In
environmental chemistry
Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source. It can be defined as t ...
, the chemical oxygen demand (COD) is an indicative measure of the amount of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
that can be consumed by
reactions
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
*Nuclear reaction
*Reaction (physics), as defined by Newton's third law
*Chain reaction (disambiguation).
Biology and me ...
in a measured
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
. It is commonly expressed in
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of oxygen consumed over
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The de ...
of solution which in SI units is milligrams per litre (
mg/
L). A COD test can be used to easily quantify the amount of
organics in
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
. The most common application of COD is in quantifying the amount of oxidizable
pollutant
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like oi ...
s found in
surface water
Surface water is water located on top of land forming terrestrial (inland) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean.
The vast majority of surface water is produced by prec ...
(e.g.
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s and
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
s) or
wastewater
Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industr ...
. COD is useful in terms of
water quality
Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
by providing a metric to determine the effect an
effluent
Effluent is wastewater from sewers or industrial outfalls that flows directly into surface waters either untreated or after being treated at a facility. The term has slightly different meanings in certain contexts, and may contain various pollut ...
will have on the receiving body, much like
biochemical oxygen demand (BOD).
Overview
The basis for the COD test is that nearly all organic compounds can be fully oxidized to
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
with a strong
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...
under
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
ic conditions. The amount of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
required to oxidize an organic compound to carbon dioxide,
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
, and water is given by:
:
This expression does not include the oxygen demand caused by
nitrification
''Nitrification'' is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of amm ...
, the oxidation of ammonia into
nitrate
Nitrate is a polyatomic ion
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
:
:
Dichromate, the oxidizing agent for COD determination, does not oxidize ammonia into nitrate, so nitrification is not included in the standard COD test.
The
International Organization for Standardization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...
describes a standard method for measuring chemical oxygen demand in ISO 606
Using potassium dichromate
Potassium dichromate
Potassium dichromate, , is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health ...
is a strong oxidizing agent under acidic conditions. Acidity is usually achieved by the addition of
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
. The reaction of potassium dichromate with organic compounds is given by:
:
where
. 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.
In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
reactions, one reagent is oxidized and the other is reduced), forming Cr
3+. The amount of Cr
3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
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 Cr
3+ can be determined with accuracy. To do so, the excess potassium dichromate is
titrated
Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...
with
ferrous ammonium sulfate
Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2(H2O)6. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a com ...
(FAS) until all of the excess oxidizing agent has been reduced to Cr
3+. Typically, the oxidation-reduction indicator
ferroin
Ferroin is the chemical compound with the formula e(''o''-phen)3O4, where ''o''-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.
Redox ...
is added during this titration step as well. Once all the excess dichromate has been reduced, the ferroin indicator changes from blue-green to a reddish brown. The amount of
ferrous ammonium sulfate
Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2(H2O)6. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a com ...
added is equivalent to the amount of excess potassium dichromate added to the original sample. Note: Ferroin indicator is bright red from commercially prepared sources, but when added to a digested sample containing potassium dichromate it exhibits a green hue. During the titration the color of the indicator changes from a green hue to a bright blue hue to a reddish brown upon reaching the endpoint. Ferroin indicator changes from red to pale blue when oxidized.
Preparation of ferroin indicator reagent
A solution of 1.485 g 1,10-
phenanthroline
1,10-Phenanthroline (phen) is a heterocyclic organic compound. It is a white solid that is soluble in organic solvents. The 1,10 refer to the location of the nitrogen atoms that replace CH's in the hydrocarbon called phenanthrene.
Abbreviated ...
monohydrate is added to a solution of 695 mg FeSO
4·7H
2O in distilled water, and the resulting red solution is diluted to 100 mL.
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.
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
2 (assume all C goes to CO
2), H
2O (assume all H goes to H
2O), and NH
3 (assume all N goes to NH
3), using the following formula:
:COD = (''C''/FW)·(RMO)·32
Where
:''C'' = Concentration of oxidizable compound in the sample,
:FW = Formula weight of the oxidizable compound in the sample,
:RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO
2, water, and ammonia
For example, if a sample has 500 Wppm (Weight Parts per Million) of phenol:
:C
6H
5OH + 7O
2 → 6CO
2 + 3H
2O
:COD = (500/94)·7·32 = 1191 Wppm
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
Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industr ...
,
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
is often the most serious source of interference. Its reaction with potassium dichromate follows the equation:
:
Prior to the addition of other reagents,
mercuric sulfate
Mercury(II) sulfate, commonly called mercuric sulfate, is the chemical compound Hg S O4. It is an odorless solid that forms white granules or crystalline powder. In water, it separates into an insoluble sulfate with a yellow color and sulfuric a ...
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
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
, a maximum oxygen demand between 200 and 1000 mg/L must be reached before waste water or
industrial water
Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. 97% of the water on the Earth is salt water and only three percent is fresh water; slightl ...
can be returned to the environmen
History
For many years, the strong
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...
potassium permanganate
Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution.
Potassium permanganate is widely used in the c ...
(
K Mn O4) 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
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. T ...
(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.
Since then, other oxidizing agents such as
ceric sulphate,
potassium iodate
Potassium iodate ( K I O3) is an ionic chemical compound consisting of K+ ions and IO3− ions in a 1:1 ratio.
Preparation and properties
Potassium iodate is an oxidizing agent and as such it can cause fires if in contact with combustible materi ...
, and
potassium dichromate
Potassium dichromate, , is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health ...
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.
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. As with all colorimetric methods
blanks are used to control for contamination by outside material.
See also
*
*
*
* discusses both
BOD and COD as measures of water quality.
References
Citations
Sources
*
*
External links
ISO 6060: Water quality - Determination of the chemical oxygen demand
{{DEFAULTSORT:Chemical Oxygen Demand
Anaerobic digestion
Chemical oceanography
Environmental chemistry
Aquatic ecology
Water quality indicators
Limnology