Chemical Oxygen Demand [COD]
Reference:- 1) Standard Methods for the examination of water & wastewater.
by A.P.H.A. ( 16th Edition - 1985.)
2) Chemistry for Environmental Engineering.
By Sawyer, Mccarty & Parkin (4th Edition – 1988.)
Aim:- To find Chemical Oxygen Demand ( COD.) of a given wastewater sample.
Apparatus: - Reflux apparatus, Hotplate, Titration set.
Reagents: - Conc. Sulphuric Acid,
Potassium dichromate solution (0.25 N),
Ferrous Ammonium sulphate solution ( 0.1 N),
Silver sulphate. Mercuric sulphate,
Glass beads.
Significance:-
The chemical oxygen demand is a measure of the oxygen equivalent of the organic matter content of a sample that is subjected to oxidation by a strong chemical oxidant.
The COD test data is used for the following
1. It is used in the analysis of industrial wastes.
2. Results may be obtained within a relatively short time and measures taken to correct
errors on the day they occur.
3. In conjunction with BOD test, the COD test is helpful in indicating the presence of
biologically resistant organic substances.
4.The test is widely used in the operation of treatment facilities because of the speed with
which results can be obtained.
Principle:-
The organic matter gets oxidized completely by K2Cr2O7 in the presence of H2SO4 (50% of the total volume) to produce CO2+H2O at an elevated temperature. The excess K2Cr2O7 remaining after the oxidation of organic matter is titrated with standard Fe(NH4)2 (SO4)2. The dichromate consumed gives the O2 required for oxidation of organic matter.
Theory:-
1. General:- The chemical oxygen demand ( COD ) test is widely used as a means of measuring the pollutional strength of domestic and industrial wastes. It is the measurement of a waste in terms of the total quantity of oxygen required for oxidation to carbon dioxide and water.
2. Relation between BOD & COD:- During the determination of COD organic matters are oxidized completely regardless of their biological assimilability . Hence COD values are greater than BOD values and may be much greater when significant amounts of biologically resistant organic matter are present. Ratio of BOD and (COD-BOD) is called treatability index.
T.I. = BOD
COD- BOD
Range of Treatability Index
|
Type Of Treatment Required
|
T.I. < 0.5
|
Chemical Treatment
|
0.5 < T.I. < 1.0
|
Biological treatment plus nutrient supplement
|
T.I. > 1.0
|
Biological Treatment
|
a) The solution must be strongly acidic
b) Temperature should be high for oxidizing the organic matter by K2Cr2O7 completely.
c) Volatile matters (originally present and those formed during digestion period) should not
get lost during digestion. Hence reflux condenser is used.
d) K2Cr2O7 should be in excess.
4. Use of Potassium dichromate: -
a). It is capable of oxidizing a wide variety of organic substances almost completely
to carbon dioxide and water.
b). It is relatively easy to measure any excess of potassium dichromate, in order to
calculate the amount actually used in the oxidation of organic matter.
5. Use of silver sulphate: -
Certain organic compounds, particularly low molecular weight fatty acids, are not
oxidized by dichromate hence silver ion acts as a catalyst in the test.
6. Necessity of ferrous Ammonium Sulphate: -
A solution of reducing agent is used for measurement of excess oxidizing agent
remained after oxidizing all organic matter.
7. Interference of inorganic matter: -
Certain reduced inorganic ions can be oxidized under the conditions of the COD test and thus can cause erroneously high results. Chlorides cause the most serious problem because of their high concentration in most wastewaters. This interference can be eliminated by addition of mercuric sulphate to the sample prior to addition of the other reagents.
Procedure: -
Standardization of Ferrous Ammonium Sulphate. (0.1 N): -
a) 10 ml of 0.25 N K2Cr2O7 solution is taken in conical flask.
b) 20 ml of distilled water and 30ml of Conc. H2SO4 is added & cooled to room temperature.
c) Then it is titrated against given ferrous Ammonium sulphate using ferroin as an indicator.
d) Normality of ferrous Ammonium sulphate is determined as follows.
N = 0.25 x 10
Volume of titrant used.
Determination of COD of Unknown sample: -
(1) Two COD flasks A & B are taken.
(2) About 0.4 gm of mercuric sulphate is placed in each flask to suppress the
interference of chloride ion.
(3) 20 ml of distilled water is added in flask “A” and 20 ml of sample in flask “B”.
(4) 10 ml of standard (0.25N) K2Cr2O7 is added to each flask.
(5) 30ml of conc. H2SO4 is added to each flask.
(6) About 0.2 gm of silver sulphate is added to each flask.
(7) Three or four glass beads are added to each flask to prevent bumping during
boiling.
(8) The contents of each flask are refluxed for 2 hours in the COD test assembly.
(9) Each condenser is cooled and washed down with 40ml of distilled water.
(10) Each flask is taken out from COD assembly and cooled to room temperature.
(11) The content of each flask is titrated against ferrous Ammonium Sulphate using
ferroin as indicator. Colour change is blue green to reddish brown at the end.
Calculations: -
COD (mg/1) = (a-b) X N x 8 x 1000 x dilution factor
ml of sample.
Where,
a = Amount of titrant used for blank flask A.
b = Amount of titrant used for sample flask B.
N = Normality of Ferrous Ammonium Sulphate solution.
Eq. Wt of oxygen = 8
Observations :-
Sr. No.
| Flask |
Dilution factor
|
Burette Reading
|
COD mg/lit.
| Remark |
1
|
A (Blank)
| ||||
2
|
B (Sample 1)
| ||||
3
|
C(Sample 2)
|
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