3. Definition
The total amount of oxygen required
to chemically oxidize the bio
degradable and non biodegradable
organic matter.
4. Principle
Chemical oxygen demand (COD) is a measure of the capacity of water to consume
oxygen during the decomposition of organic matter. COD measurements are
commonly made on samples of waste waters or of natural waters contaminated by
domestic or industrial wastes. Chemical oxygen demand is measured as a
standardized laboratory assay in which a closed water sample is incubated with a
strong chemical oxidant under specific conditions of temperature and for a
particular period of time. A commonly used oxidant in COD assays is potassium
dichromate (K2Cr2O7) which is used in combination with boiling sulphuric acid
(H2SO4). Because this chemical oxidant is not specific to oxygen-consuming
chemicals that are organic or inorganic, both of these sources of oxygen demand are
measured in a COD assay.
6. • COD digester
• COD vials with stand
• 250 ml conical flask (Erlenmeyer flask)
• Pipettes
• Burette & burette stand
• Pipette bulb
• Tissue papers
• Wash bottle
Apparatus Required
7. ➢Cod values are particularly important in the surveys designed to
determine and control the losses to sewer systems.
➢The ratio of BOD to COD is useful to assess the amenability of
waste for biological treatment . Ratio of BOD to COD greater
than or equal to 0.8 indicates that wastewater highly polluted
and amenable to the biological treatment.
➢It is useful to assess strength of wastes ,which contain toxins and
biologically resistant organic substances.
Environmental
significance
8. Preparation
Preparation Of AgSO4:
Take 0.5gm of AgSO4 powder in a test tube and add 100 ml of dil. Sulphuric
acid and stir for 10 min.
Preparation of Ferrous Ammonium
Sulphate Solution:
Take 33 gm of FAS crystals in test tub and add 1000 ml of distilled water
Solution is found to be yellow.
Add dilute sulphuric acid to remove the yellow colour.
Now the solution turns nearly transparent.
9. Procedure
Pipette out 10 ml of water sample into clean sample tube. Add 20 ml
of Potassium dichromate solution, 30ml of silver sulphate solution and
a pinch(0.5 gm.) of mercuric sulphate to it. Add one porcelain bit to the
reaction vessel. Insert reaction vessel in the holes of the heating
block, which has attained 150 degree Celsius, fix the air condenser on
reaction vessel. Reflux the content for 30 min. ,cool to room
temperature and transfer the contents in 200 ml conical flask. Transfer
the contents by washing the tubes with 10 ml distilled water. Titrate
the excess potassium dichromate with FAS solution using 4 to 5 drops
of ferroin indicator. The end point is change of color from yellow to
reddish brown.
Repeat the same procedure with blank, Pond, using distilled water
instead of sample.
10. Review 2 Readings..
Types Of water Volume OF
sample
taken(ml)
Burette
Reading
(Initial)
Burette
Reading
(Final)
Volume of FAS
Sewage Water 10 0 ml 27.9 ml 27.9 ml
Blank Water
(Tap Water)
10 0 ml 28.4 ml 28.4 ml
11. Review 3 Readings..
Sample Water Volume of
Sample
Taken(ml)
Burette
Reading
(initial)
Burette
Reading
(Final)
Volume OF
FAS used.
Blank 10 0 ml 21.7 ml 21.7 ml
Pond 10 0 ml 16.9 ml 16.9 ml
Sewage 10 0 ml 12.4 ml 12.4 ml
12. Calculations
For The Pond water:
Pond ppm= [0.1*1000*8*(21.7-16.9)]/10
= 304 ppm
For The Sewage water:
Sewage ppm=[0.1*1000*8*(21.7-12.4)]/10
= 744 ppm
