The determination of Chemical Oxygen Demand (COD) of wastewater involves a standardized procedure to measure the amount of organic and oxidizable inorganic substances in the water.
First of all,
we check the Appearance, Smell & pH of the sample water.
Reagent Preparation for COD
I. Standard Potassium dichromate (K2Cr2O7) Solution:- (0.0417M or 0.25N)
A) Dilute Primary standard grade 12.259gms K2Cr2O7.H2O (dry at
120C for 2 hours) in 1000 ml of Distilled water and mix properly.
B) Dilute 6.1295gms K2Cr2O7.H2O
(dry at 120C for 2 hours) in 500 ml of Distilled water and mix properly.
Standardization:- Take a 250ml conical flask. Add 25ml above potassium dichromate solution. Now add 2gm
of Potassium iodide (KI)
and 100ml of distilled
water. Now add 5ml Conc. HCl slowly and allow to stand for 10 min.
Titrate the
liberated iodine with 0.25M Sodium Thiosulphate. Add 3ml of Starch solution as an
Indicator.
Normality of Potassium dichromate = Vol. of thiosulphate(ml) x Normality thiosulphate/Vol. of Pot. dichromate
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Determination of the odor of a material
Sulphuric acid (H2SO4) Reagent Preparation
A) Take 10 gms of silver sulfate and add
this to 1000ml of Conc.H2SO4 and keep for 1-2
days for complete dissolution.
B) Take 5 grams of silver sulfate and add
this to 500 ml of Conc. H2SO4 and keep for 1-2
days for complete dissolution.
Ferroin Indicator Solution Preparation
Take
1.485gms of 1,10–phenanthroline monohydrate and 0.695gms of Ferrous sulfate
(FeSO4.7H2O). Add in 100ml distilled water and shake well to
dissolve it properly.
0.1N Ferrous Ammonium Sulphate Solution (FAS) [Fe(NH4)2(SO4)2.6H2O]
A) Take 39 grams of Ferrous Ammonium sulfate and add it to 1000ml distilled water. Now
add 20ml of Conc. H2SO4 slowly & Cool.
B) Take 19.5 gms of Ferrous Ammonium sulfate and add it to 500ml distilled water. Now
add 10ml of Conc. H2SO4 slowly & Cool.
Standardization:
- Take 250ml of the conical flask. Add 10ml of standard Potassium
dichromate solution and 90ml of D/M water or distilled water. Now add 30ml of
conc. H2SO4 and Cool. Add 2 drops of Ferroin
indicator and titrate with Ferrous Ammonium Sulphate (FAS).
End Point: - Orange →Green →Reddish
Brown
Titration
for this: - (In Short)
1) In Burette:
-FAS
2) In a 250ml
Conical flask
10ml K2Cr2O7solution
+
90ml D/M
water
+
30ml Conc.H2S04
+
2-4 Drops of
Ferroin indicator
↓
Reading (Reading is
near about 15+1)
Molarity of
FAS = Vol. of dichromate(ml) x Normality of dichromate
Vol.
of FAS
For Example:
- In this End Point =16.5
Vol.
of K2Cr2O7 =10 ml
Normality of
K2Cr2O7 = 0.25
M = 10
x 0.2 (FAS Consumed = 16.5)
16.5 = 2.5
16.5 = 0.1515
0.1N Sodium Thiosulphate Solution: - Dissolve
25gms of Sodium Thiosulphate in 1 liter of freshly boiled distilled water.
● Improve
its stability by adding 0.1 gms of Na2CO3.
● Store
in a clean glass bottle in a dark place.
Determination of Chemical Oxygen Demand of Wastewater
A) Potassium Permanganate Consumption Method
A rough
estimate of organic matter may be made using the fact that when potassium permanganate
is reduced by Sulphuric acid, the oxygen liberated oxidizes the organic
material.
2KMnSO4 + 3H2SO4 → 2MnSO4 + K2SO4 + 3H2O
+ 5(O)
The amount
of potassium permanganate consumed for the oxidation of organic matter can be calculated by the disappearance of a violet color.
Reagents
1) Dilute Sulphuric Acid (H2SO4) –Mix
one part of concentrated acid with three parts of Distilled water.
2) N solution of Potassium permanganate
(KMnO4) –Dilute 103.161 gms KMnO4 in 1 liter of distilled water.
Chemical Oxygen Demand Procedure
Take 10 ml of
the water sample in a test tube. Add 5 drops of dilute H2SO4 and
3 drops of N/10 solution of Potassium Permanganate. Mix well by shaking. Allow
this to stand for 6 min. Heat carefully to boiling while heating to
avoid spattering while the liquid boils.
Interpretation of Results
1) Decolorization of 5 drops of Potassium Permanganate, after standing for 5 min. without heating. Potassium Permanganate consumption above 50 mg/L. A high level of organic matter is indicated.
2) Decolorization of 3 drops of Potassium Permanganate, after standing for 5 min. without heating. Potassium Permanganate consumption 30-50 mg/L. A moderate amount of organic matter is present.
3) Decolourisation of Potassium Permanganate only after boiling. Potassium Permanganate consumption 20-30 mg/L. Only a small amount of organic matter is expected to be present.
4) No decolorization even after boiling. Potassium Permanganate consumption below 12 mg/L. Organic matter present is very little or negligible.
(B) Potassium Dichromate Method (W.B.P.C. B’s method)
The Chemical
Oxygen Demand (COD) test determines the oxygen required for the chemical
oxidation of organic matter with the help of a strong chemical oxidant. The
test can be employed for the same purpose as the BOD test-taking into
account its limitations.
The
intrinsic limitation of the test lies in its inability to differentiate between biologically oxidizable and biologically inert material.
COD determination has an advantage over BOD determination. Because in COD the result can be obtained in about 5 hours as compared to BOD in which the results are obtained in 5 days. Further, the COD test is relatively easy gives reproducible results, and is not affected by interferences as the BOD test. BOD Value is not more than 30 PPM.
Principle of Chemical Oxygen Demand (COD)
The organic
matter gets oxidized completely by K2Cr2O7 in
the presence of H2SO4 to produce CO2 +
H2O. The excess K2Cr2O7 remaining
after the reaction is titrated with Fe (NH4)2(SO4)2.
The
dichromate consumed gives the O2 required for the oxidation of
organic matter.
Interferences
Fatty acids,
straight-chain aliphatic compounds, chlorides, nitrates, and iron are the main
interfering radicals.
The
interference caused by chlorides can be eliminated by the addition of HgSO4 to
the sample before the addition of other reagents. About 0.4g of HgSO4 is
adequate to complex 40mg Chloride ions
in the form of poorly ionized HgCl2. Addition of Ag2SO4 to
conc. H2SO4 as a catalyst stimulates the oxidation
of straight-chain aliphatic and aromatic compounds.
NO2 exerts
a COD of 1.14 mg/mg NO2. Sulphuric acid in the amount of 10 mg/mg NO2 may
be added to the K2Cr2O7 solution to avoid
interference caused by NO2.
For complete
oxidation of organic matter, it is necessary to see that equal volumes of
sulphuric acid, sample, and dichromate are taken.
Apparatus
- Heating metals.
- Two round bottom flasks.
- Two condensers.
- Pipettes.
- Burette.
- Measuring Cylinders.
Reagents:
(i) K2Cr2O7 Solution (0.0417 M)/ (0.25 N)
(ii) H2SO4 Reagent
(iii)
FAS Solution i.e., Ferrous Ammonium Sulphate (0.25 N)
(iv)
Sodium Thiosulphate (0.25N)
(v)
Ferroin Indicator
(vi)
Mercuric Sulphate
(vii)
Silver Sulphate
Procedure
Take two conical flasks A for the sample and B for the blank.
(A) Sample
0.40gms Mercuric Sulphate
+
0.02gms Silver Sulphate
+
2ml Sample + 18ml Distilled Water
+
30ml H2SO4 Reagent
+
10ml K2Cr2O7 Reagent
↓
Heating for 2 hrs
↓
5 mins cooling
↓
Add 70ml Distilled Water
↓
Cooling for half an hour
↓
Titration with FAS Solution by adding 2-3 drops of Ferroin indicator. Note down the Reading
(B) Blank Sample
0.40gms Mercuric Sulphate
+
0.02gms Silver Sulphate
+
20ml Distilled Water
+
30ml H2SO4 Reagent
+
10ml K2Cr 2O7 Reagent
↓
Heating for 2 hrs
↓
5 mins cooling
↓
Add 70ml Distilled Water
↓
Cooling for half an hour
↓
Titration with FAS Solution by adding 2-3 drops of Ferroin indicator. note down reading.
Calculations of COD(mgO2) = (
B-A ) x M x 8000
ml
of Sample
Where:
(COD is
always in PPM)
A → FAS
used for Sample
B → FAS
used for Blank solution
M → Molarity
of FAS
(In Burette
FAS Solution)
End Point: - Orange
color →Light Green →Green →Bluish Green →Reddish Brown
NOTE:- Generally
a freshwater COD capacity is under 250PPM.
If this capacity is more than 250 PPM i.e., water is polluted, it means Organic & Inorganic contents are more than freshwater.
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