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TitleExperiment COD
Tags Chemical Elements Physical Sciences Materials Chemical Substances
File Size318.9 KB
Total Pages4
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Page 3

There are several errors occur while doing the experiment. The largest error is caused by using a non
homogeneous sample. Every effort should be made to blend and mix the sample so that solids are never
excluded from any aliquot. There are several precaution in this experiment that we should followed, since the
vials contain sulphuric acid, potassium dichromate, silver and mercury, it must be handled properly and not
to through away the solutions into the laboratory sink because mercury is very hazardous. We have to careful
when take out the vials from the COD reactor because they are heated to 150oC and become very hot.


From this experiment, the COD of the raw wastewater, treated water and the standard is measured.
The COD of the raw wastewater is greater than COD of the treated water and COD of the standard is the
smallest. The percentage of reduction of the COD is above 90%


1) What was the purpose of the blank sample of distilled water?
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 indicate
the absence of contamination or super saturation of dilution water with DO.

2) Why is it important to determine COD test in wastewater treatment?
COD is a measurement of the maximum organic load and this indicates the type of treatment needed

and if doing the treatment biologically, the amount of oxygen required doing it. In effluent, it is related to
the pollution level, but there are many factors included in that determination. There are different COD
tests and each has to be interpreted a different way. Hg/Cr COD gives the max number, while Mn COD
will be closer to the BOD measurement.

3) Summarize the method and write the chemical reaction involved and interpret your test result?
A catalyst, silver compound is added to promote the oxidation of certain classes of organics, and a

mercuric compound may be added to reduce interference from the oxidation of chloride ions by the
dichromate. End products are carbon dioxide, water, and various states of the chromium ion. After the
oxidation step is completed, the amount of dichromate consumed is determined by measuring its
absorbance. In the oxidation of organic materials by dichromate in sulfuric acid, most of the carbon is
converted to carbon dioxide while any hydrogen present in the organic compound is converted to water.
Other elements also may be oxidized. A blank is also prepared to ‘zero’ (to calibrate) the
spectrophotometer. The COD value is directly displayed in the window of the instrument.

The chemical equation involved is:

4) A sample is collected for COD analysis and it is to be performed on the following day.
Describe two ways by which the sample can be preserved.

2 ways to preserve the sample is by adding sulfuric acid, H2SO4 as the preservative and adjust the
pH<2. The sample also should be cooled at 4.0˚C. It is necessary to preserve the sample to avoid of

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