Chemical oxygen demand also known as COD accounts for the amount of oxygen
consumed in a solution by chemical
reactions in order to consume organic
contaminants present, typically measured
in milligrams per liter. As oxygen is a
fundamental element for degradation of
any products the amount of oxygen that
is present and consumed in water bodies
is very important.
COD allows to estimate the effect of
discharging polluted wastewater in the
environment. When effluents with high COD levels are discharged in the
environment there will be a reaction of
the dissolved oxygen (LO) available for
the biome and biota in the environment.
Consequently the ecosystem will suffer
losses in that area until LO is back to
normal levels. COD can be tested by
chemically oxidizing organic matter
under conditions of heat and strong
acids. Here the presence of a very strong
oxidant is necessary to ensure the
proper measurement. Lately the most
common acid applied for the test is
potassium dichromate which is a
hexavalent chromium salt and very strong
oxidant oxidizing between 95 to 100 % of
all organic matter. The technique
involves a two hour digestion process in
which samples are mixed with potassium
dichromate, sulfuric acid and metal salts.
The last two added to suppress
interference and catalyze the reaction.
After the digestion process is finished
the oxidation is measured considering
the electrons consumed for the reduction
of CR6+, CR3+ and analyzed by
spectrophotometry. Since the potassium
dichromate shifts from yellow to dull
green as it oxidizes the samples can be
read in
wavelengths of 600 and 420 nanometer.
Another similar and complementary
measurement is the biological oxygen
demand BOD. As the name suggests BOD
refers to the oxygen consumed in a
solution by biological organisms
typically aerobic microorganisms and
accounts for the compounds that are not
chemically degraded. Combined these two
represent basic measures that allow to
estimate the final quality of the
effluent to be discharged in the
environment by determining COD and BOD.
the impact of the organic compound
discharged into the environment can be
estimated serving as a parameter for the
assessment of the water treatment
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