Chemical oxygen demand
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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 mg/L, which indicates the mass of oxygen consumed per liter of solution.
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Overview
COD is based on the fact 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:
- <math>C_nH_aO_bN_c + \left( n + \frac{a}{4} - \frac{b}{2} - \frac{3}{4}c \right)O_2 \rightarrow nCO_2 + \left( \frac{a}{2} - \frac{3}{2}c \right)H_2O + cNH_3<math>
In contrast to biochemical oxygen demand (BOD) — another common measure of water-borne organic substances — the process of measuring COD causes the conversion of all organic matter into carbon dioxide. For this reason, one limitation of COD is that it cannot differentiate between levels of biologically active organic substances and those that are biologically inactive. One major advantage of COD, however, is that it can be measured in a fraction of the time required by BOD: while BOD takes 5-7 days to determine, COD requires just 3 hours.
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 (KMn04) was used for measuring chemical oxygen demand. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand of organic substances. Postassium permanaganate's effectiveness at oxidizing organic compounds varied widely, and in many cases 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 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.
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:
- <math>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>
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 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.
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.
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.
Calculations
The following formula is used to calculate COD:
- <math>COD = \frac{8000 (b - s)n}{sample\ volume}<math>
where b is the volume of FAS used in the blank sample, s is the volulme 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:
- <math>6Cl^- + Cr_2O_7^{2-} + 14H^+ \rightarrow 3Cl_2 + 2Cr^{3+} + 7H_2O<math>
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 | Elmination forms |
|---|---|---|
| Chloride | Mercuric sulfate | Mercuric chloride complex |
| Nitrate | Sulfamic acid | N2 gas |
| Ferrous iron | Sulfamic acid | - |
| Sulfides | Sulfamic acid | - |
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).
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 and Agriculture Organization of the United Nations)
- About chemical oxygen demand (http://www.hannainst.com/products/cod/aboutcod.htm) (Hanna Instruments)
- Chemical oxygen demand (http://www.primeindia.com/manav/mangt48.html) (technique)
References
- Sawyer, C., McCarty, P., Parkin, G. Chemistry for Environmental Engineering and Science. 5th ed. McGraw-Hill, New York, 2003. ISBN 0072480661
- Standard Methods for the Examination of Water and Wastewater - 20th Edition ISBN 0-87553-235-7. This is also available on CD-ROM and online (http://www.standardmethods.org/) by subscriptionit:COD