In environmental chemistry, the chemical oxygen demand 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 or wastewater, making COD a
useful measure of water quality. It is expressed in milligrams per liter also referred to as
ppm, which indicates the mass of oxygen consumed per liter of solution.
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 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.
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]. History
For many years, the strong oxidizing agent potassium permanganate 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
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, and potassium
dichromate have been used to determine COD. Of these, potassium dichromate 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. Using potassium dichromate
Potassium dichromate is a strong oxidizing agent under acidic conditions. 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, 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. Measurement of excess
For all organic matter to be completely oxidized, an excess amount of potassium dichromate 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 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 a reddish-brown. The amount
of ferrous ammonium sulfate 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 redish-brown upon reaching the endpoint. Ferroin indicator
changes from red to pale blue when oxidized. Preparation Ferroin Indicator reagent
A solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg
FeSO4·7H2O 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 CO2, H2O, and NH3, using the following
formula: COD =(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 CO2, water, and ammonia For example, if a sample has 500 wppm of phenol:
C6H5OH + 7O2 → 6CO2 + 3H2O COD =(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. 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 Carbonaceous biochemical oxygen demand
Theoretical oxygen demand Wastewater quality indicators discusses both
BOD and COD as measures of water quality. References
Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin. Chemistry for Environmental Engineering
and Science. 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. Washington, DC: American
Public Health Association. ISBN 0-87553-235-7. "General Chemistry Online".
External links ISO 6060: Water quality - Determination of
the chemical oxygen demand Water chemical oxygen demand