In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers), making COD a useful measure of water quality. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older references may express the units as parts per million (ppm). Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. ... Benzene is the simplest of the arenes, a family of organic compounds An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; therefore, carbides, carbonates, carbon oxides and elementary carbon are not organic (see below for more on the definition controversy... Impact of a drop of water. ... Many of the compounds which are dangerous to the environment can also be harmful to humans in the long-term range and come from mineral and fossil sources or are produced by humans themselves. ... Surface water is water on the ground or in a stream, river, lake, sea or ocean; as opposed to groundwater. ... A man-made lake in Keukenhof, Netherlands A lake is a body of water or other liquid of considerable size surrounded by land. ... For other uses, see River (disambiguation). ... Water quality is the chemical and physical characterization of water. ... The milligram (symbol mg) is an SI unit of mass. ... The litre or liter (U.S. spelling, see spelling differences) is a unit of volume. ... Unsolved problems in physics: What causes anything to have mass? The U.S. National Prototype Kilogram, which currently serves as the primary standard for measuring mass in the U.S. Mass is the property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... Dissolving table salt in water This article is about a chemical solution; for other uses of the term solution, see solution (disambiguation). ... Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. ...

Overview

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. The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by: Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ... European Union Chemical hazard symbol for oxidizing agents Dangerous goods label for oxidising agents An oxidizing agent (also called an oxidant or oxidizer) is referred to as A chemical compound that readily transfers oxygen atoms or A substance that gains electrons in a redox chemical reaction. ... Acidity redirects here. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... Ammonia is a compound with the formula NH3. ...

This expression 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. The following is the correct equation for the oxidation of ammonia into nitrate.

The second equation should be 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 Standardization describes a standard method for measuring chemical oxygen demand in ISO 6060 [1]. The International Organization for Standardization (ISO) is an international standard-setting body composed of representatives from national standards bodies. ...

History

For many years, the strong oxidizing agent potassium permanganate (KMnO₄) was used for measuring chemical oxygen demand. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand of organic substances. Potassium permanaganate'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. European Union Chemical hazard symbol for oxidizing agents Dangerous goods label for oxidising agents An oxidizing agent (also called an oxidant or oxidizer) is referred to as A chemical compound that readily transfers oxygen atoms or A substance that gains electrons in a redox chemical reaction. ... Potassium permanganate is the chemical compound KMnO4. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... General Name, Symbol, Number manganese, Mn, 25 Chemical series transition metals Group, Period, Block 7, 4, d Appearance silvery metallic Atomic mass 54. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ...

Since then, other oxidizing agents such as ceric sulfate, potassium iodate, and potassium dichromate have been used to determine COD. Of these, potassium dichromate (K₂Cr₂O₇) 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. Potassium iodate (KIO3) is a chemical compound. ... Potassium dichromate, K2Cr2O7 is used in oxidation reactions. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... Categories: Move to Wiktionary | Stub | Chemistry ...

Using potassium dichromate

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: Potassium dichromate, K2Cr2O7 is used in oxidation reactions. ... Sulfuric acid (British English: sulphuric acid), H2SO4, is a strong mineral acid. ...

where d = 2n/3 + a/6 - b/3 - c/2. Most commonly, a 0.25 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. Normality can mean Normality (chemistry) Normality (statistics) Used in the English language: Being normal. ...

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 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. Illustration of a redox reaction Redox (shorthand for oxidation/reduction reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. ...

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 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. This article or section does not cite its references or sources. ... This article or section is not written in the formal tone expected of an encyclopedia article. ...

Measurement of excess

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 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. Titration setup: the titrant drops from the burette into the analyte solution in the flask. ... Ferroin is the chemical compound with the formula [Fe(o-phen)3]SO4, where o-phen is an abbreviation for 1,10-phenanthroline. ...

Calculations

The following formula is used to calculate COD:

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.

Inorganic interference

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: Wastewater is any water that has been adversely affected in quality by anthropogenic influence. ... The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and are also called chlorides. ...

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.

The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and are also called chlorides. ... Mercury chloride is a white poisonous soluble crystalline sublimate of mercury, used as a pesticide or antiseptic or wood preservative. ... // Definition The nitrite ion is NO2−. A nitrite compound is one that contains this group, either an ionic compound, or an analogous covalent one. ... Sulfamic acid, also known as amidosulfonic acid amidosulfuric acid, aminosulfonic acid, and sulfamidic acid, is a molecular compound with the formula H3NSO3. ... Ferrous in chemistry is a term used for the iron with an oxidation number +2. ... Formally, sulfide is the dianion, S2−, which exists in strongly alkaline aqueous solutions formed from H2S or alkali metal salts such as Li2S, Na2S, and K2S. Sulfide is exceptionally basic and, with a pKa > 14, it does not exist in appreciable concentrations even in highly alkaline water. ...

Government regulation

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 [2].

See also

• Biochemical oxygen demand
• Wastewater quality indicators discusses both BOD and COD as measures of water quality.

This article or section does not cite its references or sources. ... Wastewater quality indicators such as the Biochemical oxygen demand (BOD) and the Chemical oxygen demand (COD) are essentially laboratory test measures of the amount of oxygen in a wastewater. ... Look up bod in Wiktionary, the free dictionary. ... Species Gadus morhua Gadus macrocephalus Gadus ogac Cod is the common name for the genus Gadus of fish, belonging to the family Gadidae, and is also used in the common name of a variety of other fishes. ...

References

• Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and Science, 5th edition, New York: McGraw-Hill. ISBN 0-07-248066-1. 
• Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton. Standard Methods for Examination of Water & Wastewater, 20th edition, Washington, DC: American Public Health Association. ISBN 0-87553-235-7. 

External links

• ISO 6060: Water quality - Determination of the chemical oxygen demand
• Water chemical oxygen demand (Food and Agriculture Organization of the United Nations)