Chemical Oxygen Demand (COD):
Chemical oxygen demand (COD) is used to determine the
quantity of pollution in waste water. The higher value of chemical oxygen demand
indicates the higher organic pollution in water sample. Only chemically digest
able matter can be determined by the COD test. COD determination takes less
time than the Biological Oxygen Demand test. COD is recommended where the polluted
water has toxicity and organic matter can’t be determined by biological oxygen
demand.
Principle:
The organic matter, present in water sample is oxidized by potassium dichromate in the presence of sulfuric acid, silver sulfate and mercury sulfate to produce carbon dioxide (CO2) and water (H2O). The quantity of potassium dichromate used is calculated by the difference in volumes of ferrous ammonium sulfate consumed in blank and sample titrations. The quantity of potassium dichromate used in reaction is equivalent to the oxygen (O2) used to oxidize the organic matter of water.Preparation of Potassium dichromate (K2Cr2O7) Solution:
Add 6.13 gm Potassium dichromate (previously dried at 105 °C
for at least two hours) into 800 ml distilled water. Shake the flask well to
dissolve the content and make up the solution to 1000 ml and mix well.
Preparation of Silver sulfate-Sulfuric acid Solution:
Dissolve 10 gm Silver sulfate (Ag2SO4)
in 500 ml concentrated sulfuric acid and make up the solution to 1000 ml swirl the
flask to mix well. Allow to stand the solution for 24 hours before use.
Preparation of Mercury sulfate Solution:
Dissolve carefully 0.1 gm of HgSO4 in 5 ml of
concentrated Sulfuric acid.
Preparation of Ferrous ammonium sulfate Solution (0.025 M):
Dissolve 9.8 g ferrous ammonium sulfate in a solution of 100
ml of distilled water and 20 ml concentrated Sulfuric acid. Cool the solution
and make up the solution to 1000 ml with distilled water. Standardize the solution to determine the actual concentration to calculate the chemical oxygen
demand.
Preparation of Ferroin Indicator:
Add 3.5 gm of Iron Sulfate heptahydrate and 7.5 gm of Phenanthroline
monohydrate to 400 ml of distilled water. Mix well to dissolve and make up to
500 ml with distilled water.
Test for Chemical Oxygen Demand:
1. Take 10 ml of sample into a round bottom reflex flask.
2. Add some glass beads to prevent the solution from bumping
in flask while heating.
3. Add 1 ml of Mercury sulfate (HgSO4) solution
to the flask and mix by swirling the flask.
4. Add 5 ml of Potassium dichromate (K2Cr2O7)
solution.
5. Now add slowly and carefully 15 ml Silver sulfate- Sulfuric
acid solution.
6. Connect the reflex condenser and digestboil the content using
hot plate for 2 hours.
7. After digestion cool the flask and rinse the condenser with
25 ml of distilled water collecting in the same flask.
8. Add 2-4 drops of ferroin indicator to the flask and
titrate with 0.025 M ferrous ammonium sulfate solution to the end point.
9. Make the blank preparation in the same manner as sample
using distilled water instead of sample.
Calculate the chemical oxygen demand by following formula:
COD = 8x1000xDFxMx(VB - VS)
Volume of
sample (in ml)
Where,
DF – Dilution Factor (if applicable)
M – Molarity of standardized Ferrous Ammonium Sulfate
solution
VB – Volume consumed in titration with blank
preparation
VS – Volume consumed in titration with sample
preparation
Example Calculation:
Volume of ferrous ammonium sulfate for Sample (VS)
= 23.8 ml
Volume of ferrous ammonium sulfate for Blank VB)
= 25.6 ml
Dilution Factor (DF) = 1 (sample used as it is)
COD = 8x1000x1x0.025x(25.6-23.8)
10
= 8000x0.025x1.8
10
= 360 = 36
mg/lit or ppm
10
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