
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| The following formula is used to calculate COD:
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| :<math>COD = \frac{8000 (b - s)n}{sample\ volume}</math>
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| where ''b'' is the volume of FAS used in the blank sample, ''s'' is the volume of FAS in the original sample, and ''n'' is the normality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/L.
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| The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO<sub>2</sub> (assume all C goes to CO<sub>2</sub>), H<sub>2</sub>O (assume all H goes to H<sub>2</sub>O), and NH<sub>3</sub> (assume all N goes to NH<sub>3</sub>), using the following formula:
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| :'''COD = (C/FW)(RMO)(32)'''
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| :Where C = Concentration of oxidizable compound in the sample,
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| :FW = Formula weight of the oxidizable compound in the sample,
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| :RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO<sub>2</sub>, water, and ammonia
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| For example, if a sample has 500 wppm of phenol:
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| :C<sub>6</sub>H<sub>5</sub>OH + 7O<sub>2</sub> → 6CO<sub>2</sub> + 3H<sub>2</sub>O
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| :COD = (500/94)(7)(32) = 1191 wppm
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| ==Inorganic interference==
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| Some samples of water contain high levels of oxidizable inorganic materials which may interfere with the determination of COD. Because of its high concentration in most [[wastewater]], [[chloride]] is often the most serious source of interference. Its reaction with potassium dichromate follows the equation:
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| :<math>\mathrm{6Cl^- + Cr_2O_7^{2-} + 14H^+ \rightarrow 3Cl_2 + 2Cr^{3+} + 7H_2O}</math>
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| Prior to the addition of other reagents, [[mercuric sulfate]] can be added to the sample to eliminate chloride interference.
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| 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.
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| {| border=1 cellpadding=3 cellspacing=0 style="border-collapse:collapse"
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| |- bgcolor="#cccccc"
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| ! Inorganic molecule
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| ! Eliminated by
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| ! Elimination forms
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| |-
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| | [[Chloride]]
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| | [[Mercuric sulfate]]
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| | [[Mercuric chloride]] complex
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| |-
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| | [[Nitrite]]
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| | [[Sulfamic acid]]
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| | N<sub>2</sub> gas
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| |-
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| | [[Ferrous|Ferrous iron]]
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| | -
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| | -
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| |-
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| | [[Sulfide]]s
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| | -
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| | -
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| |}
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| ==Government regulation==
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| Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in wastewater 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 wastewater or [[industrial water]] can be returned to the environment [http://www.csem.ch/corporate/Report2002/pdf/p56.pdf].
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| ==See also==
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| * [[Biochemical oxygen demand]]
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| * [[Carbonaceous biochemical oxygen demand]]
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| * [[Theoretical oxygen demand]]
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| * [[Wastewater quality indicators]] discusses both [[Biochemical oxygen demand|BOD]] and COD as measures of water quality.
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| ==References==
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| {{refbegin}}
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| *{{cite book | author=Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin | title=Chemistry for Environmental Engineering and Science | edition=5th | location=New York | year=2003 | publisher=McGraw-Hill | isbn=0-07-248066-1}}
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| *{{cite book | author=Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton | title=Standard Methods for Examination of Water & Wastewater | edition=20th | publisher=American Public Health Association | location=Washington, DC | isbn=0-87553-235-7}}
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| {{refend}}
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| ==External links==
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| * [http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=12260&ICS1=13&ICS2=60&ICS3=50 ISO 6060: Water quality - Determination of the chemical oxygen demand]
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| * [http://www.fao.org/gtos/tems/variable_show.jsp?VARIABLE_ID=123 Water chemical oxygen demand] (Food and Agriculture Organization of the [[United Nations]])
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| {{DEFAULTSORT:Chemical Oxygen Demand}}
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| [[Category:Anaerobic digestion]]
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| [[Category:Chemical oceanography]]
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| [[Category:Environmental chemistry]]
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| [[Category:Aquatic ecology]]
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| [[Category:Water quality indicators]]
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My name is Elbert Gower but everybody calls me Elbert. I'm from Canada. I'm studying at the college (1st year) and I play the Post horn for 6 years. Usually I choose songs from the famous films ;).
I have two brothers. I love Rugby league football, watching TV (Doctor Who) and Inline Skating.
Feel free to visit my webpage - dermology anti aging cream (http://shuni.org/activity/p/14955)