Chemical Oxygen Demand - Wikipedia, The Free Encyclopedia

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Chemical oxygen demandFrom Wikipedia, the free encyclopedia

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) or wastewater, 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).

Contents

1 Overview

2 History

3 Using potassium dichromate

3.1 Blanks

3.2 Measurement of excess

4 Preparation Ferroin Indicator reagent

4.1 Calculations

5 Inorganic interference

6 Government regulation7 See also

8 References

9 External links

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:

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.

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.

The International Organization for Standardization describes a standard method for measuring chemical oxygen demand in ISO

6060 [1] (http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=12260&ICS1=13&ICS2=60&ICS3=50)

.

History

For many years, the strong oxidizing agent potassium permanganate (KMnO4) was used for measuring chemical oxygen demand.

Measurements were called oxygen consumedfrom permanganate, rather than the oxygen demandof 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

mical oxygen demand - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Chemical_oxygen

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determining COD.

Since then, other oxidizing agents such as ceric sulphate, 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.

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.

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:

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.

In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all redoxreactions, 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.

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 -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.

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. and also we can determine COD by boiling the water sample and we can determine CO2

ratio by the infra-red analyzer

Preparation Ferroin Indicator reagent

A solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg FeSO47H2O in water, and the resulting

red solution is diluted to 100 mL.

Calculations

The following formula is used to calculate COD:


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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.

The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric

reaction with oxygen to yield CO2 (assume all C goes to CO2), H2O (assume all H goes to H2O), and NH3 (assume all N goes to

NH3), 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 of phenol:

C6H5OH + 7O2 6CO2 + 3H2O

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, chloride 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 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.

Inorganic molecule Eliminated by Elimination forms

Chloride Mercuric sulfate Mercuric chloride complex

Nitrite Sulfamic acid N2 gas

Ferrous iron - -

Sulfides - -

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] (http://www.csem.ch/corporate

/Report2002/pdf/p56.pdf) .

See also

Biochemical oxygen demand

Carbonaceous biochemical oxygen demand

Theoretical oxygen demand

Wastewater quality indicators discusses both BOD and COD as measures of water quality.

References


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Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and Science (5th ed.). 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 ed.).

Washington, DC: American Public Health Association. ISBN 0-87553-235-7.

External links

ISO 6060: Water quality - Determination of the chemical oxygen demand (http://www.iso.org/iso/en

/CatalogueDetailPage.CatalogueDetail?CSNUMBER=12260&ICS1=13&ICS2=60&ICS3=50)

Water chemical oxygen demand (http://www.fao.org/gtos/tems/variable_show.jsp?VARIABLE_ID=123) (Food andAgriculture Organization of the United Nations)

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Chemical Oxygen Demand - Wikipedia, The Free Encyclopedia